Wood Preservation, Second Edition - Barry a.richardson

WoodPreservation

OTHER TITLES FROM E & FN SPON Defects and Deterioration in BuildingsB.A.Richardson

Building FailuresW.H.Ransom

Building Services EngineeringD.V.Chadderton

Clays Handbook of Environmental HealthSixteenth editionW.H.Bassett

Practical Timber FormworkJ.B.Peters

Timber StructuresE.C.Harris and J.J Stalnaker

Timber EngineeringPractical Design StudiesE.N.Carmichael

The Maintenance of Brick and Stone Masonry StructuresA.M.Sowden

For more information about these and other titles please contact:The Promotion Department, E & FN Spon, 2–6 Boundary Row, London, SE1 8HN

WoodPreservationSecond edition

Barry A.RichardsonConsulting and Research ScientistDirector, Penarth Research InternationalLimited

E & FN SPONAn Imprint of Chapman & Hall

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First edition 1978

This edition published in the Taylor & Francis e-Library, 2003.

Second edition 1993

© 1978, 1993 B.A.Richardson

ISBN 0-203-47403-1 Master e-book ISBN

ISBN 0-203-78227-5 (Adobe eReader Format)ISBN 0 419 17490 7

Apart from any fair dealing for the purposes of research or privatestudy, or criticism or review, as permitted under the UK CopyrightDesigns and Patents Act, 1988, this publication may not be reproduced,stored, or transmitted, in any form or by any means, without the priorpermission in writing of the publishers, or in the case of reprographicreproduction only in accordance with the terms of the licences issued bythe Copyright Licensing Agency in the UK, or in accordance with theterms of licences issued by the appropriate Reproduction RightsOrganization outside the UK. Enquiries concerning reproduction outsidethe terms stated here should be sent to the publishers at the Londonaddress printed on this page.

The publisher makes no representation, express or implied, withregard to the accuracy of the information contained in this book andcannot accept and legal responsibility or liability for any errors oromissions that may be made.

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication data

Richardson, Barry A., 1937–Wood preservation/Barry A.Richardson.—2nd ed.

p. cm.Includes bibliographical references (p. ) and index.ISBN 0-419-17490-7 (alk. paper)1. Wood—Preservation. I. Title.

TA422.R53 1993674´.386–dc20 92–30664

C I P

To my friend John F.Levy whoseadvice and encouragement have

been highly valued for manyyears by all of us who are involved

in studies of wood deterioration andpreservation

Preface ixPreface to the first edition xi

1. Preservation technology 11.1 Introduction 11.2 Preservation principles 121.3 Wood structure 13

2. Wood degradation 232.1 Introduction 232.2 Biodegradation 232.3 Moisture content fluctuations 332.4 Fire 41

3. Preservation systems 433.1 Preservation mechanisms 433.2 Application techniques 663.3 Evaluating preservative systems 93

4. Preservation chemicals 974.1 Preservative types 974.2 Tar-oils 984.3 Inorganic compounds 1054.4 Organic compounds 1274.5 Organometal compounds 1354.6 Carrier systems 1404.7 Water repellents, stabilizers and decorative systems 1424.8 Fire retardants 1464.9 Stain control 1474.10 Remedial treatments 149

5. Practical preservation 1535.1 General principles 1535.2 Uses of preserved wood 1605.3 Health and the environment 171

Further reading 177

Contents

vii

Appendix A. Selection of a preservation system 179Table A.1 Typical preservative retentions for Baltic redwood 180Table A.2 Properties of principal construction woods used in northern

and southern hemisphere temperate zones 182

Appendix B. Wood-borers 185Table B.1 Wood-destroying termites 204

Appendix C. Wood-destroying fungi 211

Index 219

viii

Contents

Preparing a second edition of a technical book isalways interesting because the alterations thatare necessary indicate the amount of progressthat has been made since the issue of the firstedition. In this case I believe that very littleprogress has been made since I wrote my Prefaceto the first edition in December 1977, andcertainly much less progress than was made inthe previous 15 years, which were probably oneof the most exciting periods in modern woodpreservation development.

There are several reasons for this limited anddisappointing progress in recent years. It is easyto forget that the world suddenly became awarethat we were rapidly exhausting our reserves ofhydrocarbon fuels, and shortages and escalatingfuel prices then affected our lives andparticularly our industrial operations. Whilst theuse of petroleum solvents in preservatives wasobviously discouraged, the shortage of energyaffected the manufacture and application of allwood preservatives at a time when it was alsobeing recognized that forest resources were beingharvested faster than natural and plantationrenewal, and when wood preservatives weretherefore becoming particularly attractive as ameans to reduce unnecessary deterioration andconsumption. The energy crisis triggered aneconomic recession throughout the world andthe wood preservation industry was affected inthe same way as most other industries, andresearch and development expenditure has beendrastically reduced. At the same time the

development of new preservative systems hasbecome increasingly difficult due to theintroduction of more stringent health andenvironmental controls. As a result only verylarge companies and consortia can now afford todevelop new preservatives, and small companiesare suffering serious difficulties as theirestablished products become subject toincreasing restriction or even prohibition.

This increasing awareness of health andenvironmental dangers has not necessarilyresulted in the introduction of safer products,but instead the continuing use of products whichhave been widely accepted for many years. Forexample, widely used preservatives such ascreosote and the copper-chromium-arsenicsystems could not be introduced today, yet thedevelopment of safer alternatives is virtuallyimpossible because of the enormous costsinvolved, even if a new preservative is based onestablished knowledge and experience. Thepresent system is therefore actively discouragingthe development of new preservatives which aremore efficient and safer, and is insteadencouraging, through economic necessity, theextended use of established and less safe systems.

I believe that, when we look back at thisperiod in 15 or 20 years’ time, we will consider itto be perhaps the most depressing period in thehistory of wood preservation, but I also hopethat our health and environmental controlsystems will become more realistic, activelyencouraging the development of safer systems

Preface

ix

but equally actively discouraging hazardoussystems. One of the most serious problems withthe present system is the lack of understandingof the hazards; controls are based so often onparticular groups of toxic compounds withoutrecognizing that some individual compounds aremuch less toxic than others, and without ap-

preciating that the toxicity of a preservative doesnot depend on the presence of a toxic ingredialone, but also on the concentration at which itis used. Perhaps I can help a little by including,in this completely revised second edition of thisbook, an entirely new section in which I discussthese health and environmental problems.

Barry A.RichardsonLatchmere

Lainston CloseWinchester

Hampshire SO22 5HJ

x

Preface

Perhaps the most difficult task facing an authoris to decide upon the type of person for whom heis writing. This book is an attempt to provide areasonably comprehensive and non-controversialaccount of wood preservation of value to aperson approaching a study of this subject forthe first time, yet it is likely to be of equal valuefor reference purposes to the person who isalready involved in commercial woodpreservation or related research. Indeed, those inindustry would naturally tend to specialize,perhaps concentrating upon certain preservationprocesses involving the use of a particular typeof preservative applied by a particular method.Those involved in research are likely to beconcerned with a limited geographical area andtheir interest will normally be confined withintheir own scientific discipline, such asentomology or mycology. To all these personsthis book attempts to provide information on theother areas of wood preservation beyond theirdaily experience.

Wood Preservation is primarily an account ofthe situation in the principal temperate areas ofEurope, North America, South Africa andAustralasia, but the text refers in many respectsinitially to the situation in the British Isles,where wood preservation is most advanced.Wood has been imported into the British Isles forseveral centuries, so that it is widely acceptedthat it is valuable and preservation has long beeneconomically justified. Wood preservation wasfirst introduced as an industrial process in

England and it has continued to be used insituations where decay is otherwise inevitable,such as for railway sleepers (ties) andtransmission poles. However, it is not sufficientto confine this account to the British Isles alone,for even British readers require information onmany other areas. As modern trade hasexpanded preservatives and preserved woodproducts have been exported to an increasingextent to countries with substantially differentdecay hazards. In addition new borers and fungihave been introduced on imported materials.

Wood Preservation is a book on a science (oris it an art or technology?) that is steadilydeveloping, a fact that may be overlooked byscientists using this book, who will almostcertainly criticize the lack of a bibliography andreferences to specific statements in the text. Thiscannot be accepted as a serious criticism asanyone with such an advanced interest in woodpreservation will already be aware of the paperspublished in, for example, the Records of theAnnual Conventions of the British WoodPreserving Association and the Proceedings ofthe American Wood Preservers’ Association,which have extensive bibliographies and whichprovide a far more up-to-date source of furtherinformation than can be provided in any book.Other readers may criticize the failure to quotespecifications for test methods, preservativeformulations and treatment requirements, butagain these are continuously revised and vary ineach country so that the function of this book is

Preface to thefirst edition

xi

simply to establish the principles involved,leaving the individual reader to obtain copies ofappropriate specifications when required directfrom the issuing authorities, such as the BritishStandards Institution, the Nordic WoodPreservation Council and the American Societyfor Testing and Materials. No doubt a furthercriticism will be the failure to comprehensivelylist proprietary preservatives but again these aresubject to frequent changes; some are mentionedby name and a few are described in detail whenit is considered that they are particularlyimportant, but the enormous space required tolist and describe the several thousandpreservatives that are now available cannot bejustified.

Wood Preservation is concerned with woodpreservation, not with wood deterioration.Whilst it is obviously necessary for anyoneinvolved in preservation to possess at least abasic knowledge of the deteriorating agenciesthat require to be controlled, the identificationof deterioration is of limited importance. Areasonably detailed account of deterioratingorganisms is given in the appendices, but theselack the diagnostic tables that are so often afeature of such descriptions; the identification ofdeterioration is the speciality of those whoinspect structures and prepare specifications forremedial treatment, a subject that is consideredin far greater detail in Remedial Treatments inBuildings. Although a section on wood structureis included in Chapter 1, it is assumedthroughout that the reader has a basicknowledge of the properties and uses of woods.

If this creates a difficulty for any reader, he canrefer to Wood in Construction or to Appendix A,which not only summarizes the most importantpreservation treatments but also includes asummary of the properties of the more importantstructural woods.

I was introduced to wood preservation by myfather, Stanley A Richardson, and it is a subjectthat has always proved interesting to me. Themore I know about wood preservation the moreI become aware of our lack of knowledge andthe need for further observations andinvestigations. Wood preservation is aremarkably complex subject, involving so manydifferent disciplines, and my first impressionupon completing the writing of this book was ofthe enormous amount of information that it hadbeen necessary to omit and thus the very limitedamount that could be included. I have alwaysbeen encouraged in my studies of woodpreservation by my many friends in industry andthe related academic and research institutions,many of whom have kindly provided theillustrations that I have used. There are toomany of them to list, but I would like to mentionDr John F. Levy of the Imperial College ofScience and Technology, London, whosethoughts on deterioration and preservation are astimulation to so many of us. We must give himcredit for the very profound statement that ‘asfar as a fungus is concerned, wood consists of alarge number of conveniently orientated holessurrounded by food’, surely the most impressivestatement ever in support of the need forscientific wood preservation.

Barry A.Richardson

Preface to the first edition

xii

1

1.1 Introduction

It must be accepted that wood decay is inevitable.Indeed, if this were not the case our forests wouldsoon become cluttered with the giant skeletons ofdead trees. Natural durability is simply anindication of the rate of decay, but there is afurther factor of fundamental importance—whilstdecay may be inevitable in the forest it is notnecessarily inevitable in wood in service. Forexample, fungal decay is dependent on anadequate moisture content, so that a structuredesigned to maintain wood in a dry condition issufficient to ensure freedom from fungal decay,whatever the species of wood. In areas wherewood borers exist which are capable of destroyingdry wood, these structural precautions areinsufficient and it becomes essential to selectwood species which, whilst they may besusceptible to ultimate destruction from fungaldecay, possess good natural resistance to thewood-borer concerned. If wood with adequatenatural durability cannot be obtained it becomesnecessary to adopt preservation processes,although these cannot be applied universally butonly to those woods which are sufficientlypermeable to permit the required penetration andretention of preservative.

Need for preservation

Preservation involves additional cost and mustclearly be justified. The environmentalist maysee preservation as a means for reducing our

demand for replacement wood, thus conservingour forests. The economist may wish toconserve our forests for rather different reasonsbut the principle remains the same. Indeed,wood-importing countries will wish to preservein order to conserve foreign currency byreducing wood imports, whilst wood-exportingcountries will adopt preservation in order toreduce home demand for replacement wood,thus leaving the maximum possible volumeavailable for export. Even in the most primitivetropical jungle village wood preservation haseconomic importance, for in these conditionsthe ravages of fungi, termites and other wood-destroying organisms ensure that anunacceptable amount of time and effort isdevoted to replacing wooden structures such ashomes and bridges. If preservation is practised,either by the selection of more durable speciesor by the adoption of a simple preservationprocess, structures may double their life. In thisway more time and effort is available toimprove the quality of life in the community,perhaps by growing extra crops for sale. In suchprimitive communities wood has no value as itis freely available, but the labour for repair andreconstruction represents a substantial burdenon the community which is just as significant inmore sophisticated countries. For example, intemperate climates a normal transmission polepressure-treated with creosote will have atypical life of 45–60 years, whereas an identicaluntreated pole will last only 6–12 years. Asimilarly treated railway sleeper (tie) can be

Preservationtechnology

1

Preservation technology

2

expected to last more than 35 years incomparison with only 8–10 years for untreatedwood. In these conditions preservation has nowbeen universally adopted and, as a result, thereis a tendency to forget the basic economics; ifuntreated structural wood deteriorates theexpense incurred is not confined to the cost ofits replacement, or even this and the additionalcost of labour required, but it also involves theperhaps much higher cost arising throughstructural failure. It can always be argued thatfailure can be avoided through regularinspection, but this cannot reduce the amountof disruption caused whilst services areinterrupted during repair and replacement.

Preserved wood must be regarded as anentirely new structural material and must not beconsidered as just an improved form of wood, asit can be used in entirely different circumstancesand certainly in more severe exposure situations.The most obvious advantage of preserved woodis that it can be used with impunity in situationswhere normal untreated species would inevitablydecay, but it may be argued that, in manysituations, this is a property that it enjoystogether with many competitive materials. Infact, the use of wood has many advantages. It isextremely simple to fabricate structures fromwood and, even in the most sophisticatedproduction processes, the tooling costs arerelatively low compared with those forcompetitive materials. Wood is ideal if it isnecessary to erect an individual structure for aparticular purpose but it is equally suitable forsmall batch or mass production. When theseworking properties are combined with the otheradvantages of wood, such as high strength toweight ratio, its excellent thermal insulation andfire resistance, and the unique aestheticproperties of finished wood, it sometimesbecomes difficult to understand why alternativematerials have ever been considered! However,there is one feature of wood which is uniqueamongst all structural materials; it is a cropwhich can be farmed, whereas its competitors

such as stone, brick, metal and plastic are allderived from exhaustible mineral sources.

With all these various advantages wood haslittle to fear from competitive materials,provided it is efficiently utilized and eitherselected or preserved to ensure that it iscompletely durable in service. The need fordurability is obvious, yet traditions are difficultto displace and in many countries the progressivedeterioration of wood in service is generallyaccepted. It is unlikely that all owners ofbuildings and other wooden structuresthroughout the world can be educated toappreciate the actual costs of the material andlabour involved in repairing decay damage, butthe authorities in many countries are becomingincreasingly conscious of the way in which thesecosts can affect prosperity. In this connectionone current problem is the demand for woodpulp which directly competes with structuralwood for the available forest resources. A highpulp yield can be achieved after short growingperiods so that there is a tendency to fell forestswhilst they are very immature to give rapidreturn on the invested capital. This has resultedin rapid increases in the cost of wood and afurther justification for its efficient utilizationand its preservation to avoid decay.

History of preservation

Preservation is not, in fact, new. The ancientsworried little at first about decay as theirbuildings were seldom very permanent andreplacement wood was easily obtained.Probably the person earliest recorded as usingwood preservative was Noah who, whenbuilding the ark, was instructed by God to‘pitch it within and without with pitch’. In fact,various oils, tars and pitches were used fromtimes of the most remote antiquity. Herodotus(c. 484–424 BC), a Greek whose monumentalwork earned him the title of ‘Father of History’,writes of the art of extracting oils, tars andresins. Healso draws attention to a much older

Introduction

3

system of preserving organic matter, the ancientEgyptian art of mummifying or embalmingbodies. This is probably the most efficientmethod of preserving organic matter that hasever been devised. The Egyptian mummies arenow at least 4000 years old and many are aswell preserved as when originally entombed.Herodotus and Diodorus Siculus (1st centuryBC) indicate that the body was steeped innatrum (or natron) for 70 days and then in anoily or bituminous substance for a similar time.Natrum, the production and use of which was astate monopoly in Ptolemaic times, from c. 320BC, was a mixed solution of sodiumsesquicarbonate, chloride and sulphate. It wasobtained from three centres fed during the floodseason by seepage from the River Nile. Themost important centre was an oasis in theWestern Desert still known as Wadi Natrum. Itis not possible that mummifying was practisedin the simple way described as the bituminoussubstance would scarcely penetrate, yet it hasbeen found that even the interior of the boneshas been penetrated. It is probable that thebody, after steeping in natrum, was placed inthe bituminous substance which was heated totemperature above the boiling point of water sothat the water within the body volatilized andwas then replaced by the oil. Boulton carriedthis out on a piece of wood in the middle of the19th century; his results indicated thecorrectness of the theory and were also theorigin of the Boultonizing treatment which isstill in use today.

The Egyptians were not the only people touse metallic salts as preservatives. The Chinesewere immersing wood in sea water or the waterof salt lakes prior to use as a building materialbefore 100 BC. Well preserved props have beenremoved from old Roman mines in Cyprus andexamination has shown them to containmetallic copper, well distributed throughoutboth the heart and sapwood. Various theorieshave been advanced to explain its presence as aRoman attempt at preservation, but is seems

more likely that the true explanation involvesthe copper found in the soil in this area. It ispossible that the process was electrolytic, oneend of the prop being in one type of soilcontaining copper and the other end being inanother type of soil so that the damp woodformed a rather complex cell.

Marcus Porcius Cato (234–149 BC), a Romanwhose condemnation of the luxury of his timesearned him the nickname of ‘Cato Censorious’,commented on wood preservation, but by far themost informative writer was the renownedRoman naturalist Pliny. Pliny the Elder (AD 23–79), who perished at Pompeii during the eruptionof Vesuvius, mentioned that Amurca, the oil-lessby-product in the manufacture of olive oil, andalso oils of cedar, larch, juniper and nard-bush(Valeriana spp.) were used to preserve articles ofvalue from decay. He claimed that wood wellrubbed with oil of cedar was proof againstwoodworm and decay, and in his writingsdescribed the preparation of 48 different kinds ofoil for wood preserving. He also observed that themore odoriferous or resinous the wood the moreresistant it was to decay. Because the statue ofZeus (Jupiter) by Phidias was erected in a dampgrove at Olympia its wooden platform wasimbued with oil. The statue of Diana at Ephesuswas made of wood and was believed to have beenof miraculous origin. Pliny, quoting an eye-witness Musicians, notes that it was still thoughtnecessary to saturate it with oil of nard throughsmall orifices bored in the woodwork. TheRoman use of olive oil was copied fromAlexander the Great (356–323 BC), the king ofMacedon who conquered a large area of theknown world. He is said to have ordered piles andother bridge timbers to be covered with olive oilas a precaution against decay.

The previously mentioned statue of Diana atEphesus was underpinned with charred piles. Thiswas not a new idea as a prehistoric race, theBeakermen, applied charring to wood. Theaborigines called the Tiwi who live on MelvilleIsland near Darwin, Australia, and whose


4

civilization is said to be 50 000 years behind ourown, mark their graves with brightly painted poleslike American Indian totem poles, which are madefrom Bloodwood, a hard red-sapped wood whichthey have found to be resistant to termites andfungal decay. Prior to painting, the wood is charredand covered with beeswax, orchid sap or white ofturtle egg. This may be the continuation of someprimitive knowledge of wood preservation.Alternatively it may be solely a method of forminga suitable background for painting.

As soon as man began using wood as abuilding material it was only a matter of timebefore decay became domesticated. The fungusthat probably causes the greatest damage inbuildings is the common Dry rot fungus Serpulalacrymans. It has never been recorded asoccurring in nature and appears to be associatedonly with man-made structures. The name isderived from the Latin word lacrima, a tear, forSerpula lacrymans, (formerly known as Meruliuslacrymans is the weeping fungus as fresh growthcan often be observed covered with drops ofwater. It is this weeping or fretting which enablesit to be identified as the ‘fretting leprosy of thehouse’ in the Old Testament Book of Leviticus.Until comparatively recent times the priest wasthe person who was called in to deal with anykind of trouble or pestilence and consequentlyLeviticus contains full instructions on how todeal with the ‘leprosy of the house’. The priest,when carrying out his inspection, was to look for‘hollow strakes, greenish or reddish, on thewalls’. If they were present the house was to beshut up for 7 days, and if after that time ‘theplague be spread in the house, it is a frettingleprosy’ and Dry rot rather than one of the Wetrots, and he was to ‘command that they takeaway the stones in which the plague is, and…cast them into an unclean place without thecity,…the house to be scraped within roundabout, and…pour out the dust…without the cityinto an unclean place’. This may appear ratherruthless but even today an affected area must bestripped to the bare masonry to ensure the

successful application of fungicide, the Dry rotfungus spreading through masonry in the searchfor wood. It is in these verses of Leviticus, whichalso describe leprosy in man, that we may readof early ideas of contagion, and people enteringthe house were required to wash themselves andtheir garments thoroughly on leaving. If thepriest found that the fungus had not developedafter replastering the affected area he was toapply final ‘fungicidal’ treatment and ‘take tocleanse the house two birds, and cedar wood,and scarlet, and hyssop’. He was instructed tosacrifice one bird and sprinkle the house seventimes with the blood. The other bird, after beingdipped in the blood, was freed and flew away,presumably taking the pestilence or infectionwith it!

It was the belief that the words of the Bibleand several other books were completelyirreproachable that severely discouraged thedevelopment of science and technology up to theearly 16th century. Ideas not in agreement withthese standard works were considered heretical,and often people were put to death forexpressing them. Other than those alreadyquoted, references to timber preservation beforethe 18th century appear to be negligible,although timber decay was frequently describedand appears to have been a serious problem.

Early problems

In the reign of Elizabeth I, Britain’s greatest assetwas her navy. When Elizabeth came to thethrone she found that ten out of the 32 Royalships were suffering from decay. There was,apparently, no accepted method of woodpreservation, and the condition of the navy wasso bad a few years later during the reign ofJames I that a commission of inquiry wasappointed. The findings emphasized theimportance of constructing ships of seasonedwood, but little notice appears to have beentaken of this. James I was succeeded by his sonCharles I, who was beheaded, but whose son

Introduction

5

Charles II, was crowned on the restoration of themonarchy. A period of rearmament commenced,the navy’s programme of shipbuilding being inthe control of Samuel Pepys, the Secretary to theAdmiralty. In his famous diary Pepys remarkedon the shortage of wood being such that a largeamount of green and unseasoned wood was usedin shipbuilding. The result was that many shipsbegan to decay before being commissioned.Pepys very wisely suggested that this would notbe so if the ships were better ventilated. Thesetroubles were not confined to Royal Navy ships,and the merchantmen of the East India Companyseldom made more than four, and sometimesonly three, voyages to India before becominguseless through decay.

The shortage of wood that Pepys referred towas becoming serious. The almost continuousarming and rearming occurring throughout thesetimes meant that extremely large quantities ofwood were required for shipbuilding.Concurrently the rise in the use of iron saw theprogressive destruction of the Wealden and otherforests to provide smelting charcoal. DanielDefoe in 1724 wrote that the Sussex ironworkswere carried on ‘at such a prodigious expense ofwood, that even in a country almost overrunwith timber, they began to complain ofconsuming of it for the furnaces and leaving thenext age to want for timber for building’. Hesaw no justification for the complaint, for Kent,Sussex and Hampshire were ‘one inexhaustiblestorehouse of timber’. Defoe was wrong, ofcourse, for wood was being used at such analarming rate that there was already a noticeableshortage in the 16th century. Various Acts werepassed through Parliament in attempts to limitconsumption and obtain supplies from abroad.There were also attempts to transfer the ironsmelting industry to North America but thisnever came about because of the introduction ofcoal for smelting. Later a further significanteconomy occurred when prejudices against theuse of coal as a domestic fuel were finallyovercome. Softwood began to arrive in

increasing quantities from the Baltic and Canadabut, despite this, the general shortage soon raisedprices and the need for preservation becamemore apparent.

The wastage of ships in the Royal Navy onaccount of decay was becoming an extremelygrave problem. Little seems to have been doneagainst Dry rot, for in 1771 Lord Sandwich hadthe fungus dug out of the reserve ships so that hemight inspect the timbers. Dry rot was not theonly problem the navy had to contend with, forthere were also the marine-borers such asshipworm and other fouling organisms thatattacked the outsides of ships. At the time ofVasco de Gama (1469–1524) the Portuguese areknown to have charred the outsides of their shipsas a protection against shipworm, and in 1720the Royal Navy built a ship the Royal Williamentirely from charred wood. It appears that theexperiment failed for it was never repeated, butcharred wood is still used for shipbuilding by theSolomon Islanders, who apparently learned thepractice from the Portuguese. Covering the hullwith sheet metal as a protection againstshipworm was a system used as early as Romantimes. Lead was the metal that was most easilyworked into sheets but it was so heavy that itpulled away from its fixings. Copper was triedby the British Navy but this coincided with theintroduction on a large scale of iron fittings forships. Electrolytic action occurred, seriouslydamaging rudder bearings, so that the use ofmetal sheeting was discontinued. In 1782 theprevalence of Dry rot in Royal Navy ships wasmade very apparent by the tragic loss of theRoyal George at Portsmouth. At the courtmartial investigating the deaths of the 800 menon board it was disclosed that previously thebottom had fallen out while the ship was beingheeled over for slight repairs.

Early preservation

In 1784 the Royal Society of Arts offered a goldmedal ‘for the discovery of the various causes of


6

Dry rot in timber and the certain method of itsprevention’. It was awarded in 1794 to Batsonwho, in 1778, had treated an outbreak of Dry rotin a house by removing sub-floor soil andreplacing it with anchorsmith’s ashes. In the early19th century Britain was successfully negotiatinga most difficult period of history, highlighted bythe wars against Napoleon and in North America.The Royal Navy was more important than everbefore but, although the shortage of availablewood brought the decay of ships daily into clearerperspective, it was not until 1821 that theAdmiralty asked James Sowerby to investigate theproblem. He reported on the state of the QueenCharlotte, a first rater of 110 guns that waslaunched in 1810 at a cost of £88 534. Only 14months after launching she had to be re-built at acost of £94 499 before she could becommissioned. By 1859, when her name waschanged to Excellent, the total cost of repairs hadrisen to £287 837. Sowerby reported the cause asfungal rot and identified a score or more of fungi.

The Royal Navy was not the only suffererfrom Dry rot. In 1807 James Randell spoke tothe Royal Society of Arts on the subject of Dryrot and mentioned that it had destroyed thegreat dome that Robert Taylor had built on theBank of England. By this time people werebeginning to take serious interest in chemicalpreservatives. 1812 saw the first attempt atfumigation when Lukin experimented inWoolwich dockyard with the injection ofresinous vapours; the attempt was abandonedafter the apparatus exploded with fatalconsequences to the workmen.

Salt preservatives

The 19th century produced further interest inwood decay on account of the widespreadexpansion of the railways. The stone blocks firstused for supporting the rails were found to betoo rigid and wooden sleepers were substituted.These rapidly decayed and obviously a goodchemical preservative was required. The first list

of established preservatives, published in 1770by Sir John Pringle, was followed shortlyafterwards by a second list drawn up by DrMacbride. The age of chemical woodpreservation had arrived and these lists bothappeared in the 1810 edition of EncyclopaediaBritannica. By 1842 five processes wereestablished using mercuric chloride, coppersulphate, zinc chloride, ferrous sulphate with asulphide, and creosote respectively.

Mercuric chloride, first used by the Frenchscientist Homberg in 1705 to preserve woodfrom insect attack, was later recommended byDe Boissieu (1767) and Sir Humphrey Davy(1824), and in 1832 Kyan took out his patentfor this process which became known asKyanizing. Kyan’s first success was thepreservation of the Duke of Devonshire’sconservatories. The British Admiralty tested itand found it failed against marine organisms.This was more than a century after the DutchGovernment had found exactly the same result.Mercuric chloride, also known as corrosivesublimate, is soluble in water, volatile atordinary temperatures and poisonous. Its usecontinued for some time in the United Statesand Germany but probably the last large-scaleinstance of Kyanizing in Britain was in 1863.

The use of copper sulphate was recommendedas early as 1767 by De Boissieu and Bordenave.Thomas Wade recommended it in 1815, and in1837 Margary took out a patent for its use inwood preservation. Copper sulphate was by farthe most successful metallic salt and long after itdied out of use in England its popularitycontinued in France, where it was applied by amost ingenious method known as the Boucherieprocess, by which a newly felled tree isimpregnated by replacement of the sap. TheBoucherie copper sulphate process was verypopular in France from about 1860 until it wentout of favour in 1910 because of some failureson alkaline soil. It was used for preservingtelegraph poles in Britain prior tonationalization in 1870, and continued in use in

Introduction

7

the South of France and Switzerland until a fewyears ago. The Boucherie process was revived in1935 by the Deutsche Reichpost but with amultisalt preservative.

The antiseptic properties of copper sulphatehave never been questioned and it is only thesolubility in water of this and other metallic saltsthat makes them unsuitable as woodpreservatives in wet situations. It will be recalledthat the ancient Egyptians had a very effectivemethod for the preservation of bodies, whichBoulton suggested had involved steeping innatron followed by pickling in a bituminoussubstance. Boulton impregnated a piece of woodwith natron and afterwards placed it in creosoteat a temperature above the boiling point ofwater. He found that water evaporated,depositing the natron salts in the wood, and thecreosote then penetrated well into the driedwood. This process formed the basis of a patentby Boulton in 1879 except that copper sulphatewas used in place of natron. It was successful butwas little used because of the success of creosotealone, the addition of the copper sulphateserving only to increase the cost. Boultonsuggested, however, that an oil with nopreservative properties could be used in place ofthe creosote, its purpose being solely to preventleaching of the copper sulphate. ModernBoultonizing involves the use of high-temperature creosote and vacuum, but simply toboil off moisture within the wood so that thecreosote is able to penetrate.

Zinc chloride was recommended as a woodpreservative in 1815 and 1837 by Thomas Wadeand Boucherie respectively. In 1838 Sir WilliamBurnett patented its use but throughout its woodpreserving history it has suffered because of itsextreme solubility in water. Despite this it wasmuch used by the Royal Navy. Because of theshortage of creosote in the United States its usecontinued there long after it was forgotten inBritain. Even there, however, its use graduallydiminished because of the expansion of the coalgas industry and the increased imports of

creosote from Britain as return cargo inpetroleum tankers. Although extremely soluble itwas found to retain its power of preservation toa small extent. This was thought to be due to theformation of zinc oxychloride, insoluble in waterbut poisonous to fungi because of its solubility intheir enzyme secretions. This hypothesissuggested the idea of the formation of insolublepreservatives within wood by the application oftwo or more solutions, a process that has beeneffectively applied using various materials sincethe beginning of the 20th century. However, thefirst precipitation process was a complete failure.It was in 1841 that Payne was granted a patentfor his two-stage process which involvedimpregnation of wood with ferrous sulphatefollowed by calcium sulphide. It was said that adouble decomposition occurred within the poresof the wood, forming ferrous sulphide andcalcium sulphate, both only sparingly soluble inwater, but the treatment was found to havenegligible preservative effect.

Coal-tar products

Creosote is certainly the most successful of thepreservatives developed during the 19th century.The process, known as creosoting, is basedessentially on a patent granted to John Bethell in1838. Bethell’s patent lists 18 substances,mixtures or solutions, including metallic salts,oleaginous and bituminous substances. Althoughthe word ‘creosote’ is not used, mention is madeof a mixture of dead oil with two or three partsof coal-tar, and this is the origin of creosoting bypressure impregnation. It was not the firstattempt at the use of tars, for as early as 1756experiments were carried out in Great Britainand America (Knowles) on the impregnation ofwood with vegetable tars and extracts. Boultonsuggested that the term creosote originated in apatent granted to Franz Moll in 1836. Thispatent was concerned with injecting wood inclosed iron vessels with extracts of coal-tar, firstas vapour and then as oil in a liquid state. Moll


8

termed oils lighter than water ‘Eupion’ and thoseheavier ‘Kreosot’, the latter being said to haveantiseptic qualities. The process was notpractical as the light oils immediately evaporatedon the application of the heated kreosot, and itwas left to Bethell to patent the moderncreosoting process two years later. Franz Mollprobably derived his term ‘kreosot’ from theGreek words kreas for flesh and soter forpreserver but, although the term kreosot wasapplied to the heavy coal-tar oils, the term‘creosote’ was not derived directly from it. Evenin Franz Moll’s time the term ‘creosote’ wasapplied to a product of the dry distillation ofwood, and the term was applied to the heavy tar-oils in the belief that true creosote was identicalto the carbolic acid contained in coal-tar.Boulton mentions that Tidy compared these twosubstances and showed them to be dissim-ilar.He also demonstrated that coal-tar contained notrue creosote but the term ‘creosote’ is nowuniversally applied to the heavy oils producedduring the distillation of coal-tar.

By 1853 creosote had established itself as amost reliable and persistent wood preservative,and most other processes were abandoned. InFrance, however, creosote established laterbecause of the popularity of copper sulphateallied by the Boucherie process. In 1867Forestier, working for the French Governmentand also Dutch Government investigators,showed that creosote of a suitable grade,efficiently applied, rendered wood resistant toshipworm. At about the same time, Crepin,working for the Belgian Government, showedthat this applied also to other marine animals.However, there were distinct failures resultingfrom the use of the wrong type of creosote andthese focused attention on studies of thecomposition and effectiveness of various grades.

In 1834 the German chemist Rungediscovered phenol (carbolic acid) in coal-tar, andin 1860 Letheby attributed the preservativeproperties of coal-tars to this component.Carbolic acid was recognized as an effective

fungicide and it was due to investigations intothe wood-preservative properties of coal-tar thatthe world gained the benefit of a most importantmedical development. A young surgeon, JosephLister, was discussing with a railway engineer hismethod for preserving sleepers (ties) and wasinformed that it was the carbolic acid whichprevented decay. Lister, worried by the highdeath rate due to infection during operations,immediately saw that carbolic acid might beused to prevent it. He started operating under acontinuous carbolic acid spray, all hisinstruments and his own hands having beenwashed in the acid. Conditions were mostunpleasant but he thought it worthwhile if hecould save a few lives. He was surprised anddelighted to find that, instead of the slightimprovement that he had hoped for, he hadachieved almost complete success. There wasopposition to his idea at first, but his dramaticresults eventually gained him universalrecognition and it was not long before carbolicacid was described in medical textbooks as the‘aerial disinfectant par excellence’.

Letheby, appreciating the antiseptic propertiesof carbolic acid, specified that naphthalene andpara-naphthalene should be excluded as far aspossible from creosote as he considered them tohave no preservative value. However, in 1862Rottier concluded that carbolic acid, althoughan energetic antiseptic, had little persistent effectdue to its volatility and solubility in water. Heattributed the durable success of creosote to theheavier and less volatile components of coal-tar.Despite the interest in wood preservationresulting from the expansion of the railways andlater of the telegraphs, it was not until 1863 thatreally instructive experiments were carried out.These experiments, which were conducted byCoisne on behalf of the Belgian Government,were repeated in 1866. Coisne treated woodshavings with various grades of creosote andplaced them in a putrefying pit for 4 years. Theresults were entirely in favour of the use of theheavier oils, tar acids by themselves having no

Introduction

9

persistent effect. These results were confirmed bylong-term experience and the BelgianGovernment adopted the recommendations forits successful creosoting specifications.

About this time there were two theories on thecause of decay. The accepted theory was that ofthe great scientist Liebig, enunciated in greatdetail in 1847 and 1851, which claimed thatputrefaction or decay of an organic material wasa form of slow combustion which he termed‘Eramacausis’ and that it was initiated oncontact with bodies which were already affected.He discovered that it could be prevented by lackof moisture and atmospheric air, and from thishe deduced (and later showed) that it wasprovoked by oxygen. Dalton’s atomic theory,proposed in 1808–1810, was by this time one ofthe foundation stones of science and Liebigclaimed that the method of transfer was thecommunication of motion from atoms of theinfected matter to atoms in the contactingmaterial. He denied that fermentation,putrefaction or decomposition was caused byany fungi, animalcules, parasites or infusoriathat might be present, their presence beingcoincidental or due to a preference for feedingon decaying matter.

The parts of animals and plants which decaymost rapidly are the blood and the sap. It wassuggested that decay could be prevented bycoagulants of albumin, such as mercuricchloride, copper sulphate, zinc chloride and thetar-oils. In 1854 Louis Pasteur was appointedProfessor and Dean of the Faculty of Science atLille. Here he concentrated on his study offermentation in the production of beer and wine.Three years later he moved to the Ecole Normaleat Paris as Director of Scientific Studies, andwhile there he proclaimed that fermentation wasthe result of the action of minute organisms. Iffermentation failed to occur it meant that theorganism was absent or unable to developproperly.

Liebig had observed that decay requiredatmospheric air and deduced that this was

because oxygen was necessary. He confirmedthis theory by showing that oxygen accelerateddecay. Pasteur repeated the experiments and in1864 announced that decay was caused byminute organisms that were not spontaneouslygenerated but were instead present inatmosphere, and this was one reason whyatmospheric air was necessary. It was only a yearafter this that Lister appreciated the significanceof infection in surgery and, as previouslymentioned, initiated the use of carbolic acid as asurgical disinfectant. Pasteur’s theory wasconfirmed by Koch and soon gained supportamong authorities such as Tyndall. This wasimmediately thought to be a simplification of thetheory of preservation as the only problem wasto discover substances toxic to the decay-causingorganisms.

Pasteur was not the first to declare decay tobe caused by living organisms. As far as woodwas concerned there was the obvious damage byborers and also the presence of fungi. In 1803Benjamin Jonson had declared Dry rot to be theresult of a ‘visit from a plant and is and ever wasso’ but he left it to Theodore Hartig in 1833 torecognize the general association between fungiand decay. This association had been noticedbefore but it was Theodore’s son Robert who in1878 showed fungi to be directly responsible fordecay. He continued his studies in 1885 andmade a thorough investigation into the Dry rotfungus and its effect on wood.

By 1884 the wood-preserving industry hadbeen established long enough for serious interestto be taken in long-term effectiveness. Themetallic salts had broken down completely andtheir use had been largely discontinued in favourof creosote. It was many years before interest insalt preservatives revived with the developmentof the multisalt preservatives, described in detailin Chapter 4. Creosote had been known to failbut because of the careful records of treatmentkept by some of the impregnating companies,coupled with the work of people like Coisne,Boulton and Tidy, specifications had been


10

developed which could be relied on to give goodprotection. Boulton carried out tests in 1884 ona 29-year-old fence in London Docks, apparentlyas sound as when it was erected. He detected notar acids but found the semi-solid constituents oftar-oils, including naphthalene, to be present. Hefound very little distilling below 232°C (450°F)and 60–70% distilled above 316°C (600°F). Hemanaged to detect acridine solidified in the poresof some of the specimens. This is an acrid andpungent substance, neither volatile nor soluble inwater, that had been discovered by Graebe andCaro. Greville Williams also examined samplesfrom the fence and, although he managed todetect traces of tar-acids, the indication was veryslight and was probably due to the heaviest tar-acids trapped within solidified portions of theoil. In nearly all of the specimens he detectednaphthalene and in all he detected acridine andbasic substances. He concluded that thepreservative action was due more to the latterthan to the tar-acids. Tidy experimented onnaphthalene, finding that it remained in thepores of the wood. Although not as powerful anantiseptic as the tar-acids it was far morepersistent. He decided that the para-naphthaleneor anthracene contained in tar-oils was probablywithout wood-preserving properties and drew uphis creosote specification accordingly. Thisstandard, introduced in 1883, was the basis ofnearly all British specifications until the BESA(now BSI) specification was introduced in 1921.

In 1824 Hennell had synthesized alcohol and 2years later Wohler was responsible for thesynthesis of urea. These achievements opened thedoor to tremendous developments in industrialsynthesis of organic compounds. Coal is averitable treasure chest of raw materials for theseprocesses, and it was not long before coal-tarbegan to suffer from the extraction of some of itscomponents. Typical of this was the use made ofanthracene. From the earliest times the roots ofmadder (Rubia tinctoria) had been used as adyestuff in India and Egypt. The principal dyeinvolved is alizarin which is present in the root as

a glucoside, ruberythric acid. This can behydrolysed to glucose and alizarin, and wasextensively employed until towards the end of the19th century in the production of Turkey Red dyefor dyers and printers. However, in 1868 Graebeand Lieberman found that alizarin could bereduced to anthracene by heating with zinc dust.They suggested a rather expensive process forsynthesizing alizarin from anthracene, which wassoon relinquished in favour of an alternativeprocess they discovered simultaneously withPerkin, the ‘Father of Dyeing’.

Whilst the increasing sophistication of thechemical industry threatened to reduce theeffectiveness of creosote, it was also ultimatelyresponsible for the development of compoundssuch as pentachlorophenol and the organo-chlorine insecticides which made the formulationof organic solvent-based preservatives possible,as described in Chapter 4. Fortunately, Tidy hadalready shown that anthracene had only weakwood-preserving properties, so that there was noconflict between dye manufacturers and creosoteusers. Other changes in the composition ofcreosote were caused by the different methods ofcoking and the varying grades of coal. All thismade it more important that the principal wood-preserving components in creosote should beidentified. Work has continued to the presentday, but despite improved methods thepreservative action of creosote is still imperfectlyunderstood. In 1951 Mayfield concluded that‘the toxicity of creosote is not due to one or avery few highly effective materials but is due tothe many and varied compounds which occurthroughout the boiling range. The value ofcreosote as a wood preservative depends largelyon whether or not it remains in the wood underthe conditions and throughout the period ofservice’. Essentially this means that a particulargrade of creosote cannot be said to be efficienton the merits of its chemical composition alone.The only true test is to use it and see how itperforms in normal service, but the difficulty isthe length of life expected of creosote; the fence

Introduction

11

tested by Boulton in 1884 lasted about 70 years.Even then it was demolished only to make wayfor another structure and was still reasonablypreserved. Any field test would take as long, sothat evaluation of new preservatives is oftenbased on laboratory comparisons of preservativetoxicity.

Application methods

Little has been said of the methods used forapplying preservatives. An effective preservativecan be a complete failure if inefficiently applied,and this is the explanation of the early failures ofcreosote in the United States. Vacuum andpressure methods of impregnation undoubtedlygive the greatest certainty of lastingpreservation. Breant is said to have been theinventor of this process when he took out apatent in 1831, but in Great Britain Bethell wasgranted a patent in 1838 which includedamongst other substances creosote applied bythis means. The method soon became known asthe full-cell or Bethell process, although it wasmodified to its present commercial form, whichwill be described in detail in Chapter 3, by Burt,who was granted a patent for his improvementsto the method. With creosote the method isineffective when applied to unseasoned or wetwood, so that extensive storage facilities arerequired for drying and seasoning. In 1879Boulton was granted a patent for his Boilingunder Vacuum process, using hot creosote to boiloff the water in the wood. This process may befollowed by the full-cell process or an empty-cellprocess such as the Rüping process. Steamingand steaming-and-vacuum processes were triedas alternatives to the Boulton process but withno great success.

There are several difficulties encountered withthe full-cell process. Creosote bleeding is likelyto occur, an annoying factor with fences andpoles that pedestrians and animals are likely toencounter. Another aspect is the quantity ofpreservative used, a very important point in

countries where preservatives, especiallycreosote, are scarce and expensive. The empty-cell processes are a great improvement asbleeding is less likely to occur and there is a 40–60% reduction in the use of preservative. Thelatter is especially important in the case ofparticularly permeable woods and those with ahigh proportion of sapwood. The empty-cellmethods in common use, the Rüping and Lowryprocesses, will be decribed in Chapter 3.

The Rüping process was initially patented byWasserman in Germany in 1902, althoughRüping applied the process commercially andAmerican patents were subsequently granted inhis name. The process is commenced by theapplication of an initial air pressure. When theentire process is complete the pressure isreleased, the compressed air in the cells drivesout some of the preservative and a short periodof vacuum recovers more preservative, so thatthe net retention in the wood is only about 40%of the gross absorption, a saving in preservativeof 60%. The Lowry process, which was patentedin America in 1906, differs only in that it relieson compression of air at atmospheric pressurefor return of excess preservative, so that there isno initial compression stage. The recovery ofpreservative is about 40%.

Other similar processes due to Hülsbert,including the Nordheim process of 1907, havebeen entirely superseded by the Rüping process.In 1912 Rütgerswerke AG were granted a patentfor treatment of insufficiently dry timber by theRüping process. It is identical with Boulton’spatent except that an oil, used for evaporating thewater, is drawn off before the Rüping process isapplied. The vacuum and pressure methods arethe most important and most effective methodsused for the application of wood preservatives.They suffer, however, from the great disadvantagethat special plant is required and it is oftenimpossible or uneconomical to send wood to theplant for treatment. Numerous non-pressuremethods are available but are suitable for use onlywith specially developed preservatives such as the


12

low-viscosity organic solvent products for sprayand dip treatment of dry wood, and theconcentrated borate solutions which can be usedfor diffusion treatment of freshly felled woodwith high moisture content. Preservationprocesses are discussed in detail in Chapter 3.

1.2 Preservation principles

The simplest method to avoid deterioration is touse only naturally durable wood. Durability isan embarrassment in nature as it delays thedisposal of dead trees, and it can therefore beappreciated that only a limited number of woodspecies are truly durable. This durability isalways confined to the heartwood but theelimination of sapwood, coupled with selectionfrom a very limited range of species, isunrealistic unless very high costs can betolerated. It is far more realistic to select a woodspecies for its physical properties and then totake suitable precautions to ensure thatdeterioration is avoided. This does notnecessarily mean the use of preservativetreatments. For example, the most efficientmethod to avoid fungal decay is to keep wooddry, and this is most simply achieved bystructural design, such as the incorporation ofoverhanging eaves and gutters to dispose ofrainfall and damp-proof membranes to isolatestructural wood from dampness in the soil orsupporting structure. However, there are somesituations, usually termed severe hazardconditions, where deterioration is una-voidableunless naturally durable or adequately preservedwood is used. The most important severe hazardrisk is the ground contact condition which arisesin transmission poles, fence posts and railwaysleepers (ties). In some areas insect-borer attackis virtually inevitable whatever the serviceconditions, such as in areas subject to the DryWood termites. In some parts of Europe theHouse Longhorn beetle, sometimes known as theHouse borer, represents a severe hazard to

softwood. In other situations deterioration maynot be inevitable, yet it may be possible or evenprobable, representing a moderate hazard. Thusthe Common Furniture beetle is a particularlywidespread cause of damage, yet it seldomresults in structural collapse. Similarly, fungaldecay may not normally present a risk, yet itmay be able to develop if structural woodworkbecomes wet through accident or neglect.

It is obviously important to identify thedeterioration hazard before deciding on theprecautions that are necessary. However, thehazard does not vary only with the conditions towhich the wood is exposed but also with thewood species. For example, a group ofBasidiomycetes are responsible for the fungaldecay that is commonly known as Wet rot. TheCellar rot Coniophore puteana occurs inpersistently damp conditions, such as when adamp-proof mem-brane is omitted and whenplates under floor joists are in direct contact withdamp supporting walls. If the moisture contenttends to fluctuate, as in wood affected by aperiodic roof leak, the White Pore fungus Poriavaporaria is far more common in softwoods and,for example, the Stringy Oak rot Phellinusmegaloporus in oak. Coriolus versicolorsometimes develops when non-durable tropicalhardwoods are used as drips or sills on windowand door frames, and Paxillus panuoidesgenerally occurs where the conditions are too wetfor these other fungi. A knowledge of the basicnature of various wood species, and perhaps evenof the principles for their identification, istherefore essential for a proper understanding ofthe decay hazard. The reliability of preservationprocesses also varies widely with the woodspecies. The most important requirement is toachieve an adequate retention of the preservativewithin the wood. In many species this can beachieved relatively easily in the sapwood but theheartwood may be completely impermeable. InBaltic redwood (Scots pine) the treatment of thesapwood may be all that is necessary as theheartwood possesses significant natural

Wood structure

13

durability. In other species such a Balticwhitewood (spruce) even the heartwood is non-durable, yet neither heartwood or sapwood issufficiently permeable to permit adequatepreservative penetration. Preservative efficacyalso varies with the microscopic structure of thewood. Thus the usually reliable copper-chromium-arsenic water-borne preservatives aremuch less efficient in hardwoods than insoftwoods, apparently through the inadequatemicro-distribution of the preservative within thecell walls. Clearly a detailed knowledge of the finestructure of wood is necessary if these variousproblems are to be fully understood.

1.3 Wood structure

The tree

The basic structure of wood, the variationbetween softwoods and hardwoods, thedifferences between species and the significanceof various features ar all described in greaterdetail in the book Wood in Construction by thepresent author. Many of the features are ofimportance in wood decay and preservation.Wood is the natural supporting skeleton of largerplants and it is important to understand theorigin of the skeletal parts in order to fullyappreciate their properties. A plant consists of acrown of leaves, a supporting stem and the rootsthat anchor it within the soil. A tree is specialonly in regard to the scale of its development,and thus the need for a supporting skeletonwhich ultimately becomes the wood ofcommerce. However, the trunk does not performsolely this passive supporting function but alsoacts as a storage area, and the outer zones arethe conducting routes between the crown andthe roots. In addition, the growth of the crownmust be accompanied by similar growth in theroots, and an appropriate enlargement in thetrunk to enable it to continue to perform itssupporting function.

The growth arrangement of a tree comprising acrown of leaves connected to the roots by usuallya single main stem, trunk or bowl is known as thedendroid habit. The sole purpose of this veryelaborate structure is simply to survive and tosupply the cells within the plants. This is achievedfirstly by the roots, which absorb watercontaining dissolved mineral salts which is thenconveyed by the trunk, branches and twigs to thecrown and the individual leaves. The function ofthese leaves is to absorb atmospheric carbondioxide, which is combined with the water fromthe roots to form simple sugars by the processknown as photosynthesis; the chlorophyll in greenplants is the essential catalyst which enables thisprocess to proceed whenever adequate ultravioletradiation is received from the sun. The sugars arethen conveyed throughout the plant to the leaves,twigs, branches, trunk and roots. The primaryfunction of the sugars is to provide an energysource or food for the individual living cellswithin all these components of the tree, but asecondary function is to provide the basic unitsfrom which the skeletal structure of the tree isformed. Whenever there is sufficient sunlight thesimple sugars will be produced by the leaves andwill be found distributed throughout the livingtissue of the tree. Some of this sugar will beformed into starch and deposited within the livingtissue as a reserve energy source which can beutilized by the cells whenever sugar is unavailablefrom the leaves, such as at night or during thewinter months when deciduous trees shed theirleaves. Finally the sugar units are joined intocellulose chains which are then assembled into themain skeleton of the woody parts of the tree.

Wood formation

A tree is continuously increasing in size (Fig.1.1) and this is the function of the embryonictissue distributed around the whole plant. Theincrease in the overall size of the crown is theresult of the activity of the apical meristem or


14

bud at the end of each twig which achieves theprogressive extension in length. The detailedstructure of this bud has little significance on thestructure of wood but the meristem tissueactually extends over the entire surface of thetree, just beneath the bark of the twigs, branchesand trunk but extending similarly over the entireroot system. The purpose of this lateral meristemis to enable all the structural components of thetree to increase in girth so that they are capableof supporting the enlarged crown. Each twig asit lengthens consists initially only of pith formedby the apical meristem, but it is coveredexternally by the lateral meristem to permitsubsequent increase in girth, although it alsoprovides a protective covering to the new shootto control water loss and to prevent diseasedamage. As this meristem tissue generates newcells which increase the girth the protectivecovering splits, exposing inner tissue, but themeristem tissue then generates a new protectivelayer which becomes the rough outer bark of thebranches and trunk (Fig. 1.2).

A trunk in its simplest form could thusconsist of single twig, progressively increasing in

FIGURE 1.1 Diagrammatic trunk showing annualrings.

FIGURE 1.2 Wood zones in a trunk.

Wood structure

15

length or height as it is extended by the bud atits apex, the lateral meristem also increasingthe girth, so that the trunk possess a steepconical shape which is essentially the portionof the tree which is the wood of commerce.The trunk consists of a central pith enclosed,in effect, by a series of cones, each conerepresenting the annual growth increment orannual ring. The wood tissue around the pithis the heartwood and consists of dead cells.The heartwood is surrounded by the livingcells and the sapwood or xylem which iscovered by a thin layer of phloem cells and theprotective bark. The interface between thexylem and phloem cells is known as thecambium, the term used by wood technologistsfor the actively dividing cells which aredescribed by botanists as the lateral meristem.These dividing cambium cells are known asfusiform initials, the cells splitting off on theinner side of the cambium forming becomingxylem or wood tissue and those on the outerside forming phloem or bark tissue. Inconiferous trees the xylem cells are termedtracheids whilst in dicotyledons or broad-leaftrees they are termed fibres. The cambium alsopossess additional active cells known as rayinitials which generate horizontal radial bandsof cells known as rays or parenchyma tissuewithin the xylem.

This description is not very complex, yet itexplains the development of wood within thetrunk of a tree. Xylem can be formed only whenthe entire tree structure is active. The xylemdeposits thus tend to be seasonal, particularly intemperate areas, so that a trunk will increaseeach year by the addition of an outer cone oftissue representing the growth for a season. Thisgrowth varies from a wide band of large butthin-walled cells known as the early or springgrowth, to much smaller thick-walled cells whichrepresent the summer or autumn growth. Thislate growth terminates sharply as cell formationdiscontinues with the onset of winter and thissharp terminal line is then followed by the large

cells of the early growth for the followingseason.

These details explain the basic structure ofwood but they have additional significance. Thexylem or sapwood is the tissue that conductswater and dissolved salts from the roots to thecrown of the tree, whilst the phloem is the tissuethat conducts sugars from the crown to thevarious growing cells throughout the structure ofthe tree. When a xylem cell, a tracheid or fibre, isfirst formed it consists of a thin wall of sugarswhich have polymerized into cellulose. Thesugars from the phloem continue to be suppliedto xylem cells, the rays perhaps providing routesfor this transfer, so that successive secondarylayers of cellulose are formed within the originalprimary wall. Once the cell structure is complete,a relatively rapid process occurring within theoriginal growth season, the much slower processof lignification commences. This consists of theprogressive deposition of lignin, initially withinthe middle lamella, an amorphousundifferentiated region between the cells, butthen within the cellulose cell walls. Thislignification serves to stiffen and strengthen thecells, occurring progressively whilst the cellsremain alive. The sapwood or living xylem cellsconsist of a reasonably constant number of ringsor depth of xylem, presumably controlled by theavailability of food and oxygen to maintain theliving cell processes, so that further annualgrowth at the cambium is accompanied at theinner surface of the sapwood by the death ofcells and their conversion into heartwood. Theonly significant difference between sapwood andheartwood is the large amount of material that isdeposited in the latter, apparently waste arisingfrom the living processes of the tree. Thesedeposits in the heartwood cells reduce theirporosity and are often significantly toxic, so thatheartwood is usually more resistant to insect andfungal attack than sapwood. These deposits alsotend to make heartwood more stable, so that it ismuch more resistant to swelling and shrinkagewith changes in moisture content.


16

Softwoods and hardwoods

There are fundamental differences in the natureof the wood of the conifers or softwoods andthe dicotyledons or hardwoods. In softwoodsthe principal longitudinal cells are known astracheids and serve both conducting andmechanical support functions. In transversesection a piece of wood appears as ahoneycomb, with the annual rings arisingthrough the change in density of appearancebetween the early wood with its large thin-walled cells and the late wood with its smallthick-walled cell. The transverse section mayalso show occasional vertical resin canals whilstthe radial and tangential sections may showhorizontal features such as rays. In contrast thehardwoods possess fibres, similar to softwoodtracheids, to provide structural support but theconducting cells are termed vessels or pores.These vessels are distributed singularly or inclusters throughout the wood, or in radially ortangentially orientated groups. For example,tangential distribution results in the ringporouswoods, a term that results from the observationthat a distinct ring can be seen with the nakedeye on a transverse section of species of thistype such as oak, ash or elm. Whilst thetransverse and radial sections of bothhardwoods and softwoods show annual ringswhich may appear to be superficially similar,they actually result from rather differentvariations in the structure.

In order to examine these microscopicfeatures of a piece of wood it is necessaryeither to macerate the sample or to preparethin sections. For maceration the sample isfirst treated with chromic acid in order todissolve the middle lamella and thus releasethe individual components. Thin sections areprepared by soaking the wood until it is softand then cutting the sections with amicrotome. In both cases stains can be usedwhich have an affinity for various individualcomponents so that they are more readily

visible under the microscope. Maceration hasthe advantage that it is possible to examineindividual components in their entirety, but thedisadvantage that their relative positions arecompletely unknown. Thin sections give anindication of the relative positions but it isnecessary to prepare a large number of serialsections in order to encounter individualcomponents and thus construct a three-dimensional picture of the entire wood.

This description is not intended to be acomprehensive account of the microscopicfeatures of wood but simply a contribution tothe understanding of wood structure. As anexample Scots pine, Pinus sylvestris, has beenselected to represent softwoods with Europeanoak, Quercus robur, to represent hardwoods(Fig. 1.3).

The maceration of Scots pine gives a largenumber of thin needlelike units about 0.8 mm(1/32 in) long. These principal longitudinalstructural elements are hollow, four-sided andpointed at each end. ‘Pits’ are scattered alongopposite sides of the tracheids; the bordered pitsare circular whilst the simple pits arerectangular. It is also possible to distinguishparenchyma cells in the macerated sample, eachoblong, box-shaped and about 0.1 mm (1/200in) long. Examination of sections shows thatScots pine is composed predominantly oftracheids which were laid down in regular rowsas they were formed by the cambium at the outerlimit of the sapwood. The tracheids are fittedend-to-end with an overlap to give both strengthand continuity through the pits in thelongitudinal conduction of fluids. The springwood tracheids are large in cross-section andthin-walled compared with the summer woodtracheids which are distinguishable by their verythick walls. The only other longitudinal featuresare vertical resin canals which appear as spotsin cross-section and as fine white hair-lines inthe radial and tangential sections. These resincanals are narrow tunnels lined with smallrectangular cells. The other principal features are

Wood structure

17

the medullary rays running as horizontal ribbonsin the radial direction; with luck they might bevisible in a radial section but the ends of the rayswill certainly appear in a tangential section. Indepth they consist of three to ten small oblongcells with their length in the horizontal direction.The top and bottom rows consist of ray tracheidswith walls of irregular thickness whilst the middlerows are parenchyma cells which are connected tothe vertical tracheids via the simple pits. The raymay also incorporate horizontal resin canals.

The structure of a hardwood such asEuropean oak is entirely different. In mosthardwoods the vessels are the dominant featuresand in cross-section they appear in the oak aslarge pores. These vessels run for a distance ofseveral metres vertically within the tree andconsist of the many squat tubular cells that canbe seen in a macerated preparation. These cellspossess thin walls so that increasing porositynecessarily leads to decreasing strength in ahardwood. The tubular cells also possessnumerous pits connecting with the adjacent

tracheids and fibres. The tracheids are rather likethose in softwoods but less regular in form.Fibres occur in clumps and are responsible forthe principal longitudinal strength of the wood.Each fibre is spindle-shaped, long, thin andtough with a thick wall and only a small cavity.The fibres are interlocked or cemented to eachother to give a hard tough wood. In additionthere are small vertical rays and very largehorizontal medullary rays, usually composedentirely of regular-sized parenchyma cellswithout the ray tracheids or resin canalsassociated with medullary rays in softwoods. Inoak there are two sizes of medullary ray, onebroad and the other narrow and barely visible tothe naked eye. These medullary rays are a sourceof weakness as the vertical tracheids and fibresare deflected around them so that shrinkage inoak and other hardwoods is often associatedwith splitting through the medullary rays. Insome hardwoods the medullary rays give aregular pattern, the ripple marks or silver figurefrequently seen in quarter-sawn oak.

One of the features of the hardwood vessels is

FIGURE 1.3 Reconstruction of 3 mm cubes of wood, (a) A softwood, Scots pine Pinus sylvestris.(b) A hardwood, European oak Quercus robur.


18

the formation of tyloses as the sapwood isconverted into heartwood. Living cells bulgethrough the pits to give the appearance initiallyof small balloons in the vessel cavities. Thesegrow until eventually the vessel is completelyblocked. Tyloses occur only in certain species;white oak possesses tyloses which block thevessels so that it is particularly suitable for barrelmaking, whereas red oak has no tyloses andsmoke can actually be blown through the vessels.

Annual rings

The annual rings represent the amount of woodformed each year but the structure in softwood isentirely different from that in hardwoods. In thesoftwoods a wide ring of thin-walled spring cellsis formed, followed as the season progresses by anarrower ring of thick-walled summer or late-wood cells. In hardwoods the spring wood isformed with very large vessels but as the seasonprogresses the vessels become smaller with anincreasing density of fibres and a smaller numberof tracheids. In fast-grown softwoods the woodis generally of low density and inferior quality,whereas in fast-grown hardwoods the woodtends to have a high density and superior quality.This is, of course, only a generalization; veryslow-grown softwoods have excellent work-ability but inferior strength due to theircomparatively short tracheids, whilst very fast-grown hardwoods tend to lack the durabilityassociated with slower grown wood.

Wood structure

These are the various microscopic features whichare visible under a normal light microscope butit is also necessary to consider the sub-microscopic features which can only be seenwith an elec-tron microscope, as well as theultimate chemical structure, in order tounderstand some of the features of wood decayand the explanations for the action of the moresophisticated wood preservative. The

microscopic fibres or tracheids consist oforientated microfibrils and it has been deducedthat these in turn consist of bundles of cellulosechains. Crystalline and amorphous cellulosestructures have been identified, as well as relatedcarbohydrates such as hemicellulose and starch,all these components being assembled fromsugar molecules. All the characteristic features ofwood are found to be derived from these sugar-based structures and it is therefore hardlysurprising that the entire purpose of the tree is tosupport and supply the leaves where sugars aregenerated through photosynthesis.

The trunk or stem of the tree is, of course, thewood of commerce. The initial twig isrepresented by the central pith which issurrounded by the heartwood consisting of cellswhich are so far removed from the bark thatthey have died and become filled with variousextractives. Around the heartwood is thesapwood of living cells, the inner zoneconducting water upwards to the leaves and theouter or phloem conveying sugars from theleaves to the living cells throughout the tree,providing them with energy to sustain life andsugar components to form cellulose,hemicellulose and starch. The outer sapwoodcells immediately beneath the phloem are knownas the cambium and are distinctive as they candivide to form new cells. The cambium consistsessentially of two types of dividing cell, thefusiform initials and the ray initials. Thefusiform initials adjacent to the sapwood giverise to xylem or wood cells which ultimatelybecome the tracheids of conifers or softwoodsand the fibres of dicotyledons or hardwoods.The outer fusiform initials give rise to the bark.The ray initials also produce xylem but in theform of parenchyma or ray cells. In this way thewood is formed so that the vertical tracheids andfibres give longitudinal strength, as well asvertical transport routes within the tracheids inconifers and within the pores surrounded byfibres in dicotyledons. In contrast, the ray cellsare orientated along horizontal radial paths in

Wood structure

19

order to provide conducting tissue between thephloem and the cells deep within the sapwoodand heartwood, apparently conductingextractives towards the heartwood and sugars tothe living cells in the sapwood, and also oftenproviding tissue for the storage of starch,particularly in deciduous hardwoods which shedtheir leaves in winter and thus require a sourceof stored energy to enable them to survive.

The strength properties of wood can beattributed to the principal components,particularly the basic longitudinal cellularelements which are the tracheids in softwoodsand the fibres in hardwoods. When a fusiforminitial divides the resultant new xylem cell issurrounded by an amorphous undifferentiatedsubstance which subsequently becomes themiddle lamella. The cell rapidly achieves itsultimate length and rectangular cross-section,squeezing the middle lamella to form a thin layerbetween adjacent rectangular cells. The initialor primary cell wall (P layer, Fig. 1.4) consistsof loose and irregularly orientated microfibrils,an important feature as this thin P layer mustbe capable of extension as the cell grows to itsultimate dimensions. The microfibrils areorientated in a predominantly shallow spiral,perhaps at about 60° to the vertical axis of thecell. Once the P layer has achieved its ultimatedimensions the secondary wall (S layer) iscommenced and is formed typically in threeseparate stages. The first secondary wall (S1

layer) is thin with a predominantly shallow

microfibril spiral, perhaps at about 50° to thelongitudinal axis of the cell. The S1 layer sometimesconsists of two or more lamellae spiralling inopposite directions and is morphologically andstructurally intermediate between the P and S2

layers. The second secondary wall (S2 layer)consists of very regular and closely packedmicrofibrils at a very steep spiral angle, perhapsonly 10–20° relative to the longitudinal axis, and itis also it is also very thick and the dominant cellwall. Finally a third secondary wall (S3 layer) issometimes formed consisting of a thin shalloworientated layer of microfibrils, perhaps at about50° to the longitudinal axis. In all cases thesecondary walls are more regularly orientated thanthe primary wall.

These cell walls account for the cellulosewhich comprises about 60% of the woodsubstance. The S2 layer is always dominant andit is therefore hardly surprising to find that thebasic longitudinal orientation of the microfibrils,and thus the cellulose chains, within this layeraccount for many of the basic physical propertiesof wood. This S2 layer is also rather moremassive in late wood than in early wood, againaccounting for the different properties betweenthese zones of the annual rings.

The rectangular fibres and tracheids that areso readily observed when a cross-section of apiece of wood is examined under the microscopeare comparatively large but their componentmicrofibrils are quite small. It is difficult toimagine microfibrils without a knowledge of theAngstrom (Å), the unit of dimension that mustbe used at this scale. By definition an Angstromis 1×10-8 cm, or 0.0000001 mm. Once thedefinition of the Angstrom is appreciated it canbe said that a microfibril consists of elementaryfibrils having a diameter of about 35 Å, so thatmicrofibrils have typical diameters of 35, 70,105, 140, etc. Å, although some microfibrils areflattened with dimensions such as 100×50 Å. Ifthese measurements are now converted tosomething familiar, such as a fraction of amillimetre, it will be appreciated that theFIGURE 1.4 Cell wall layers in a softwood tracheid.


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elementary fibrils are very small, yet eachconsists of a bunch of about 40 cellulose chains,and thus the basic structure of the wood cell hasbeen reduced to its ultimate chemicalcomponents.

It is usually considered that the cellulosechains consist of between 200 and 2000 glucoseunits, although it is sometimes reported that asmany as 15 000 units may be involved. As eachglucose unit has a length of about 5 Å the chainsare relatively long, perhaps 10 000 Å (0.001mm) or perhaps far longer. As wood is a naturalmaterial some discontinuity of the structureoccurs but the chains lie parallel for considerablelengths, perhaps over 120 units (600 Å) or more,and it is these essentially parallel cellulose chainsin the dominant S2 layer that account for theprincipal properties of the wood.

Figure 1.5 illustrates the way in which glucoseunits are assembled to form cellulose and thusthe principal structural elements of wood. Thisstructure has considerable importance indetermining the properties of wood. Firstly thesugar units are formed from water and carbondioxide within the leaves by the process knownas photosynthesis:

6H2O + 6CO2 �� C6H12O6 + 6O2

water cabon dioxide glucose oxygen

The water is obtained from the surrounding soilby the roots and is conveyed up the tree throughthe living xylem cells in the inner sapwood to theleaves. Carbon dioxide is then absorbed from theatmosphere by the leaves, glucose is formed byphotosynthesis and conveyed down the tree inthe phloem between the xylem and the bark.

Production of glucose by photosynthesis canoccur only in the presence of the catalystchlorophyll, the green pigment in leaves, and isdependent upon the absorption by the leaves oflight energy, particularly ultraviolet light.

Energy in wood

The glucose product thus has a far higherenergy level than the water and carbon dioxideconstituents, and this energy can be released ina variety of ways. For example, animals eatglucose or other sugars, converting them backto the original water and carbon dioxide bythe addition of oxygen and releasing energy inthe process. This energy is used formaintaining life processes or for generatingheat, and the burning of sugars is perhaps themost obvious illus-tration of the way in whichoxygen can be combined with glucose toreverse photosynthesis and release energy. Inthe formation of cellulose chains a smallproportion of the energy is lost but aconsiderable amount remains, and thisexplains why wood burns and why it isattractive to some insects and fungi as a sourceof nourishment.

The glucose produced by a tree is largely usedfor formation of structural cellulose but theenormous mass of living cells in the roots,sapwood and leaves all requires energy tomaintain life, this being obtained from theglucose and associated sugars such as xylosewhich are also produced by the leaves. Duringdarkness or the winter months the leaves are notproducing sugars, yet the cells still requireenergy and obtain this from glucose accumulated

FIGURE 1.5 Glucose and the formation of cellulose.

Wood structure

21

within them or, for longer periods such as thewinter, from deposits of starch which is formedfrom glucose and fairly readily reconverted whenrequired. Some attacking insects are unable toutilize cellulose as a source of nourishment butthey will attack wood just for the starch orsimple sugar content; even mammals such asrabbits and squirrels will strip the bark fromtrees to gain access to the sugary sap in thephloem.

Water and wood

It has already been explained that the cellulosechains are assembled into microfibrils whichare orientated in a predominantly longitudinaldirection within the cell walls of softwoodtracheids and hardwood fibres. The physicalproperties of wood largely result from thislongitudinal orientation of the cellulosechains, and this is particularly the case in therelationship between wood and water. Eachglucose unit in a cellulose chain possesses threehydroxyl groups which have an affinity forwater. This ensures firstly that cellulose chainswill wet easily but in addition water will beheld between the chains, pushing them apartas illustrated in Fig. 1.6. As the chains becomeseparated by water the bond between thembecomes weaker, so that a high moisturecontent in wood is associated with loss ofstrength, particularly a loss of sheer strengthand rigidity, so that a beam is more flexibleand wood will cleave more readily when wet.If this separation of the cellulose chains werepermitted to continue indefinitely the wood

would eventually break down into individualcellulose chains and thus disintegrate, butmovement with moisture change can be largelyattributed to the predominantly longitudinalorientation of the microfibrils and thecellulose chains in the S2 layer which accountsfor most of the mass of the cell wall, butseparation of these chains is limited by thechains in the P and S1 layers that are wrappedaround them.

As orientation of the microfibrils in the S2

layer steepens from the pith to the bark of thetree the cross-sectional movement with changeof moisture content increases approximately inproportion to the cosine of the angle oforientation. In addition, it would be expectedthat the longitudinal movement would beproportional to the sine of the angle oforientation, so that an angle of 10–20° to thelongitudinal direction would suggest alongitudinal movement of 17–36% of the cross-sectional movement. In fact the longitudinalmovement is less than half this calculated figure,apparently due to the restraining influence of theplanes of lignin in the middle lamella. Themiddle lamella appears to have much less controlon the more massive cross-sectional movement,perhaps largely because it is orientated then as athin envelope over the swelling material incontrast with the vertical tubes in which itobstructs longitudinal movement. However, thecross-sectional movement is still restrained whenit reaches the fibre saturation point, but this canbe attributed largely to the very shallowmicrofibril angle in the P and S1 layers whichphysically restrain and prevent further swelling.

FIGURE 1.6 Water absorption forcing cellulose chains apart.


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Prolonged waterlogging introduces slow butprogressive hydrolysis of the cellulose and thusweakening, which eventually permits the S2 layerto absorb water beyond the original fibresaturation point and swell to a greater extent;this is the explanation for the weakeningobserved in archaeological wood which has beenwater logged for many centuries. Carefulmicroscopic observation discloses that waterlogging permits ballooning where the S2 layer isapparently bursting through the restraining Pand S1 layers; this damage is also observed whenwood is soaked with more powerful swellingsolvents such as alcohols.

The loss or gain of water between thecellulose chains does not occur instantaneouslywith changes in the surrounding relativehumidity but tends to lag. The reason is thatchanges will occur only under the influence ofexcess energy. In fact, energy is liberated whencellulose is wetted and this becomes apparent asthe heat of wetting. This is virtuallyimmeasurable and certainly insignificant in mostcircumstances, but it means that wetting willproceed only if this heat is removed and dryingwill proceed only if heat is provided. Whilst thedimensions of a piece of wood will depend on itsmoisture content, this lagging effect or hysteresiswill mean that the wood is larger than might beexpected during the drying stages and smallerthan might be expected during the wettingstages, as shown in the hysteresis diagram in Fig.1.7.

Only 1% of the mass of wood consists ofextractives, extraneous materials and mineralssuch as silica, the remainder being lignin andholocellulose or the carbohydrates and relatedcompounds derived from sugar units. Thecrystalline structural cellulose which is theprincipal component of the microfibril is knownas a-cellulose and accounts for abut 50% of themass of softwoods and temperate hardwoods,the remaining holocellulose, principallyhemicellulose, contributing about 25%. Ligninusually contributes about 25%, although intropical hardwoods the lignin content is often farhigher, displacing the holocellulose in proportionand giving greater rigidity but less resilience.

FIGURE 1.7 Variation in moisture content of woodwith changes in atmospheric relative humidity.

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2.1 Introduction

This book is entitled Wood Preservation. It is nottherefore intended that it should contain acomprehensive and detailed account of wooddegradation as this is not essential to the studyof preservation chemistry and technology.However, the action mechanisms of variouspreservatives cannot be fully understood withouta basic knowledge of deterioration processes,and it is obviously desirable that the mostimportant causes of deterioration should beunderstood.

2.2 Biodegradation

It must be accepted that wood is perishable.Indeed, if this were not the case our forestswould soon be cluttered with the uselessskeletons of dead trees. Unfortunately thevarious wood-destroying insects and fungi areunable to distinguish between forest waste andwood in useful service. In the past wood wasused principally in forest areas where it wasreadily available. Degrade was accepted butreplacement was comparatively simple andinexpensive. In contrast, wood is now a valuablecommodity and transported considerabledistances between the production forest and theultimate user. It is essential for wood to beutilized efficiently in order to conserve worldresources but also to avoid unnecessary cost,both to the individual user and to importingnations as a whole.

Wood quality

Wood has been an article of commerce for manycenturies, some areas exporting exotic decorativewoods and others supplying straight and strongstructural woods. In the past high transportationcosts were justified only for the most valuablewoods and even normal construction woodimported, for example, into Great Britain in the19th century was generally slow-grown and freefrom sapwood. Dwindling resources have sinceresulted in the introduction of thinning toencourage maximum yield, giving rapidly growntrees with wide rings, principally composed ofspring wood. As the trees are felled whencomparatively small in diameter a largeproportion of the wood is now sapwood;Swedish redwood (Scots pine) supplied assawnwood for building is now approximately50% sapwood. Whilst the strength propertiesare not significantly affected by wide rings andthe presence of a high proportion of sapwood,the wood is far less durable and far less stablethan the slow-growing heartwood that wascommonly used in the past.

Whilst fibre and moisture content changesmust be recognized as causes of degrade inwood, it is the various living organisms, bothmicrobiological and animal, which are usuallymore important. The tree in the forest passesthrough various stages, originating with thedeath of the living tissue and passing through adrying stage until the protective bark is lost sothat rewetting is then able to occur. Commercial

2

Wooddegradation

Wood degradation

24

wood passes through similar stages and it ispossible to establish a sequence of degradingorganisms to which wood is susceptible at thevarious stages in its progress from the living treeto the wood in service and then beyond to thevarious stages of destruction.

Green wood

SapstainFreshly felled green wood has a very highmoisture content, 60–200% for most species butas high as 400% in extreme cases. Sugars andstarch are present, particularly in the phloemand parenchyma tissue, and these conditions areparticularly attractive to the sapstain fungi suchas Ceratostomella species. The hyphae, theminute strands comprising the growing tissue ofthe fungus, are brownish in colour but whenwood is affected by a mass of these hyphae lightdiffraction gives it a blue or black colouration,often known as blue staining or blueing.Sapstain problems can be largely avoided inconiferous woods by winter felling, whichensures a lower moisture content. Float-ing logscan be the cause of staining problems and, withwet wood, an increase in temperature duringshipment can cause sweating if the cells are stillalive and oozing of sugar-rich sap from the endsof logs followed by the development of aluxuriant fungal growth. Sapstain damage isrelatively unimportant as it does not destroy thewood cell or cause any structural damage but itis unsightly and leads to down-grading and thusreduced value.

In recent years the excessive demand forsoftwood has caused the felling period in theconiferous forests to be extended throughout thewhole year instead of being confined to thewinter months alone. Wood felled during theactive growing season, particularly the earlyspring, contains both higher moisture and sugarcontents than winter-felled wood and there is acorresponding increase in the danger of staining.Indeed staining is virtually inevitable for fellings

in late spring or early summer unless they areconverted and kiln-dried immediately. Even thenthere is still a danger that stain will developduring the early stages of kilning before themoisture content has been significantly reduced.The best solution to the problem is to convertthe logs immediately into sawnwood which isthen given a stain-control treatment.Alternatively the sawnwood is transferredimmediately to kilns for drying and a toxictreatment is introduced into the kiln sprays.Many of the most widely used stain-controltreatments are significantly volatile, particularlyat the high temperatures used for kiln-drying, sothat much of the treatment is lost during theprocess. A further treatment is thereforedesirable when drying is complete in order toprevent the recurrence of staining should thewood become accidentally rewetted.

In many cases this final treatment is not usedbut instead the wood is packaged and wrappedin plastic sheeting in order to retain the lowmoisture content achieved by kiln-drying andthus prevent the development of staining. In fact,packaged wood wrapped in this way can oftenbecome very seriously affected by staining of theouter sticks in contact with the sheeting, shouldit be exposed to wide temperature changes whichcan induce evaporation of moisture from thewood during warm conditions but condensationunder the plastic wrapping during a subsequentcold period. For this reason complete packagesare frequently dipped in a stain-controltreatment before wrapping but this achievesonly limited success as it can give littleprotection to the inner pieces within thepackage. This often results in mirror imagestaining (Fig. 2.1) which develops on adjacentpieces where tight contact prevents penetrationof the stain control treatment. The only propersolution is to treat pieces individually beforepackaging and wrapping, but most of thecommonly used stain control treatments areapplied in water and increase the moisture contentof the wood, partly defeating the object of the

Biodegradation

25

original kiln-drying. Clearly there is scope fordevelopment of improved and more reliable staincontrol treatments.

Bark-borers

Bark-borers frequently attack freshly felled logs.The Wood wasps, particularly Sirex noctilio,attracted attention a few years ago when theAustralian authorities feared that waspsintroduced in softwood packaging from thenorthern hemisphere would damage valuable pineplantations; all wood imported into Australia isnow subject to quarantine regulations to avoidthe introduction of wood-borers of any type, andmust be either inspected or destroyed on arrivalor accompanied by a certificate to show that ithas received an approved treatment. Wood waspsbore through the bark with their ovipositor andlay their eggs in contact with the phloem. Thelarvae hatch from the eggs and then explore thephloem, living on the sugar content of the sap andat the same time loosening the bark; this is the

first stage in the destruction of a dead tree in theforest. Some Longhorn beetles behave in a verysimilar manner, laying their eggs in cracks in thebark. The developing larvae either explore thephloem in search of nourishment in the same wayas the Wood wasps or alternatively tunnel in thesapwood, gaining nourishment from starchdeposits or, in few cases, from the cellulose of thewood.

Pinhole-borers

Both Wood wasps and Longhorn beetles can bequite large, with fully grown larvae being perhaps25 mm (1 in) or more in length. In contrast theAmbrosia beetles are much smaller, only about3mm (1/8 in) long. These insects, the Scolytidsand Platypodids, tunnel through the bark toproduce extensive galleries in which they lay theireggs. At the same time they introduce fungi whichthrive on the high moisture, sugar and starchcontent of the freshly felled wood, thus providingthe larvae with food; the browsing of the larvaeon the fungus accounts for the name Ambrosia.The galleries either extend under the bark or intothe sapwood, perhaps following the less densespring wood of the annual rings, and thedistinctive pattern of the galleries is frequently acharacteristic of a particular species. Ambrosiaattack can be very severe in the tropics, frequentlyintroducing stain; the ambrosia fungus causes astreak of stain along the grain on either side ofeach gallery, even if the stain does not spreadmore widely. The borer damage to the wood isoften described as pinholes or shotholes,depending on the size, and the damaged wood isknown as pinwormy. Because of the danger ofAmbrosia beetle attack coupled with staining it isnormal to spray logs immediately after felling inthe tropics with a mixture of a contact insecticideand a stain-control treatment, whilst sawnwoodproduced close to the forest before shipment alsoreceives a further stain-control treatment as it isnormally shipped with a relatively high moisturecontent.

FIGURE 2.1 Mirror image staining through tightstrapping during stain-control treatment. (PenarthResearch International Limited)

Wood degradation

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Powder Post beetles

As the moisture content of the wood falls itbecomes immune to fungal decay but it may besusceptible to Powder Post beetle damage. TheBostrychid adult beetles bore tunnels into thesapwood in which they lay their eggs, thehatching larvae relying on stored starch in thewood tissue for their nourishment. The Lyctidsin constrast lay their eggs in large vessels orpores, although again the hatching larvaedepend on the starch content in the wood.Powder Post attack is most common inhardwoods because of this starch requirement,with Lyctid attack confined to large-poredhardwoods. All wood is immune from PowderPost beetle attack if it is free from starch;temperate hardwoods are most susceptible iffelled in the early winter when the starch contentis at its highest level. Starch naturallydegenerates slowly so that wood is virtuallyimmune several years after felling. In additionair-seasoned wood is far less susceptible thankiln-dried, apparently because the cells remainalive during air-seasoning and thus utilize thestarch, whereas the cells are killed in kiln-dryingso that the starch deposits remain and attractPowder Post beetle attack.

Furniture beetles

The Anobids or Furniture beetles, are a veryimportant family. The bark-borer Ernobius mollislays eggs in cracks in the bark of dry softwood logsor bark still attached to the waney edges ofsawnwood. The hatching larvae explore thephloem in search of nourishment and loosening thebark, but the galleries sometimes extend for adistance of up to 12 mm (1/2 in) into the sapwood.These holes in the sapwood are sometimesconfused with those of the Common Furniturebeetle Anobium punctatum but they tend to berather larger in diameter and they are invariablyassociated with galleries under the bark; the barkmay have fallen away from the adjacent waney

edge but the galleries can still be recognized.Confusion with the Common Furniture beetle isthe most important aspect of Ernobius mollisattack. It is dependent on sugar in the phloem andto a lesser extent starch in the sapwood. Thisdeteriorates naturally a year or two afterconversion, so that Ernobius is eliminatednaturally without any need for treatment.

If small holes are observed in the sapwood of apiece of softwood with no evidence of either barkor waney edge, then the Common Furniture beetleAnobium punctatum is most likely to beresponsible. Common Furniture beetle attack willoccur in the sapwood of most soft-woods andhardwoods, as well as in the heartwood of sometemperate hardwood species such as birch andbeech. Eggs are laid, usually in the summermonths, in cracks on the surface of the wood or onfurniture in open joints. The larvae hatch bybreaking through the base of the egg and boringstraight into the adjacent wood. The insect usuallyremains in the larval stage for almost a year beforeforming a chamber just below the surface, in whichthe full grown larva about 5mm (1/5 in) longpupates into an adult beetle. The adult bores to thesurface and escapes by a circular flight hole about1.5 mm (1/16 in) in diameter. The adults mate andthe female then initiates a fresh attack on the woodby laying eggs, sometimes in the old flight holes.Obviously the damage is caused entirely by thelarvae boring through the wood, in contrast toAmbrosia beetle attack in which the adults areresponsible for tunnelling.

Whilst the life cyle of the Common Furniturebeetle is often as short as 1 year, it can beextended if conditions are unfavourable to 4 yearsor more before the larvae are able to storesufficient energy to survive through the pupationstage when they develop into adult beetles. Ifwood becomes damp it will considerablyencourage the activity of the Common Furniturebeetle, enabling it to attack normally resistantheartwood. It appears that, althoughdeterioration may not be obvious, fungal orbacterial activity is converting the wood to a form

Biodegradation

27

that is more readily assimilated by the insect. TheDeath Watch beetle Xestobium rufovillosum is anAnobid related to the Common Furniture beetlewhich is entirely dependent on the presence ofmicrobiological attack. Indeed, wood attacked byDeath Watch beetle is often brownish in colour,indicating prolonged incipient fungal attackalthough there may be no other evidence. TheDeath Watch beetle is far larger than theCommon Furniture beetle and in the British Islesit is generally associated with old buildings withoak chestnut timbers but particularly historicchurches, apparently because the periodic heatingcombines with the traditional lead roof coveringto encourage condensation, incipient fungalattack and thus Death Watch beetle attack in theroof sarking boards. However, the rather sinistername of the Death Watch beetle is not associatedwith churches but with the characteristic tappingwhich is used by the adult beetles as a mating calland which is produced by striking the top of thehead against the surface of the wood on which thebeetle is standing. On a calm day the sound isdistinctly audible in a quiet building, perhapsparticularly in one that is silent through thepresence of death.

House Longhorn beetle

Whilst it is logical to treat the Anobid beetles asa group progressing from the Ernobius barkborer to the Common Furniture beetle attackingdry wood and ultimately the Death Watch beetledependent on some decay, this unfortunatelydisplaces the House Longhorn beetle from itsposition in association with the CommonFurniture beetle in this sequence of attack. TheLonghorn or Cerambycid beetles are a largegroup of considerable importance in the forestbut the House Longhorn beetle Hylotrupesbajulus, known in some countries as theHouseborer, has adapted to dry conditions inbuildings. The attack is confined to the sapwoodof dry softwoods and is thus of particularimportance where wood with a large sapwood

content is used for structural purposes. The lifecycle is basically similar to that of the CommonFurniture beetle but varies typically from 3 to 11years. This is a large insect with a fully grownlarva perhaps 30mm (l¼in) long. A female beetlemay lay up to 200 eggs and, if most of thesehatch within the same roof or floor structure,very substantial damage can be caused duringthe years before the adult beetles ultimatelyemerge. Indeed, the entire sapwood may beriddled by oval galleries, leaving a thin intactveneer over the surface of the piece of wood. Theadult beetle emerges from the wood through anoval exit hole about 10 mm (3/8 in) across, butthe presence of this flight hole is a certainindication that severe damage has already beencaused and the structural integrity of the piece ofwood must be checked by probing.

The Death Watch beetle is only one of severalwood-borers which are dependent on damp orwet conditions. The presence of wood-boringweevils is often indicated by small holes in dampwood attacked by Death Watch beetle orCommon Furniture beetle. If large pieces ofrelatively durable wood such as European oakremain in a damp and slowly decaying conditionfor a prolonged period they may attract theattention of other insects such as Helopscoeruleus, a large blue beetle with a long larvapossessing two recurved spines on its tail, oreven Stag beetles which are more commonlyfound in decayed roots or fence posts belowground level. These insects are, of course,dependent on fungal decay which is a far moresignificant factor in wood degradation than theinsect damage.

Fungal decay

Fungal damage can originate within thestanding tree. Whilst the very high moisturecontent in the living sapwood tissue generallyprevents fungal decay and insect attack,physical damage to the bark and cambium orscars from the removal of branches may permit

Wood degradation

28

the moisture content to fall to a level at whichfungal decay is possible. Branch and root scarsare particular danger points as they mayexpose relatively dry heartwood which, whilstit is relatively durable in most species, willpermit decay in others such as beech. In thecase of a branch scar the first stage is probablyrapid drying from the exposed end-grain andthe development of checks or splits.Subsequent rainfall is then trapped withinthese cracks and produces a gradient ofmoisture content which permits spores in theatmosphere to encounter precisely theconditions that they require for germination.The fungal infection then spreadsprogressively through the heart of the tree.

Dote

These heart rots are variously known as dote orpunk, or they are referred to by the name of theattacking fungus, such as Honey fungusArmillaria mellea. There are no obvious externalsigns of damage until the decay is far advancedand a sporophore or fruiting body appearsthrough the bark, usually indicating that theheartwood of the tree is virtually hollow. In dotyheart the attack is widespread but damage islimited perhaps because the heartwood moisturecontent is too low. Whilst the infection will bekilled if the wood is kiln-dried at a sufficientlyhigh temperature, failure to take this precautionintroduces the danger that the infection willbecome reactivated if the moisture contentreaches an adequate level. For example, logs arefrequently stacked in the open and if they aredoty they are likely to become covered with thefluffy white hyphae or growing elements of thereactiviated fungus. Washing floors can alsoreactivate dote, the upper surfaces softeningprogressively until they eventually break up andfail. Dote normally causes a colour change in thewood, most frequently to a brown colouration inthe centre of the heart, although occasionallyoff-centre, when the effect is referred to as false

heart. There is usually a smell characteristic ofthe particular fungus involved.

Brown and White rots

The only special feature about dote is its originin the standing tree or in a log that has beenpermitted to remain for a protracted period inthe forest. In all other respects dote ischaracteristic of the most important group ofwood-destroying fungi, the Basidiomycetes. Inthe case of Brown rots the fungus destroys thecellulose, leaving the lignin which gives thewood a characteristic brown colouration andusually cross-grain cracking. Common dote,Trametes serialis, is a Brown rot but other dotesare described as White rots because the fungusdecays both cellulose and lignin causing lesscolour change and a fibrous appearance. Fomesannosus is a dote found in spruce, larch and redoak heartwood, giving a purplish colouration.This colour is particularly intense in spruce andlighter in larch. The attack is typical of dote,appearing on the cut surface as tiny whitepockets of rot filled with growth like smallpieces of cottonwool. If dote is suspected but notapparent in this way it can usually be detectedby lifting the fibres with a knife; if they are longand springy the wood is sound but decay must besuspected if they are brash and break readilyacross the grain.

Any dote activity that redevelops in wood inservice can be ultimately attributed to thegermination of a spore, perhaps on a branch scaron a standing tree or a log lying in the forest.The other principal decays of wood in servicediffer only in the fact that spore germinationoccurs on the surface of the wood in service,when the conditions result in an appropriatemoisture content within the wood and relativehumidity in the surrounding atmosphere. A vastselection of spores is invariably present in theatmosphere and a fungal infection will inevitablydevelop which is appropriate to the conditionsand the wood species. For example, a post

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standing in moist ground will generally provideall conditions varying between the high moisturecontent in the ground and the low moisturecontent in the well-ventilated aerial parts but itis the intermediate conditions at the ground linethat prompt the most serious decay; in the forestthis ground line damage ensures that therelatively dry tree falls to the ground whereundergrowth inhibits evaporation and theincreasing moisture content then causes decay inpreviously dry parts of the tree.

Wet rots

There are a very large number of wood-destroying fungi that occur throughout theworld. However, it must be emphasized at thispoint that there is never any need for decay inwood used in building as simple designprecautions are usually adequate to ensure thatthe wood never becomes sufficiently damp tosupport decay fungi. There are some specialconditions, such as external window frames,piles in the ground and frame walls and roofs inwhich condensation can cause wetting, wheredecay is highly probable but it can be avoided bytaking proper precautions such as the selectionof suitable durable wood or the use of anadequate preservative treatment.

Fungal decay frequently occurs in buildingsdespite the ease with which it can be avoided,usually through neglect in design ormaintenance. The Cellar rot, Coniphoraputeana, is a particularly good example of adecay that can occur as a result of neglect. Itoccurs in persistently damp conditions, such aswhen a dampproof course is omitted and whenplates under floor joists are in direct contactwith damp supporting walls. If the moisturecontent tends to fluctuate as in wood affected bya periodic roof leak, the White Pore fungus,Poria vaporaria, is far more common insoftwoods whilst other fungi occur inhardwoods, such as the Stringy Oak rot,Phellinus megaloporus, in oak. Coriolus

versicolor sometimes develops when non-durabletropical hardwoods are used for drips or sills onexternal joinery (millwork), and Paxilluspanuoides generally occurs where the conditionsare too wet for the Cellar fungus.

These are only a few of the fungi that may beencountered but they serve to illustrate the wayin which a fungus is often associated with aparticular combination of wood species andconditions. The Coniophora, Poria and Paxillusspecies are all Brown rots giving affected wood adistinct brown colouration and a varying degreeof cross-grain cracking. The Phellinus andPolystictus species are White rots causing onlylimited change in the colour of the wood butvery pronounced softening and loss of strength.

Dry rot

Perhaps the fungus that is most widely known ascausing serious damage in buildings is the Dryrot fungus Serpula lacrymans. Dry rot sporeswill germinate only when the atmosphericrelative humidity is suitable. Usually accidentalwetting, perhaps from a plumbing leak or a roofdefect, has caused wood to become very wet andsubsequent slow drying, perhaps a seasonaleffect or through correction of the defect,permits the wood moisture content and therelative humidity in contact with the surface toreduce slowly through the optimum conditionfor spore germination. Spore germinationconsists of the development of hyphae or threadswhich penetrate into the wood, radiating fromthe original point of germination and branchingso that the affected area is covered with a softwhite growth like cottonwool. If favourable andunfavourable conditions alternate successivemasses of hyphae will become compacted on thesurface of the wood to form dense skin ormycelium.

The growth will not be confined to wood butwill spread across and through plaster, brickwork,masonry and concrete in an attempt to discoverfurther supplies of wood for nourishment. This

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exploratory growth must be provided with foodfrom adjacent wood attacked by the fungus andthe hyphae develop into rhizomorphs orconducting strands which convey food as well aswater absorbed from damp masonry. The funguswill use this water and water formed during thedestruction of cellulose to form globules on thesurface of the growth which, if ventilation isrestricted, will maintain the atmospheric relativehumidity at the optimum level for growth. Thishabit of forming ‘tears’ on the surface of thegrowth explains the Latin name lacyrmans andthe description in the Old Testament Book ofLeviticus, Chapter XIV, of Dry rot as the ‘frettingleprosy of the house’. As an attack progresses thecellulose is destroyed, giving the wood thecharacteristic dark brown colour of a brown rot,accompanied in the case of Dry rot by verypronounced cross-grain cracking which, incombination with longitudinal cracking, gives thedecayed wood a characteristic cuboidalappearance. The preference of Dry rot forunventilated situations in which it can control thehumidity ensures that it generally remains largelyconcealed and the first sign of damage other thana characteristic odour is perhaps the buckling andcracking of a painted skirting. By then the attackmay well be very extensive, perhaps spreadingthrough masonry for considerable distances in alldirections in the search for wood. When the woodsupply is nearing exhaustion or when thehumidity falls unexpectedly the fungus mayspread onto the surface of the concealing wood orplaster and form a sporophore or fruiting bodywhich will produce millions of red-brown sporesin an attempt to infect any other wood that maybe in suitable condition in the vicinity.

Soft rot

Wood that is continuously immersed in waterbecomes saturated and immune to the attack ofBasidiomycetes, the Brown and White rots, butSoft rot can occur which is caused byAscomycetes and the Imperfect fungi. Soft rot

takes the form of a softened layer of wood on allexposed surfaces, the damage progressivelyincreasing in depth at a very slow rate.Unprotected wood immersed in fresh or sea wateris invariably affected in this way but the damageis comparatively insignificant in large sectionssuch as those used for piling and the constructionof groynes. In wood of thinner section the loss instrength can be significant, as in a neglected boatwhere planking has been exposed throughabrasion damage to the paintwork, or in cooling-tower slats where the high temperature results inparticularly rapid Soft rot attack, even in thepresence of some preservative treatments.

Marine-borers

Continuous immersion in sea water alsointroduces the danger of marine-borer attack.Several animals can infest wood in this way butdamage is most commonly due to crustaceanscalled Gribble (Limnoria species) and molluscscalled Ship worm (Teredo species). Gribbleattack takes the form of superficial tunnellinginto the wood. Small holes less than 2.5 mm (1/10 in) in diameter are produced and intensiveattacks seriously weaken the surface of the woodwhich progressively erodes, exposing fresh woodto attack. Gribble is a major marine problem inmost parts of the world. In contrast, shipwormattack is very difficult to detect. The animalenters the wood by boring a hole about 0.5 mm(1/50 in) in diameter. This is then extended toform a long and progressively widening tunnelwith a characteristic calcareous lining. Severeattack may considerably reduce the strength ofthe wood but this may not become apparentunless abrasion or Gribble attack exposes thetunnels and their characteristic white lining.Shipworm is generally confined to relativelywarm and saline water so that in Europe it is notfound in the north except on coasts warmed bythe Gulf Stream and it is absent from riverestuaries and the Baltic Sea where the water isinsufficiently saline.

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In boats most normal anti-fouling paintsprovide efficient protection against marine-borerattack but in areas where marine-borers are aparticular problem regular inspections areadvisable to ensure that unprotected wood is notexposed by abrasion. For example, woodenrudder trunkings cannot be effectively coatedwith anti-fouling paint and shipworm frequentlybecomes established as a result. Paint cannot beapplied regularly to heavy marine woodworksuch as piles, fenders and groynes, so thatnaturally durable or adequately preserved woodmust be used. Wood is relatively easy to preserveagainst shipworm attack but gribble is a fargreater problem; most preservatives will reducethe rate of attack but will not ensure completeprotection.

If wood is waterlogged and completelysurrounded by an impervious clay or mud layerthere is no possibility of fungal decay or borerattack because of the complete lack of oxygen.However, some anaerobic bacteria are able tosurvive in these conditions, obtaining thenecessary oxygen by their reducing action onsuitable chemicals that may be present, such asby the conversion of sulphate to sulphide andthus the generation of the hydrogen sulphideodour which is a typical feature of theseconditions. These bacteria do not cause anyobvious damage to wood, although it becomesvery dark brown in colour, but protractedwaterlogging results in slow hydrolysis of thecellulose, which is thus progressively lost. Theamount of loss can be assessed by determiningthe dry density of the wood but this is verydifficult as any attempt to dry the sampleinvariably results in considerable distortion,often rather reminiscent of the shrinkage thatoccurs from Brown rot attack which is, ofcourse, another method for removing thecellulose from wood and leaving the brownlignin. The alternative is to measure thesaturated moisture content of the sample byweighing firstly when wet and subsequentlyafter over-drying. The amount of water loss is

then related to the dry mass to give thesaturated moisture content and this can bedirectly related to the period of immersion. Thistechnique is sometimes used by archaeologiststo estimate the age of an ancient structure whenthey are able to obtain waterlogged woodsamples from, for example, a sunken boat orpiles driven into a bed clay.

Termites

Termites or White Ants are probably the mostserious wood-destroying pests. They are not antsbut belong to the Isoptera whereas true ants areHymenoptera, an order which also includes thebees and the wasps. The termites are socialinsects like the true ants, living in communitieswith specialized forms or castes, the workers andsoldiers, as well as male and female reproductiveindividuals. Termites are widespread in tropicaland sub-tropical countries, and the rate andseverity of their attack make them a serious pestand economically significant wherever theyoccur. There are approximately 1900 identifiedspecies of termites and more than 150 are knownto damage wood in buildings and otherstructures. With such a vast number of speciesinvolved it is essential when consideringpreservation systems to have a knowledge oftheir basic behaviour and differences betweenthe various families.

Termites are principally tropical indistribution but are encountered as far south asAustralia and New Zealand and as far north asFrance and Canada. Improvements intransportation and world trade have beenresponsible for the wider distribution of termitesand this is particularly apparent in France andGermany. The termite of Saintonge,Reticulitermes santonensis, is native to the westcoast of France between the rivers Garonne andLoire, but it has spread to Paris where it isconcentrated around the Austerlitz station whichserves this coastal region. The North Americanspecies Reticulitermes flavipes occurs in

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Hamburg where it was introduced in shipcargoes. Both these species are sensitive toclimatic conditions and are not expected tospread widely; they are largely concentrated inheating pipe ducts and other rather warm areasin buildings. However, it is still remarkable thatthe British Isles remain free from termites.

A common feature of the six families oftermites within the order Isoptera is the lack ofcellulase in their digestive enzymes, despite thefact that all six families possess members whichare wood-destroying. Three of the families are ofonly limited significance; the Mastotermitidaeare represented by only a single primitive speciesin northern Australia, the Termopsidae includethree species that infest buildings in NorthAmerica, and the Hodotermitidae are confinedto semi-desert areas of South Africa, NorthAfrica and the Middle East. The remaining threefamilies are of considerable significance as theycontain the more important wood-destroyingtermites.

The family Termitidae is a large and mixedgroup which includes the subterranean and themound termites, which construct nests under theground, on the sides of trees or as mounds on theground. They are able to digest wood withoutthe assistance of an intestinal symbiont becausethey rely on fungus for the production of acellulase to convert the wood to a form suitablefor their own digestion. The Termitidae can bedivided into two distinct groups. The first groupcontains the Microcerotermes, Amitermes andNasutitermes species which depend on priorinfection by a fungus to convert cellulose to adigestive form. Lignin is not digested and ispassed through the gut, providing the rawmaterial for the construction of the typicalhoneycomb nests and covered walkways. Incontrast the group containing the Macrotermes,Odontotermes and Microtermes species is notrestricted to wood already infected by fungus butinstead these termites convert all cellulose to adigestible form in fungal gardens within theirnests. Fresh wood is gnawed into fragments

which are then chewed to paste and placed in thenest where they become infected by the termitewith a fungus which causes deterioration of bothcellulose and lignin. The wood is thereforeconverted to fungal tissue which is then eaten bythe termites.

The Kalotermitidae family includes the DryWood termites which, as this name implies, areable to digest wood possessing a very lowmoisture content. Symbiont protozoa in thehindgut provide cellulases sufficient to enablethese termites to digest wood cellulose in anormal manner, although lignin is not digestedand is passed through the gut. Colonies of thesetermites inhabit sound dry wood and rarely enterthe ground; for this reason all the other familiesare sometimes collectively known assubterranean termites. Dry Wood termite attackis spread by winged egg-laying females and, inareas where there is a risk of Dry Wood termiteattack, it is essential to use naturally durable oradequately preserved wood.

The termites in the Rhinotermitidae familyare sometimes described as the Moist Woodtermites; in contrast those in the minor familyTermopsidae are sometimes described as DampWood termites. The Rhinotermitidae causedamage to buildings and other structures buthave far less economic significance than eitherthe Termitidae or the Kalotermitidae. Theypossess protozoan intestinal symbionts but theyprefer moist wood which is already infected byfungi or bacteria, thus achieving more effectivedigestion and assimilation than the Dry Woodtermites.

In many areas the main termite hazard isconfined to the subterranean species of theTermitidae. The first group of this family dependson prior infection of the wood by fungus and theyare therefore a particularly serious hazard tofence posts, transmission poles and all other woodin soil contact. Attack can be readily prevented bytaking the conventional precautions to avoidfungal decay, such as ensuring that wood remainsdry in buildings and using only wood that is

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naturally durable or adequately preserved inground contact. In the case of the second groupcontrol is more difficult as they will physicallydestroy undecayed dry wood, ingesting fragmentsand transporting them back to the nest wherethey process them in fungal gardens and thusconvert them into useful food.

Physical barrier systems provide the mostcommon method for protecting buildingstructures from this particular hazard. TheTermitidae are unable to fly and protection canbe obtained by isolating wood from the soil bythe use of shields of metal or plastic between thewood and the footings, by introducing a barrierof poisoned soil or concrete, or by paintingstructural components white if they provide aroute to the wood; the termites dislikeconstructing their walkways on light-colouredsurfaces. These barrier systems are reasonablyeffective provided they are conscientiouslyconstructed to prevent the termites discoveringor constructing an alternative route to the woodstructure. Subterranean termites are capable ofconstructing unsupported tubular walkwaysspanning distances of 300 mm (12 in) or moreand this may enable them to bridge shields. Inaddition, barriers of this type are completelyuseless if bridged by negligence in subsequentconstruction or maintenance, such as the carelessinstallation of electric cables and plumbing.

Many other termites, particularly the DryWood termites or Kalotermitidae, are able to flyand cannot be realistically controlled by physicalbarrier systems. If wood is to be used instructures exposed to attack it must be naturallyresistant or adequately treated with preservativeif it is to survive.

2.3 Moisture content fluctuations

The properties of wood are profoundlyinfluenced by the presence of water. Themoisture content is very high in standing treesor freshly felled wood, usually known as greenwood, varying typically from 60 to 200%. The

green moisture content tends to vary inverselywith the normal dry density of a wood so thatblack ironwood with a specific gravity of 1.08has a green moisture content of only about26% whereas South American balsa with aspecific gravity of 0.2 has a green moisturecontent of about 400%. During drying freemoisture is first lost from the cell spaces andthis involves little change in properties exceptfor a change in density. Eventually drying willresult in the wood reaching the fibresaturation point, normally at a moisturecontent of 25–30%, and further drying resultsin the loss of bound moisture from the cellwalls.

Fibre saturation point

The loss of bound water reduces the separationbetween adjacent cellulose chains and causesshrinkage as well as progressive changes inphysical properties. The amount of bound waterremaining in the wood is approximatelyproportional to the relative humidity of theatmosphere, although changes in moisturecontent lag behind changes in relative humidity,a phenomenon known as hysteresis. Most of thechanges in properties with variation in moisturecontent can be attributed to the submicroscopicor chemical structure of wood, as explainedpreviously in Section 1.3.

Movement

The swelling or shrinkage with changes inmoisture content is known as movement. In thelongitudinal direction the movement is generallyvery low, only about 0.1% for a change ofmoisture content from normal dry wood at about12% to the wet condition at fibre saturation pointor above. In contrast the movement between theradial and tangential direction can be largelyattributed to the fact that early or spring woodshrinks less, so that in the radial direction themovement can be attributed to the average of the

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early and late wood shrinkage, whereas in thetangential direction the dense and strong ribs oflate wood tend to control the physical behaviourand thus generate higher movement. Indeed, if apiece of softwood is planed smooth at a lowmoisture content and then wetted, the late woodwill swell to a greater extent than the early woodand a corrugated surface will be produced.

Generally, a higher lignin content in aparticular wood species will result in lowermovement or greater stability. The highershrinkage in the tangential direction, coupled insome species with weakness induced by largemedullary rays, sometimes results in surface splitsdeveloping during drying. However, it has beenexplained in Chapter 1 that the movementcharacteristics can be largely attributed to themicrofibril angle in the cells, and as this changesprogressively from the pith outwards there is alsoa progressive change in movement properties.Heartwood close to the pith is most stable but, inaddition to this progressive change, there is a farlarger alteration in movement between heartwoodand sapwood in many species, the sapwood oftenpossessing extremely high movement (Fig. 2.2).These changes in movement relative to theoriginal position in the tree combine withdistorted annual rings and twisted grain to causewarping and splitting, defects that will bedescribed in more detail later in this section.

Water vapour is lost or gained most rapidlythrough end-grain surfaces because of theirpermeability, a factor which also encourages thevery rapid absorption of liquid water. Thismeans that many defects arising throughmoisture content changes, such as splits and thedevelopment of decay or stain fungi, areconcentrated around end-grain surfaces. Ingeneral, wood is used as long pieces with onlysmall end-grain surfaces so that the side-grain isusually more significant, particularly in servicein dry conditions where it is the hygroscopicityof the wood coupled with changes inatmospheric relative humidity that are likely tohave the greatest effect. Wood softens asmoisture content increases towards the fibresaturation point, a property that is very valuablein some circumstances such as when bendingwood or peeling veneers from logs, although itsignificantly reduces strength so that instructural uses wood must be employed inadequate dimensions to tolerate the strength lossthat will occur should the moisture contentapproach the fibre saturation point.

Shrinkage in buildings

When trees are first converted to sawnwoodthe moisture content is significant because ofthe danger of fungal damage and also because of

FIGURE 2.2 Shrinkage on drying.


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the excessive transport costs that are involved ifthe weight of wood is perhaps doubled by a highmoisture content, but if this wet wood is used inbuildings the most serious problem, other thanthe danger of fungal decay, is the cross-grainshrinkage that will occur as the wood graduallydries and achieves an equilibrium moisturecontent with its surroundings. This is a veryserious problem in certain uses such as windowand door frames, floorboarding, panelling andother situations in which cross-grain shrinkagewill result in the development of unacceptablegaps between the individual pieces of wood. It istherefore usual to air-season or kiln-dry wood

before installation, to the average moisturecontent that it will encounter in service. Themain problem is that wood may be processed tothe required moisture content but this may thenchange during storage, delivery or installation,particularly in buildings where the wet tradessuch as bricklaying and plastering result in veryhigh humidities during construction. Indeed,floors are sometimes laid in new buildings verypromptly after kiln-drying and the highhumidity then causes them to expand so thatsometimes the entire floor lifts in a dome or theexpansion causes serious damage to thesurrounding walls (Fig. 2.3).

FIGURE 2.3 Parquet flooring distorted through moisture absorption after being laid. (CSIRO, Australia)

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Stacking the wood for a period inside thebuilding before laying will avoid this particularproblem as the wood will thus reach equilibriumwith the high humidity in the damp new buildingbut when the building is ultimately occupied therelative humidity will fall and shrinkage defectswill develop. Quarter-sawn (radial cut)floorboards will simply shrink in width leavinggaps at the joints but flat-sawn (tangential cut)boards will cup in addition; they should be laidwith the heart surface upwards so that thecupping causes each board to be raised in itscentre rather than at its edges where it can causetripping. The integrity of the floor and thus itsability to act as a fire-break or thermal insulatoris naturally affected by this shrinkage, unless theboards are joined by tongues to span theshrinkage gaps.

Panel products

A floor is a rather large panel normallyassembled out of separate boards and theproblems encountered in floors are obviouslyencountered in other panels such as wall linings,doors and furniture. Several panel materials arenow available in which wood has been processedto minimize these shrinkage defects. In plywoodthe wood has been cut into veneers which areorientated at right angles to one another so thatthe stable longitudinal grain is used to physicallyrestrain the movement in the cross-graindirection. This method for reducing shrinkage isparticularly successful in normal buildings butsometimes fails under extreme conditions such aswhen used in boat building. Adhesives have beendeveloped which will withstand the very highstress that is developed when the moisturecontent fluctuates from fibre saturation pointdown to low levels in very dry conditions but ifthis occurs regularly there is a tendency for thewood to rupture on either side of the glue lineand this can be avoided only by using wood ofmoderate or low movement in the manufactureof the plywood. It should be appreciated that the

plywood still expands normally in thickness butthis is seldom a problem unless it affects theweather seal when plywood is used as anexterior panelling.

One particular advantage of plywood is that,as the grain runs longitudinally in bothdirections within the plane of the board, it isequally strong in both directions, although notas strong as the longitudinal direction for normalwood. In contrast it could be said that thealternative process for producing a panel by themanufacture of particle-board results in amaterial that is equally weak in both directions.In particleboard manufacture the principle is todivide the wood into small particles which arethen randomly orientated so that the movementis shared equally in all directions, although it isrestrained to some extent if sufficient adhesive isused. The movement is higher than for plywood,and, whereas plywood presents a completelysmooth surface, a particle board iscomparatively irregular, a defect that isparticularly noticeable with changes in relativehumidity as the surface chips expand andcontract across the grain. Smaller particles aresometimes used for the surface of the board inorder to minimize this problem or the surface isveneered, although veneers applied on boardswith comparatively large surface chips will notconceal this movement which will show throughthe veneer as shadowing.

The only alternative is to reduce the wood toindividual fibres which are then randomlyoriented and reconstructed into a board whichis, in effect, a rather thick sheet 6f paper. Low-density boards of this type are; known asinsulation boards and are used for lining wallsand ceilings, but the medium- and high-densityboards or hardboards present a reasonablysmooth surface for painting. Whilst fibre-boardsmight appear to be similar to plywood inprinciple, although the wood components arereorientated on a rather different scale, they donot achieve the same end results. In fibre-boardsthe movement tends to be randomized in all


37

directions, whereas in plywood the movement inthe plane of the board is physically restrained.Plywood is therefore preferred for externalpanelling and other situations where it mayachieve a high moisture content as movementwithin the panel dimension remains small,whereas a fibre-board will swell significantly insimilar conditions and this may lead to seriousdistortion.

Seasoning

Shipping specifications often require wood tobe properly seasoned for shipment to thecountry of destination. This generally meansthat the moisture content of the wood must besufficiently low to ensure that no deteriorationoccurs within the hold of the ship. In theory thewood must be dried in some way to a moisturecontent below about 22% compared with anaverage moisture content of 60–200% whenfreshly felled. Seasoning is the term that isgenerally applied to the processes that areadopted for reducing the moisture content.Various shrinkage defects can develop if woodis dried too rapidly so that much of theexpertise in seasoning is devoted to achievingthe maximum drying rate whilst still avoidingthe development of defects. Shrinkage andswelling will also occur as the moisture contentvaries in service so that seasoning shouldattempt to dry the wood to the averagemoisture content that it will achieve in finaluse, in order to minimize movement defects.

The traditional method for seasoning or dryingwood is to stack it in the open air, although it willbe appreciated that drying will be achieved only ifthe stacks are protected from rain, either byproviding a roof to each individual stack or by theuse of large open-sided seasoning sheds. Oneserious problem with air-seasoning is theexcessive drying rate during hot weather. This canbe a very serious problem as drying occurs mostrapidly from the porous end-grain which thusshrinks in advance of the wood further along a

piece, causing the development of severe splits orseasoning checks. This defect can be partlyavoided by the use of end cleats, generally piecesof wood or metal nailed across the end-grain inorder to physically restrain the splitting. Thismethod for controlling checks is very unreliableand a more efficient technique is to seal the end-grain with a bitumastic or wax formulation sothat drying is confined to the side-grainthroughout the length of each piece of wood.When the wood has a high moisture content in itsinitial green state it is comparatively flexible andit must be carefully stacked in order to avoidsagging. Stickers or piling sticks must be placedbetween the pieces to permit a proper circulationof air, or alternatively the pieces may be stackedwith each alternate layer in a different direction, amethod known as self-piling which is frequentlyused for the drying of round wood such astransmission poles.

In kiln-seasoning the wood is placed incontainers or kilns in which the temperature andhumidity can be controlled in order to achievethe maximum rate of drying consistent withfreedom from the development of defects. Thekiln must be designed and the wood carefullystacked so that a uniform air circulation can beachieved. If the air is always passing in the samedirection it is obvious that the temperature willbe lower and the relative humidity higher at theexit, so that the wood nearest the exit will have ahigher moisture content than that close to theinlet. In many kilns the air flow can be reversedin order to reduce these differences. Oneadvantage of kiln-seasoning is the hightemperatures that are used towards the end ofthe drying sizes of the pieces of wood that arebeing dried when controlling a kiln, althoughcross-section dimensions will be important if it isdesired to estimate the time to complete thedrying process.

Fibre saturation point usually occurs at amoisture content of about 25–30% andshrinkage occurs in wood dried below this point.Air-drying will normally reduce the moisture

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content to 17–23%, and if lower moisturecontents are required kiln-drying is essential.Wood can be pressure-treated with preservativesat moisture contents below 25%. Carcassing orframing timber in buildings can tolerate amoisture content of up to 23%, mainly becausethe cross-section dimensions in which movementoccurs are relatively unimportant and drying isnecessary only to achieve resistance to fungaldecay. Wood intended for use in ships, boats andvehicles should be dried to 15%. In a house withreasonable central heating the average moisturecontent should be 12%, although bedroomfurniture would perhaps be better manufacturedwith a moisture content of 14%, more closelyrelated to the lower heating level in these rooms.In buildings which are more intensively heatedsuch as offices and hospitals, a moisture contentof 8% may be necessary, and perhaps less inflooring installed on top of floor heating.

In climates with cold dry winters theatmospheric relative humidity is very low andresults in moisture contents as low as 4% inbuildings with central heating, yet in the springthe high humidity may increase the moisturecontent to as much as 12% so that the choice ofa preferred moisture content for kilning is ratherdifficult. Indeed these required moisture contentsare all largely theoretical as it is very difficult toensure that wood remains completely protectedduring the numerous handling andtransportation stages after leaving the kiln. Theworst stage is perhaps installation in a newdamp building in which any wood willautomatically recondition to a higher moisturecontent in equilibrium with the new conditions.

It has already been explained that woodshrinks when its moisture content is reduced,that the outer zones of the trunk shrink morethan the inner, and that the sapwood shrinksmore than the heartwood. In addition, the outerlayers of a log or piece of wood are ventilatedand thus dry more rapidly than the inner cycle asthese eradicate any insect infestation as well asmany fungal infections.

Atmospheric relative humidity

The ability of air to absorb moisture varies withthe temperature so that the moisture content atwhich air becomes saturated increases with thetemperature. When air is completely saturated itis said to have a relative humidity of 100%, anda relative humidity of 50% naturally means thatthe air is half saturated. If air increases intemperature the relative humidity will decreaseor the air will appear to become drier as it iscapable of holding a larger amount of moistureat the higher temperature, yet the actualhumidity or moisture content of the air willremain unchanged. Thus the drying power of airwill be increased as the relative humidity isreduced, either by reducing the actual humidityor moisture content or simply by increasing thetemperature to improve the moisture holdingcapacity. Wood cannot dry if the relativehumidity of the air is 100%. In addition, if woodis allowed to remain in air possessing a constantrelative humidity, it will eventually acquire amoisture content in equilibrium with thesurrounding air. The moisture content inequilibrium with 100% relative humidity isknown as the fibre saturation point and lowermoisture contents are in equilibrium with loweratmospheric relative humidities. It will thereforebe appreciated that the actual operatingtemperature of a kiln is comparativelyunimportant as it is the relative humidity that isresponsible for the rate of drying.

Various kiln schedules have been devised to drywood as rapidly as possible without thedevelopment of seasoning defects. Obviously therate of drying will depend on the cross-section ofthe pieces of wood and it is therefore importantthat a kiln should always be loaded with wood ofsimilar dimensions. In addition, kiln schedules arenormally devised so that they are controlled by themoisture content of the wood at the air inlet side ofthe kiln. In a typical schedule it will be specifiedthat, when this moisture content reaches a certainlevel, the relative humidity of the incoming air


39

must be reduced by raising the temperature inorder to continue the drying process. In this waytime is ignored in kiln schedules and it is alsounnecessary to consider the actual layers, acombination of all these effects leading to thedistortion and tissue rupture defects which are acharacteristic of faulty seasoning or kiln-drying.One particularly important point is that wood isplastic when wet but set when dry so that it isessential that wood should be carefully stacked flatto avoid unnecessary distortion.

Warping

Warping is the general term for seasoningdistortion and takes several forms. Cupping iswarping across the grain or width of a boardand arises in flat-sawn boards through the outerzones of the trunk and particularly the sapwoodshrinking to a greater extent than the innerzones, giving a planoconcave surface on theouter side of the board relative to the trunk. Flat-sawn boards used for flooring should always belaid with the heart upwards to give aplanoconvex surface with the prolonged dryingthat will occur in service and thus avoid tripedges. The alternative is to use more expensivequarter-sawn boards which are resistant tocupping and also more hard wearing. Twistingarises in boards through the presence of spiralor interlocking grain and is really the result ofwood selection rather than a seasoning defect,although spiral grain is quite common andperhaps unrealistic to reject. Bowing islongitudinal curvature arising perpendicular tothe surface of a board, either through spring in aflat-sawn board or more usually through saggingin the stack during drying. Spring or crooking islongitudinal curvature within the plane of theboard and can be severe in some species suchas elm or kempas grown in swampy areas,giving decreased strength. Finally diamondingoccurs in pieces of wood of rectangularsection when the annual rings pass diagonallyacross the end-grain, as shown in Fig. 2.4; the

radial shrinkage is less than the tangentialshrinkage so that the diagonals in a squaresection piece of wood become different in lengthand give a diamond shape.

Splitting

Drying occurs most rapidly at the end-grainthrough its very high permeability. Theshrinkage close to the end-grain may thereforeoccur whilst the rest of the piece of wood stillhas a high moisture content and retains itsswollen dimensions. The stress between theshrunk end-grain and the swollen inner woodfrequently results in splits on the end-grainsurfaces, although the term splits is normallyused to describe cracks passing right through thepiece of wood. In contrast, checks are moreshallow, occurring as end-checks on the end-grain or surface checks on the other faces,perhaps diagonal to the edge of the board ifspiral grain is present. Splits and checks mayclose with rewetting but, although they may thenbe virtually invisible, the strength has been lost.Serious splits are sometimes described as shakes,although this term is more correctly used todescribe defects inherent in a particular treerather than those developing as a result of faultyseasoning or kiln-drying.

The stresses and strains resulting from the rapiddrying of end-grain in advance of the rest of a pieceof wood have already been described but anidentical situation occurs in respect of the entiresurface of the piece of wood which naturally dries

FIGURE 2.4 Diamonding or unsymetrical shrinkagein tangential and radial directions.

Wood degradation

40

more rapidly than the inner wood and thus shrinksin advance. This defect is known as case hardening.Shrinkage of the outer zones tends to result in thedevelopment of surface checks. If rupture does notoccur it is possible for the outer zone to be tension-set so that subsequent sawing will release thistension and result in spring or cupping; if tension-set is suspected it can be relieved by steaming. Thetension in the outer zone may alternativelycompress the inner zone which is wetter and thusmore plastic, sometimes squeezing water out of theend-grain. Further drying may then result in thesubsequent shrinkage of the inner zone withoutassociated shrinkage of the dry set or casehardened outer zone, giving internal rupturesknown as honeycomb checks or hollow horning.

Seasoning defects can still occur, despite greatcare in applying the most suitable dryingschedules. Case hardening is perhaps the mostcommon defect in certain species such as beech.When case hardening is anticipated or it hasactually occurred the kiln must be operated at ahigh temperature and high humidity in order toplasticize the outer layers of the wood so that thecase hardening is relieved and the wood recoversits proper dimensions. This treatment is appliedonly for a very short period, sufficient to relievethe case hardening without significantlyaffecting the moisture content of the piece as awhole. High temperature is also used duringkilning in order to ensure sterilization ofhardwoods against Lyctid beetle attack. In thisconnection it is interesting to note that kiln-seasoned wood is then far more susceptible tosubsequent Lyctid beetle attack than air-seasoned wood, apparently because it possesses ahigher starch content, as explained earlier in thischapter.

Joinery failure

Although it is normal to dry wood to a moisturecontent equivalent to the average atmosphericrelative humidity anticipated is use, it is commonto encounter movement problems. Faults such as

gaps appearing between floor blocks or boards aredue to the wood drying after installation, eitherthrough inadequate kilning or perhaps rewettingbetween kilning and installation. A door or drawerjammed in humid weather may be exceedinglyslack in drier conditions. Frames which introducean end-grain surface in contact with side-grain willinevitably result in cracking of any surface coatingsystem. In other situations the cross-sectionalmovement may become apparent as warping. Theobvious solution to all these problems is to use onlywood with low movement but this is not alwaysrealistic. The alternative is to impregnate the woodwith chemicals which induce stabilization,although processes of this type are also frequentlyunrealistic because of the difficulty in achievingcomplete penetration.

Preferential wetting

The only theoretical alternative is to enclose thewood within a protective film in order to stabilizethe moisture content. Paint and varnish coatingswill act in this way, provided they completelycover the wood and are not damaged in any way.Unfortunately, whilst these coatings give goodprotection against rainfall, they are unable toprevent moisture content changes resulting fromslow seasonal fluctuations in atmospheric relativehumidity. As a result the painted wood will shrinkor swell with changes in relative humidity, causingthe surface coating to fracture wherever a jointinvolves stable side-grain in contact with unstableend-grain. Rain is absorbed by capillarity into thecrack, yet the remaining paint coating restrictsevaporation so that the moisture content steadilyincreases until fungal decay occurs if the wood isnon-durable. It is frequently suggested thatpreservation provides a simple solution to thisproblem by reliably preventing decay but thisignores the fact that water also damages the paintcoating. Wood is a hygroscopic material, coveredwith hydroxyl groups which have a strong affinityfor water so that penetrating water will tend tocoat the wood elements, displacing paint and

Fire

41

varnish coatings. This failure is known aspreferential wetting and is responsible forblistering and peeling in paintwork as well as lossof transparency in varnishes.

2.4 Fire

It is well known that wood is combustible, and anatural reaction to a fire disaster in a building isto demand that only non-combustible materialsbe used in construction in the mistaken belief thatthis will ensure fire safety. The reputation of non-combustible structural materials originates fromobservations on the behaviour of, for example,traditional heavy masonry but the same excellentfire performance is not necessarily achieved byother non-combustible materials. Fire in abuilding usually initiates in and involves mainlythe contents, and the contribution from woodenstructural members is small and usually limited tothe later stages of the fire due to their size andposition within the structure. Even in acompletely wood-framed house there is a total ofonly one third more wood than in traditionalmixed construction. It can therefore beappreciated that the combustibility of structuralmembers is of limited importance compared withability to withstand fire originating in furnishingsand other building contents.

Flaming and charring

Whilst wood is certainly combustible,ignitability and rate of combustion areparticularly important in most situations.Combustion cannot occur in the absence ofoxygen so that large-section wood membersburn only slowly at their surface which thenprogressively chars. As the temperature rises thewood first releases volatile components whichflame on the surface and the residue then charsas a further increase in temperature occurs. Thethermal conductivity of wood is very low, only0.4% of that of steel or 0.05% of copper, andthe same order of conductivity as for cork,

gypsum plaster and other insulation materials.Indeed balsa, a very low density wood, is used asan insulation material. The natural thermalinsulating properties of wood therefore limit therate at which heat can be transferred inwardsfrom a burning external surface, and theseinsulating properties improve steadily asmoisture is lost from the wood and charringprogresses; charcoal is an even better insulatorwith a conductivity of only 30 to 50% of that ofnormal wood. Eventually the heat transfer fromthe surface is insufficient to release volatilecomponents from the interior and the surfaceflaming then ceases. The charring rate alsoprogressively slows down, unless heat iscontributed from surrounding burning materials;the significance of the contents of the structure isagain apparent.

Fire resistance

The charring rate is loss of dimension, whilst theburn-through rate is loss of weight. In largestructural members the charring rate isimportant as the rate of change of dimensionnaturally controls the ability of the woodencomponent to continue to support the structuralload. The charring rate varies with woodproperties such as thermal conductivity anddensity which must therefore be consideredwhen constructing a wooden barrier to achievefire resistance, although design is of even greatersignificance. For example, there must be no gapsaround doors or windows through which the firecan penetrate, or thin areas where earlypenetration can occur. Despite its combustibilitywood possesses excellent fire resistance, largelybecause of its low thermal conductivity, and thisnot greatly affected by any form of treatment.

Ignition

The ignition point, usually about 270°C (518°F)for wood, is the temperature at which the rate ofheating exceeds the rate of supply of heat and

Wood degradation

42

the fire becomes self-supporting with perhapsvisible flame or glow. There is usually noignition, even on the superficial surface, if themoisture content of deeper areas of woodremains above about 15%. A pilot source offlame is necessary in order to initiate ignition asspontaneous ignition of wood is possible only atexceptionally high temperatures. Large pieces ofwood do not ignite easily, and ignition of smallerpieces is achieved only because of the rapid rateat which they reach the ignition temperature.

Flame spread

Flame spread across the surface of a piece ofwood is really a series of ignitions, one area onfire acting as pilot ignition for the adjoiningarea. Flame spread is influenced by moisturecontent as in normal ignition but it also dependson the density and chemical nature of theparticular wood. Chemical treatments canachieve considerable success in resisting ignitionand thus preventing flame spread. Indeed, thepropagation of a fire by flame spread is the mostserious criticism of the use of wood as astructural material, yet this is the one propertythat can be readily modified by comparativelyinexpensive treatment.

Smoke

Smoke generation represents the most serioushazard to human life during fires in buildings.The most harmful smokes arise from the plasticsand synthetic fibres which are often used infurnishings, whilst the smoke from wood iscomparatively innocuous. It is thus important toensure that dangerous smokes are not generatedby chemical treatments applied to wood to limitflame spread and propagation.

Despite its combustibility, wood thus possessdistinct advantages when used as a structuralmaterial. The contribution of the structuralwoodwork to a fire is minimal, at least in the earlystages, and wood has excellent resistance to firepenetration and suffers neither significantdistortion nor rapid loss of strength. In comparisonsteel collapses when it reaches its yield temperatureand concrete shatters or spalls, particularly if itencloses a steel frame. Where reinforcement barsare present in concrete they may be stressed at hightension and a rise in temperature will result inyielding with distortion of the structure even ifcomplete collapse is avoided. Even if fire iscontrolled before the yield temperature of steel isreached there is still a danger that excessiveexpansion will rupture the building structure.

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3.1 Preservation mechanisms

The science, art and technology of preservationare concerned with the design, development andadoption of systems for preventing the variousforms of wood deterioration. Decay by the wood-destroying Basidiomycete fungi is certainly themost important form of deterioration as it isinevitable whenever wood is exposed todampness. It is true that some species of wood aremore durable than others but none are completelyresistant to decay, and a species that is said to benaturally durable will simply decay at a slowerrate than one that is said to be non-durable.


There are three important factors whenconsidering decay caused by Basidiomycetes. Thedamage caused by these fungi can be divided intotwo distinct types known as Brown and White rotsrespectively. A Brown rot generally destroys thecellulosic skeleton of the wood, leaving the ligninlargely intact so that the wood becomes brown incolour and generally rather friable, perhapsdeveloping longitudinal and cross-grain cracking.A White rot decays both the cellulose and lignin sothat the colour remains virtually unaltered and thewood becomes soft and perhaps linty.

Fungus spread

A second important feature is the development ofrhizomorphs by some fungi. These are composed

of hyphae modified into strands which are able toconduct water and food from one part of thefungus to another. Some fungi are able in this wayto transport moisture absorbed by one part of thefungus to another part where it can be used, forexample, to condition wood prior to decay. In asimilar way, nourishment or energy can betransported from an area of active wood decay toother parts of the fungus which are attempting tospread in a zone lacking nourishment. This mightinvolve spread over masonry which may, ofcourse, be a source of moisture but which cannever be a source of nourishment. Alternativelythe fungus may be attempting to spread intopreservative-treated wood. In the case of somehighly fixed preservative treatments the fungusmay spread through the treated zone withoutcausing damage but also without being affecteditself, so that there is a danger that it may be ableto spread to deeper zones which are unprotectedthrough limited penetration of the preservativetreatment. In other cases the availability of energyenables the fungus to actively detoxifypreservatives, particularly certain toxic metalssuch as copper which can be de-toxified by theformation of oxalate. Highly developedrhizomorphs are a feature of only a fewBasidiomycetes, particularly Brown rots, the bestknown being the Dry rot fungus Serpulalacrymans.

Fungus requirements

The third factor of importance concerns the

3

Preservationsystems

Preservation systems

44

essential needs of a fungus for spore germinationand development. The first requirement is for asource of nourishment, usually wood, but it mustbe appreciated that fungi are also capable ofgrowing on a variety of other cellulosic materials.The second requirement is for water, mostBasidiomycetes requiring an adequate moisturecontent within the wood that they are attemptingto decay, as well as perhaps a sufficiently highatmospheric relative humidity. IndeedBasidiomycete attack is often invisible simplybecause the fungus avoids attacking the externalsurface of the wood where it is likely to be exposedto the dehydrating effect of an atmosphere of lowrelative humidity. The third requirement is foroxygen and this clearly operates in the oppositesense, tending to inhibit fungal activity deep withinlarge pieces of wood. The fourth requirement is theneed for a suitable temperature; low temperaturesgenerally inhibit development whilst hightemperatures may initially encourage thedevelopment of sporophores or fruiting bodies buteventually lead to the death of the fungus. If thetemperature increase is slow the sporophores maybe able to develop to a sufficient extent to producespores which will be able to resist a furthertemperature increase. This will permit the fungusto redevelop when suitable conditions return, evenif the original growth has been killed by the hightemperature.


Preservation systems rely on the elimination ofone of these essential requirements in order toprevent fungal development. Chemicalpreservation treatment is essentially theelimination of a source of nourishment and willbe discussed in detail later. The elimination ofoxygen is a system that is actually used inpractice where wood is waterlogged; forexample elm is particularly susceptible to decayin damp conditions, yet it is widely used inmarine and river defence works and even in boatplanking where it gives excellent service

provided that it remains saturated with water.Wood can survive for many centuries in marshyareas in this way, although unfortunately thereare other chemical changes that slowly takeplace so that the recovery of archaeologicalwood presents special problems.

Structural design

The most widely used preservation system is toensure that wood remains dry by takingappropriate structural precautions. Thusbuildings are designed with roofs to protect thestructure from rainfall. Projecting eaves, guttersand fall pipes are all devices to ensure thatrainwater is dispersed clear of the structure. Wallsare designed to resist penetrating rain, perhapsthrough cavity construction. Rainwater is stillabsorbed by the outer layers of walls constructedfrom porous materials and damp-proofmembranes, flashings and soakers are provided toensure that this dampness cannot penetrate to theinterior. Damp-proof membranes are alsoprovided to isolate the walls, joists and floorsfrom dampness rising by capillarity.

These precautions are adopted primarily toensure that the interior of the building remainsvisibly dry but it is essential to extend theprinciples to ensure that wood componentsremain dry. The main danger areas are wood incontact with porous external brickwork ormasonry such as window and door frames androof structures supported on outer walls. Theseprecautions are not sufficient in themselves andit is generally also necessary to ventilate deadspaces under floors, in wall cavities and roofspaces, particularly flat roofs and frame walls.Unfortunately the Building Regulations in theUnited Kingdom usually limit ventilation inorder to reduce heat losses so that condensationand wood decay problems frequently occur;these are discussed in more detail in the booksRemedial Treatment of Buildings and Defectsand Deterioration in Buildings by the presentauthor. It must not be imagined that these

Preservation mechanisms

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problems arise solely through misconceivedregulations, as one of the most common causesof condensation under floors is not limitedventilation but the use of air-conditioningequipment within the living space so that floortraffic surfaces are kept cool during warmweather and condensation is encouragedbeneath, particularly in tropical areas, causingWet and Dry rot problems similar to thoseencountered in buildings in temperate regions.

Decorative coatings

The simple precautions which have beendescribed are usually sufficient to prevent decayin the carcassing or framing components ofbuildings but decay may still occur in woodwhich is exposed directly to the weather orwhich is in contact with porous brickwork ormasonry exposed to the weather. Reference hasalready been made to the difficulties encounteredin practice in ensuring that window and doorframes are isolated from surrounding porousmaterials by proper damp-proof membranes.

As far as rain is concerned it is normal to rely onpaint or varnish to protect exposed wood surfaces.Paint manufacturers often claim that paintfunctions as a wood preservative but fluctuationsin atmospheric temperature, particularlydifferences in temperature between the interior andexterior of a building, result in redistribution of thewater within wood window and door frames byevaporation and condensation so that this waterbecomes concentrated immediately beneath thecooler paint or varnish coat, usually the externalsurface during winter weather. Water penetrationthrough a small damaged area of paint or absorbedthrough contact between an inadequately paintedsurface and adjacent porous brickwork or masonrywill contribute to the moisture within the wooduntil eventually the moisture content reaches alevel at which a fungal attack can develop.

Whilst it is true that an intact paint filmprevents rain penetration, it is equally true thatpaint traps moisture within the wood. Random

surveys carried out in England have shown thatafter a few years most window frames havemoisture contents of perhaps 20 or 30% aroundthe joints with the sills, often causing decay. Thisproblem is usually due to the failure of the paintat joints where it is unable to tolerate thedifferential movement between side-grain andend-grain. In this way a crack develops whichallows water to penetrate but the remainingpaint inhibits evaporation so that moistureaccumulates and decay ultimately occurs.Coating systems may thus actively encouragedecay and in some countries it has becomenormal practice in recent years to require allwindow and door frames to be adequatelypreserved. Although this prevents deteriorationof the wood it is not the complete answer to theproblem; water accumulations still occur andtherefore damage the adhesion between thesurface coating and the wood. This problem isknown as preferential wetting failure and will bediscussed later in this chapter.

Preservation by isolation

The greatest value of paint may be to physicallyisolate wood from the attacking fungus as apainted wood component will appear to thefungus to be solid paint. Unfortunately the paintfilm can be penetrated by physical damage—thedevelopment of splits at joints, movement of thewood causing splits at edges, and thedevelopment within the wood of staining fungiwhich can bore outwards through the coating oralternatively settle on the coating and boreinwards. The various methods for avoiding thesedefects will be described later. In pursuing thetheoretical preservation techniques of isolation itis apparent that these defects can be best avoidedby impregnation rather than by superficialsurface coating. Naturally it is also better if theimpregnated material is toxic to the organismsthat are likely to cause deterioration. The use oftar is perhaps the best example of thedevelopment of this type of treatment. Tar was


46

originally applied as a superficial coating butlater hot tar or impregnation treatments wereadopted to encourage penetration and ultimatelya special tar distillate, creosote, was selected forits toxicity towards fungi and its relatively lowviscosity. Unfortunately all isolation treatmentsnecessarily involve substantial alterations in theappearance of wood and the only alternative tothe isolation principle is to adopt one of thevarious toxic systems.

Natural durability

Structural systems to ensure that wood remainsdry represent the most important and mostwidely used preservation processes but there aresituations where these cannot be used, such aswhere wood is exposed directly to the weather,ground contact or other hazardous conditions. Itis possible to find woods which possess naturalresistance to almost all biodeterioratingagencies; an indication of the durability of thesewoods which are most widely used is given inAppendix A. Generally, heartwood is much moredurable than sapwood and those species withdarker coloured and denser woods are usuallymost durable. Denser woods are less porous withmore wood substance and less access for waterand oxygen, while darker coloured woods oftencontain extracts which may be toxic to decayfungi or resins which may reduce waterabsorption. Unfortunately these generalprinciples do not always apply. Thus resinousScots pine or spruce is more durable than lessresinous Silver fir, yet resinous Weymouth pine isnot as durable as less resinous larch. Resincontent is evidently not the entire story andcertainly cannot account for the durability ofmost hardwoods.

The density principle is also frequentlycontradicted. Certainly the dry density ofsusceptible sapwood is invariably less than thatof more durable heartwood, yet very low-densityWestern Red cedar is much more durable thansome heavier softwoods. The cedar is darker due

to the presence of extractives which are knownto be toxic but it is clear that the naturaldurability is not due to this alone but to acombination of numerous factors. Unfortunatelydurability is not always consistent within aspecies, although it is sometimes possible tojudge probable durability just by colour anddensity. For example, in Scots pine the bestdurability is associated with slow growth andalso with greater density and darker colour.

In all cases natural durability is a measure ofresistance to decay; there are no woods that arecompletely durable in all circumstances as it isclearly necessary that nature should be able todispose of dead trees, but the selection ofnaturally durable wood is a technically realisticway to avoid an unacceptable rate ofdeterioration and thus ensures the life of astructure. For example, the heartwood ofEuropean oak is frequently used for fence postsand Western Red cedar is used for windowframes, greenhouses and external cladding.Unfortunately the use of naturally durable woodis not always possible; even if supplies areavailable the non-durable sapwood cannotalways be rejected. It is frequently more realisticand less expensive to select wood for itsdesirable physical or aesthetic properties andthen apply preservation treatment in order toensure its durability.

Toxic preservation

Creosote has already been described as a woodpreservative, developed from the isolationprinciple where reliability is improved byimpregnation and by the use of a material that istoxic to the attacking organism. All isolationtechniques result in a fundamental change in theappearance of wood and may be aestheticallyunacceptable or even dirty, limiting their uses.The obvious alternative is to abandon theisolation principle with high retentions ofmaterials of low toxicity and adopt instead lowretentions of compounds of very high toxicity.


47

Although creosote possesses only limited toxicityits retentions can be reduced for many uses,achieving cheaper and cleaner treatments.Originally, creosote was only applied inimpregnation plants by the full-cell system whichis designed to achieve the maximum retentionpossible but for many purposes empty-cellsystems are now adopted which, whilst achievingalmost the same penetration, reduce retentions byperhaps 40%; impregnation systems are describedlater in this chapter and creosote treatments aredescribed in detail in Chapter 4.

There are many different toxic preservationsystems. In principle, a preservative can befungistatic in the sense that it can prevent thefungus from attacking the wood but will notnecessarily kill the fungus. In contrast, in remedialtreatment wood preservation, where fungus mayalready be present, a preservative must havefungicidal properties in order to ensure eradication.Most preservative systems rely on a direct toxicfungicidal action and on absorption by the fungusof toxic materials in sufficient quantities to provefatal. Whilst this absorption is taking place someminor damage may be caused to the wood andsome of the preservative components may beremoved into the dying fungus. An adequatereservoir of fungicide is therefore essential toprovide the required treatment-life in situationswhere there is continuous fungal attack as in postsand poles in ground contact. Thus the treatmentmust be permanent with good resistance to lossesby leaching, volatilization and oxidation, yet itmust remain available to the attacking fungus. Thesimplest toxic system may involve a compound oflow solubility which, if applied at high retentions,will give an adequate life even when exposed tolimited leaching. At the same time the limitedsolubility will be sufficient to ensure that if thetreated wood becomes damp an attacking funguswill encounter a toxic solution. The best exampleof this type of treatment is the Timbor boratesystem which is applied either by diffusion into wetgreenwood or by conventional pressureimpregnation into dry wood. Timbor is a highly

soluble sodium borate but the sodium ions areprogressively neutralized by atmospheric carbondioxide to give a boric acid deposit of relativelylow solubility. Treatments of this type are suitablefor situations where severe leaching is unlikely,such as within buildings where the decay risk isgenerally associated with either rainwater orplumbing leaks which are rectified when theybecome apparent, or condensation where leachingrarely occurs.

Fixation

One way to achieve good resistance to leachingis to make use of a preservative deposit which isinsoluble in water but soluble in the presence ofa fungal enzyme. In some cases pH alone may beinvolved. For example, the enzymes exuded byBasidiomycete hyphae are usually acid andtherefore require a preservative that is soluble inthe presence of acids. However, this creates theproblem that the preservative may also be madesoluble if the treated wood is naturally acid or ifit is in contact with acid ground-water. A furtherpoint of interest with regard to pH is thatenzymes may function only in acid conditions, sothat wood with a high pH may have excellentresistance to decay. This principle has beenapplied in the preservation of wood chip piles atpulp mills by treatment with sodium hydroxide,although the same system cannot be used as ageneral wood preservative because of the causticdanger and the colour changes that result.However, some fungicides such as the variousphenols are distinctly more toxic when appliedas high pH alkali metal phenates than whenapplied as phenols.

One further effect of pH is its influence onspore germination. The spores of most wood-destroying Basidiomycetes germinate mostreliably in slightly acid conditions, ensuring thatgermination occurs only on a wood surface whichis acid when it is damp. In some cases the acidconditions are virtually essential for germinationas in the cases of the Dry rot fungus, Serpula


48

lacrymans, and it can be suggested that this maybe because the fungus wishes to ensure that itsspores can germinate only on wood which hasalready been subject to earlier Wet rot attackwhich generates the required acidity. Forexample, a plumbing leak in a building may resultin a steady high moisture content in wood whichwill permit the development of a Wet rot such asConiophora puteana but this fungus will beinhibited if the leak is repaired and the moisturecontent decreases. As the activity of the Wet rotceases the moisture content will become moresuitable for the germination of the spores of Dryrot, and this germination will be distinctlyencouraged by the acidity of the wood caused bythe Wet rot attack. An interesting point about thisprogression of events is that it apparently ensuresthe development of Dry rot in drying conditionsin which this fungus alone is able to survive; ifventilation is restricted Dry rot can maintain theatmospheric humidity at the optimum for growthby exuding moisture on the superficial hyphae.

Detoxification

A Basidiomycete hypha exudes an enzymesolution which attacks the surrounding woodand the solution of the wood componentsobtained in this way is then absorbed by thehypha for nourishment. The tip of the hyphagrows progressively into a cavity that is formedin advance by this enzyme activity. A fungicidalpreservative treatment may restrict the ability ofthe enzymes to decay the surrounding woods, asin the case of a high pH treatment, oralternatively the enzyme may dissolve a toxicdeposit which is then absorbed into the hypha,perhaps resulting in the death of the fungus.However, if a fungus which is well established onuntreated wood spreads into a treated area itmay have sufficient energy to tolerate theabsorption of the toxicant, perhaps byconverting it to a non-toxic form. Thus coppermay be detoxified by the formation of copperoxalate, a process that is often apparent on

wood treated with copper naphthenate solutionas the characteristic green colour disappears ashort distance in advance of the visible spread ofthe fungal hyphae. Detoxification requiresenergy and slow decay still occurs, so that thepreservative treatment is entirely wasted.

Test methods

Failure of a preservative treatment in this way isnot necessarily an indication that the preservativeis unsuitable but usually that it has been appliedat an inadequate retention and that the fungus hasbeen able to develop on untreated wood, perhapsexposed by cross-cutting, drilling or other wood-working, after treatment. Alternatively it ispossible that moisture content changes haveresulted in the development of splits or checkswhich have permitted fungus to penetrate throughthe treated zone; it is essential that all preservativetreatments should penetrate to a sufficient extentto avoid this danger. These factors must be clearlyappreciated in any attempt to test woodpreservatives. Normal laboratory testing methodsinvariably involve the establishment of a toxiclimit or the retention at which the fungus is justcontrolled. Spores have little spare energy andgermination is thus readily prevented on a treatedsurface, resulting in a low toxic limit. A slightlyhigher toxic limit generally occurs if treated woodis exposed to fungus with a limited energy sourceas in the European test EN 113 (British Standard6009) in which treated wood blocks are exposedto a fungal culture on malt agar. A higher toxiclimit is established where the fungus is spreadingfrom an untreated wooden block which providesa more generous energy reserve as in theAmerican and Nordic tests where the treatedblock is placed on top of an untreated feederblock. It is obviously desirable to ensuremaximum reliability for any preservative systemand these feeder block tests must be preferred asother tests may give unrealistically low toxiclimits. In fact preservative approval systems aregenerally based on prolonged stake trials in


49

natural ground contact conditions, andlaboratory tests are used only to assess thepreservative activity of individual toxicants or anewly developed formulation in comparison withan established preservative which is known toperform reliably in actual service.

Whilst referring to test methods it is worthnoting that it is established practice in manylaboratories to assess new wood preservativefungicides by dispersing them in malt agar andexposing them to fungi. This method iscompletely unrealistic as many toxicants aresubstantive on wood and may be detoxified bythis process of fixation. This affinity of sometoxicants for wood is one method of fixation butthere may be other complex chemical processesinvolved before fixation occurs and it is essentialthat preservative chemicals should always beassessed as a treatment on wood and neveralone. It has already been explained that, inaddition, the spread of a fungus from untreatedto treated wood represents the most severe riskbecause of the ability of the fungus to use energyfrom the untreated wood to detoxify thetoxicant. The initial hyphae from germinatingspores do not have access to such energy reservesand are therefore more readily controlled. Somefungicides, particularly types developed foragricultural use, are extremely efficient as sporegermination inhibitors but this does notnecessarily mean that they are equally efficient inresisting the detoxification mechanisms that areavailable to a well-established growth.

Application systems

Some wood preservative systems rely on arelatively superficial treatment applied bybrushing, spraying or dipping. Any treatmentwith limited penetration is likely to be relativelyinefficient where there is a danger of splitsdeveloping through movement resulting fromchanges in moisture content, as described inChapter 2. This danger is particularly acutewhere the treatment is exposed to rainfall, as

water is then trapped at the base of a split where,as evaporation is restricted, it can be absorbedinto the adjacent unprotected wood and cancreate ideal conditions for both sporegermination and fungal development. Even inthe absence of splits superficial treatments canprovide only limited protection against fungalattack as in many cases hyphae will be able topenetrate through the treated zone, sometimescausing decay of untreated wood beneath.

Repellants

This hyphal invasion can be prevented only bythe use of preservative systems with a directtoxic action or a repellent action; a repellentfungicide inhibits growth at a distance because itreaches the fungal hyphae by volatilization orleaching. The most efficient systems forsuperficial treatments are certainly those whichmake use of fungicides which have goodresistance to both leaching and volatilization butwhich become toxic in the presence of a fungalenzyme. Another important feature of efficientsuperficial treatments is the use of penetratingsolvents, particularly non-polar organic solvents,for the treatment of dry wood. These basicprinciples are also applicable to remedial-treatment wood preservatives.

Persistence

Organic solvent preservation systems generallyinvolve the deposition of the toxicant as thesolvent disperses by volatilization. The toxicantsthat are usually applied in this way are selectedfor their solubility in the carrier solvent systemand this generally means that they are relativelyinsoluble in water and thus resistant to losses byleaching, although they must possess somesolubility in water or in fungal enzyme systemsin order to function as fungicides. The volatilityof organic compounds depends on severalfactors but molecular weight is the mostimportant. Lower volatility is associated with


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higher molecular weight but also usually loweractivity so that it is necessary to selectcompounds possessing optimum combinations ofpermanence and activity.

In many cases a series of organic compounds isavailable with progressively changing propertiesin terms of toxicity, volatility and water solubility.For example, the chlorination of phenol typicallyresults in compounds containing from one to sixchlorine atoms. Generally the toxicity increaseswith the degree of chlorination but with aneffective optimum fungicidal activity at thepentachlorophenol. As increasing chlorinationalso reduces volatility and water solubility it willbe apparent that the pentachlorphenol is thereforethe preferred compound, achieving optimumproperties of activity and permanence. In thechlorination of naphthalene the optimumfungicidal activity occurs at only thedichloronaphthalene which is distinctly volatileand possesses limited permanence, but stomachpoison insecticidal activity is still apparent in thetetrachloronaphthalene which is much morepermanent and this compound has therefore oftenbeen used as a persistent insecticide in organic-solvent wood preservative formulations.

In all cases the surface deposits may be lostrelatively rapidly by volatilization but the rate ofloss of the total preservative deposit steadilydecreases as the deeper deposits disperse muchmore slowly. The rate of loss depends on thediffusion gradient between the deposit and thefree atmosphere, the gradient being shallowerfor deposits volatilizing at increasing depths, sothat the permanence of a volatile treatmentdepends directly on penetration.

Fixation

These comments apply to simple deposits oforganic compounds within treated wood butsome organic systems involve more complexfixation. For example, copper naphthenatetreatment suffers for an extended period fromthe characteristic odour of the appreciably

volatile naphthenic acid which is liberated byslow hydrolysis. At the same time the copperbecomes fixed within the cell walls andcompletely resistant to leaching, although it canbe made soluble and detoxified by fungalenzymes. One practical problem with a system ofthis type involves evaluation. In short-term teststhe naphthenic acid remains and contributessubstantially to the activity but in long-termservice the naphthenic acid is lost andpreservation is reliable only if there is anadequate retention of copper. This is certainlythe reason why many older copper naphthenatetreatments proved inefficient and were activelydetoxified by fungal growth; the retentions hadbeen calculated on the basis of short-termresults, influenced by the naphthenic acid residueand not on the basis of long-term service trials.

Chemical modification

The copper radical is a cation but there are manyother cations which can preserve the cellulosestructure in cell walls. Zinc is often used,particularly where a colourless preservative ispreferred in place of a green copper product.Another cation that is now widely used is tri-n-butyltin, a highly active radical which fixesstrongly to cellulose and is thus very resistant toleaching. In fact fixation is so good that somefungal hyphae can grow through the treated zone.This is not achieved through detoxification of thetri-n-butyltin but through failure of the fungalenzymes to activate the system. The treated wooddoes not decay despite the absence of toxicity andit is clear that the tri-n-butyltin has acted in acompletely different way, modifying the substrateso that it has become resistant to the fungalenzyme system. Studies of tri-n-butyltin systemssuggest that two groups are required for eachcellulose chain, and at this ratio the cellulose isresistant to decay but hyphal invasion is able tooccur. Unfortunately the lignin components in thewood remain unprotected so that some White rotfungi are still able to cause limited decay. One


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way to prevent hyphal invasion and White rotdecay is to use higher retentions of tri-n-butyltinbut another alternative is to use a preservativeformulation containing an additional toxicantsuch as a phenol which is particularly efficient inprotecting the lignin.

Throughout this discussion, detailed referencehas been made only to the tri-n-butyltin cation.In fact there are a variety of alkyl- and aryltincations available but, as is explained in Chapter4, tri-butyltin represents the optimum cation.These organotin compounds are available ashalides, acetates naphthenates, etc. but thechoice of anion is unimportant unless it is toxicin itself as all these compounds tend to hydrolyseto the oxide, so that tri-n-butyltin oxide isgenerally the most economic way to introducethis cation into a preservative formulation.

It is interesting to note that tri-n-butyltin oxidewas first considered as a wood preservative as aresult of fungicide tests in the laboratory, yet it isdetoxified in the presence of wood and performsas a wood preservative only through its ability tomodify cellulose, unless excessive loadings areused to ensure that unreacted compound ispresent. This clearly illustrates the need to includewood in any laboratory assessment of newcompounds. It will also be appreciated that woodpreservatives are not necessarily toxic and acandidate compound need not be rejected simplybecause it does not possess fungicidal activity.Acetylation has been considered as a method forstabilizing wood and it has been found that it alsofunctions as a preservative, but it is not a toxicsystem and will function reliably only on woodsthat are sufficiently permeable to be impregnatedthroughout their thickness.

Toxic precipitates

Aqueous preservative systems must possess afixation mechanism as there is otherwise a dangerof loss through leaching whenever dampconditions occur. Indeed, an aqueous preservativewhich lacks a fixation system is entirely

unsuitable for use against Basidiomycete fungiwhich represent a risk only when the conditionsare damp and when there is therefore a danger ofleaching. Aqueous preservatives can achievefixation in several different ways but many havebeen designed, at least initially, to give insolubleprecipitates within wood. Copper can besolubilized in ammonium solution andsubsequently precipitated through loss ofammonia. Sodium pentachlorophenate isprecipitated as pentachlorophenol through theneutralization of the sodium ion by absorption ofcarbon dioxide from the atmosphere. In someother multisalt formulations precipitation isachieved by a double decomposition, typically byapplying a toxic solution such as copper sulphatefollowed by a fixation solution such as sodiumchromate to give a precipitate of insoluble copperchromate. Although this is apparently a realisticfixation system it does not actually perform inthis way within wood and, whilst it is found thatsodium chromate precipitate is present, it is alsoapparent that the copper and chromium are fixedindependently to the wood elements. Indeed it hasbeen shown that the application of coppersulphate solution alone will achieve limitedfixation of copper in this way.

Two-stage treatments are commerciallyunrealistic as they involve double treatment costsso that modern multisalt preservative systemsinvolve single treatments, relying on loss of avolatile component or the influence of the woodelements to achieve the required fixation; thiswill be described in greater detail in thediscussion of the fixation of acid copper-chromium and copper-chromium-arsenicpreservatives in Chapter 4. One interestingobservation is that a copper cation appears topreserve the cellulose whilst a chromate anionappears to preserve the lignin, a system exactlyparallel to that described in connection withorganicsolvent preservatives where copper andtri-n-butyltin cations were described as cellulosepreservatives and phenol anions were describedas lignin preservatives.


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Resistant fungi

It must be emphasized that fixation systems mustachieve resistance only to losses byvolatilization, leaching and oxidation, leavingthe preservative accessible to fungal enzymesystems or alternatively modifying the substrateso that it is unaffected by the fungal enzymes. Inthe case of toxic systems the sensitivity of theBasidiomycetes varies greatly. Presumably thefungi are likely to be most sensitive to thetoxicants that are most readily solubilized bytheir enzyme systems and most resistant to thosethat they are most readily able to detoxify,perhaps by oxalate formation. For example,Coniophora puteana is resistant to manypreservative toxicants and, as it also occurswidely and causes substantial damage, it shouldalways be included in laboratory tests toevaluate preservatives. Lentinus lepideus isknown to be resistant to tar-oils whilst Lenzitestrabea is resistant to arsenic and Poria species tocopper; in initial laboratory evaluation ofpreservatives containing these toxic componentsthe appropriate resistant fungi should always beemployed.

Soft rot tolerance

The Soft rotting Ascomycetes and fungiImperfectii are generally controlled in the sameway as the Basidiomycetes but their distinctlydifferent behaviour causes some unusualdifficulties. The Soft rots were first discovered inBritain as a result of extensive investigations intothe failure of wood fill in water cooling towers.This fill consisted of softwood impregnated withWolman (FCAP) salt-type preservatives butdeterioration had still occurred under the warmsaturated conditions and the cause could not beidentified. The decay commenced on the externalsurface of the slats, producing the surfacesoftening that is characteristic of this form ofdecay. There was a complete absence ofsuperficial visible fungal growth but eventually

hyphae were identified within the cell walls. Asthe hyphae were visible only under themicroscope the organisms were at first describedas micro-fungi.

It was soon established that these Soft rottingfungi were resistant to several types ofpreservatives, particularly the fluorine types widelyused at that time. It was also observed that coolingtower fill treated with copper-chromiumpreservatives was generally free from serious Softrot attack, and the ultimate solution to the coolingtower problem was to treat all softwood fill withcopper-chromium or preferably copper-chromium-arsenic preservatives, the arsenic contentimproving preservation against the copper-resistant Poria species that are encountered in thedrier components of the towers, such as structuralsupports and mist eliminator slats.

It has since been observed that ground-lineattack in poles is often largely caused by Soft rotfungi. Whilst copper-chromium-arsenicpreservatives are reasonably efficient inpreventing Soft rots in softwoods they are not soefficient in hardwoods, a serious problem inAustralia where Eucalyptus poles are extensivelyused. Damage is particularly severe in tropicalareas and can be attributed largely to the factthat although the copper-chromium-arsenicpreservative has been distributed on allaccessible surfaces within the wood, it has notdeeply penetrated the cell wall so that the Softrot micro-fungi are able to explore within thecell wall without being affected by thepreservative treatment. This micro-distributionproblem is a matter of very serious concern andthe subject of extensive current research.

Staining fungi

The control of staining fungi presents entirelydifferent problems. These fungi invariablydevelop on the sugar and starch cell contentswithout affecting the cell walls or the structuralstrength of the wood. A preservative that fixes tothe cell wall is therefore completely ineffective


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against stain fungi which respond only topreservatives which can exert an influence overthe cell contents.

Stain in greenwood

There are two separate stain-control problems.The first problem concerns the development ofstain in freshly felled greenwood possessing ahigh moisture content. It is frequently suggestedthat stain can be avoided by rapid kiln-dryingbut the rate of moisture content reduction isoften too slow and the elevated temperaturesfrequently encourage heavy stain developmentbefore drying eventually achieves control. Ifdrying is sufficiently rapid to prevent staindevelopment the cells are killed by the elevatedtemperatures before they have been able toutilize the sugar and starch cell contents, so thatthere is then a distinct danger that stain willredevelop if the wood subsequently becomesdamp.

Some stain-control treatments rely on sporegermination control or direct toxicity to hyphalinvasion. Pentachlorophenol is one of thetoxicants that has been most widely used instain-control treatments; it has the advantage ofbeing slightly soluble in water and also slightlyvolatile so that it is able to diffuse to a limitedextent and achieve some control beyond the limitof physical penetration. This ability to exert adistant action is particularly important as staincontrol treatments are generally superficial dipor spray applications as more deeply penetratingtreatments cannot be justified as only temporaryprotection is required.

Freshly felled wood has a very high moisturecontent and it is therefore normal to apply thetreatments in the form of an aqueous solution, sothat pentachlorophenol is generally used assodium pentachlorophenate. The sodium ion israpidly neutralized by the natural acidity of thewood and carbon dioxide in the atmosphere butit still ensures an increase in the pH of thetreated wood which enhances the activity of the

pentachlorophenol. Indeed, the addition ofexcess sodium hydroxide, sodium carbonate orsodium tetraborate ensures the maintenance of ahigh pH and much greater efficacy, so that areliable treatment can be achieved at far lowerpentachlorophenol retentions. The most efficienttreatment is probably a combination of sodiumpentachlorophenate and sodium tetraborate, theborate providing efficient pH control but alsofunctioning as an additional toxicant andbroadening the spectrum of activity of thesystem.

Borate gives excellent control of many stainingfungi but unfortunately allows other fungi andparticularly surface moulds to develop if it is usedalone. The addition of a limited amount ofpentachlorophenol is sufficient to avoid theseproblems; a system containing only about 20% ofthe normal pentachlorophenol content provides asafer, less expensive and more reliable treatmentthan sodium pentachlorophenate alone.Organomercury compounds have also been usedfor stain control but they have limited persistence.

Recent developments have included the use ofdispersions of insoluble fungicides such asCaptafol and Benomyl, but these are generally farless efficient than the sodium pentachlorophenatesystems, apparently because their activity isconfined to the superficial surface, a characteristicwhich is generally unacceptable as a stain-controlsystem must prevent the development of internalstain and not simply keep the surface clean. Onefurther recent development is the addition tostain-control formulations of water repellents andother components such as polyethylene glycol inorder to reduce the development of splits throughthe treated surface and improve the control ofinternal stain.

Bark-borers

Stain-control treatments are generally applied tosawnwood immediately after conversion butthere is also a danger that stain may develop inlogs if conversion is delayed. This is a serious


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problem in tropical areas where there is also adistinct risk of bark-borers, particularly theScolytid and Platypodid Ambrosia beetles, whichcause physical damage in addition to introducingstain fungi. Logs in tropical forests are thereforeoften treated immediately after felling withformulations containing both stain controlfungicides and contact insecticides.

Stain in service

Stain may also develop in service throughcondensation beneath paint or varnish coatings.Problems with decorative systems will bedescribed in detail later but it is appropriate atthis point to consider the staining problem alone.The stain fungi are often the same mixed speciesthat are responsible for stain in freshly felledgreenwood, although the same species do notnecessarily dominate.

Aureobasidium pullulans is a particularlyimportant stain in service. It may develop in woodthrough condensation under surface coatings butit then attacks the surface coating medium,eventually causing small unsightly black pustulesto form on the coating surface. The formation ofthese pustules is associated with the developmentof a hole through the coating which allowsrainwater to penetrate and thus to accumulatebeneath the coating. This species is also able todevelop on the surface of the coating, boringthrough it into the wood beneath.

This is a relatively new problem, at least indecorative paint systems in the British Isles,apparently because it has been usual in the past forprimers and undercoats to contain lead pigmentswhich were sufficiently toxic to control these fungi.The problem is particularly acute in countries suchas Germany, where it is normal to prime woodwith dilute linseed oil before applying a coatingsustem, thus directly encouraging the developmentof Aureobasidium pullulans. Attempts have beenmade to treat wood and also to add toxicants topriming oil systems but generally these precautionshave been relatively ineffective, apparently due to

their limited persistence. It is probable thattoxicants or toxic pigments must also be added tothe paint system if comprehensive control of staindevelopment is to be achieved.

Natural control

There have been several attempts to achievenatural control of staining and wood-destroyingfungi. Some bacteria and mould fungi which arethemselves virtually harmless can inhibit thegrowth of fungi. The most efficient system ofnatural control developed so far involves theinoculation of freshly felled green sawnwoodwith the mould Trichoderma. Unfortunately thismould growth is very unsightly but it issuperficial and is removed if sawnwood issubsequently machined. At this stage it cannotbe envisaged that a naturally antagonisticcontrol system will be developed which will becommercially realistic, although structurallyharmless bacteria are already used as a meansfor increasing the permeability of sprucesapwood which is usually resistant topreservative penetration, enabling it to acceptpressure-impregnation preservative treatments.

Borer control

Preservation of wood against wood-boring insectattack is similar in principle to preservationagainst wood-decaying fungi but there are severalpractical differences. Insects are, of course, farlarger than the exploring hyphae of a fungus. Inaddition, an insect is capable of movement andthanks to this ability and its various sensoryorgans it is able to select the wood that it willattack. With a fungus the situation is quitedifferent; spores are dispersed at random and thedeveloping fungus must then attempt to infectwhatever substrate happens to be available.Despite this fundamental difference between thetwo types of attack the principle of isolation iscertainly one of the most important techniques ofpreservation in both cases. Surface coatings are


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perhaps more effective against insect attack thanagainst fungi as the development of cracks doesnot generally permit wood-borer infestation tobecome established, unless the female insect isable to lay eggs through the defect in the coatingand larval development can subsequently takeplace in the wood beneath. This is unlikely asthere is no reason to suppose that an insect canunderstand that a coating is concealing woodwhich might be an attractive site for egg-laying,and this slight danger is further reduced if animpregnation treatment is used instead of asuperficial coating.

Termite shields

Fungal spores are dispersed widely in theatmosphere and similarly wide-spread damagecan be created by flying insects, but many wood-borers are wingless. The most importantwingless group is probably the subterraneantermites which can be prevented from infestingbuilding wood by the provision of physicalshields which they are unable to negotiate; theseshields are described in Chapter 5. Termiteshields suffer from the serious disadvantage thata slight error such as subsequently installedplumbing or wiring can render them entirelyuseless. For this reason it is more usual toprevent subterranean termite attack in buildingsby soil poisoning. Although this techniqueinvolves the use of toxicants the actual system isstill a shield involving the isolation of structuralwoodwork from the source of infestation bypoisoning the soil with an insecticide possessinga repellent action.

Repellants

With fungi, a repellent action invariably involvesthe volatilization or solution of the toxicant to asufficient extent to affect growth and thusinhibit the spread of the fungus towards thetreated area, but insects have senses and are ableto move away if they dislike the conditions.

Generally, repellants affect the olfactory sensesbut they do not necessarily involve toxicity.About 200 years ago it was suggested that snuffwas such an unpleasant material that it wouldrepel any type of borer, although the results ofactual tests against marine borers showed that ithad no action whatsoever.

Repellency is not confined to the olfactorysenses. For example, some species of termite willnot explore or even build galleries across whitesurfaces, and there are many other reports ofinsects which will not lay eggs on surfaces of aparticular colour. It is also known that femaleinsects frequently taste wood before laying eggsin order to ensure that the substrate will besuitable for the emerging larvae. In the same wayinsects which bore in the adult stages may alsobe discouraged from causing damage, evenwithout the use of toxicants. Natural durabilityagainst insect attack for many tropical woodscan be attributed to their high silica content, andthis observation has resulted in the simplestmethod for protecting polyvinyl chloride (PVC)wiring against termite attack as the addition ofsilica results in greatly extended life.

The concept of using silica to prevent borerattack is not new; it was suggested as early as1862 that silica deposits should be used toimprove resistance to marine borer attack. Theidea was revived in about 1950 with littlesuccess, although this may be due to the failureto deposit the silica in the correct form. It isprobable that silica acts as an irritant orabrasive, affecting the mouth parts of the borer,but in a memorable debate at a British WoodPreserving Association Convention inCambridge in 1959 it was suggested that boroncompounds might also be non-toxic in theiraction, perhaps a ‘sharp boric acid crystal up theovipositor’ being the true explanation for theexcellent resistance to egg-laying with boratetreatments! This suggestion may appear to berather amusing but wood-borer insecticidedevelopment has generated even strangersuggestions. Persons who answered an


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advertisement publicizing a ‘completely reliablemethod for killing woodworm’ were sent a smallbox containing two blocks of wood and a sheetof instructions which told them to ‘place thewoodworm on the first block and hit it with thesecond block’!

Insect traps

A more serious suggestion involved the exposureof blocks of alder, a wood species that is verysusceptible to Common Furniture beetle attack.The intention was to distract egg-laying femalesfrom furniture or structural woodwork, burningthe blocks annually to prevent the development ofa new generation of adult insects. The destructionof the blocks was often forgotten and theirpresence actually encouraged the infestation.

The sex life of adult wood-borers has alsoattracted attention. With flying insects it ispossible to release hormone from a trap whichattracts insects of the opposite sex which can thenbe electrocuted or controlled by contact with atoxic deposit. A more efficient technique is tosterilize the attracted insects which can then bereleased to ensure sterile matings as most femalebeetles mate only once. Another serious proposalfor an insecticide is based on the observation thatkidney tubule function is stimulated by aparticular hormone so that its use may cause thedeath of the insect through dehydration, but thisis really a direct toxic insecticide.

It is obvious that wood-borers require woodbut it must be of a suitable species, an appropriatepart of the tree and in the right condition.Removal of bark will be sufficient to preventattack by borers that are dependent on bark suchas wasps, longhorn beetles, Bostrychids and otherbark-borers, such as Ernobius mollis, and rapiddrying after bark removal will also inhibitAmbrosia beetles; details of these and otherborers are given in Appendix B. The removal ofthe bark is not sufficient to prevent damage bysome borers such as the Lyctids, which generallyattack any large-pored wood containing adequate

starch. Even if sapwood is not susceptible toLyctids it is usually susceptible to other borerssuch as the Anobids but it is entirely unrealistictoday to remove all sapwood in an attempt toavoid this danger and the only alternative is theuse of toxicants to provide protection.

Toxic preservation

There is no application in wood preservation forinsecticides which must be applied topically inorder to achieve control and it must be generallyassumed that the treatment must achieve controlthrough contact with treated wood. With contactinsecticides, control may be achieved throughsimple contact as their name implies. A larvaboring into treated wood will tend to absorb theinsecticide wherever the body is in contact withtreated wood but the situation is rather differentwith an adult beetle which has a hard protectivecovering and which may only absorb theinsecticide through the mouth or claws. It is notsufficient to develop an insecticide as it may alsobe necessary to provide an access system to theinsect, perhaps involving a special oil which willenable it to be absorbed through the insect cuticleor claws, or which will encourage tasting.

In contrast, the stomach insecticides must beingested by the insects while boring so that somedamage will occur before the insect dies,particularly when an indirect-action toxicant isinvolved. Many insects do not possess enzymeswhich can convert wood to an absorbable formand rely instead on intestinal symbionts such asbacteria or yeasts. A preservative may act bycontrolling these symbionts, causing the insect todie of starvation, but it will continue to eat andmay cause significant damage before death. Manyinsects are also encouraged by or dependent onfungal attack. With some borers such as woodweevils damage can be prevented by ensuring thatfungal decay is unable to develop. Some termitesattack dry wood, removing it to their nest wherethey convert it to food in fungal gardens; woodtreated with a fungicide will eventually destroy


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the fungal gardens and control the termite colonybut significant damage may occur before controlis achieved. Even direct-action stomach poisonscan act in many different ways, perhaps affectingthe neural system or alternatively affecting cellmetabolism either by retardation or acceleration.The availability of adequate energy will permitdetoxification as with fungi, particularly in largerinsects where the available stored energy may beconsiderable relative to the absorption of toxicantfrom preserved wood, and it is essential to useadequate retentions of preservatives in order toachieve reliable control.

It is relatively easy to protect freshly fellednon-infested wood. Any treatment will tend todiscourage a female from laying eggs, but egglarvae are small with limited reserves of energyso that they are very sensitive to toxicants. It istherefore best to evaluate preservativetreatments using larger established larvae whichare likely to be more resistant.

Eradicant insecticides

The eradication of an established infestation isnot easy. It is difficult to ensure that the treatmentpenetrates sufficiently deeply to be lethal to largerlarvae. Indeed, many eradication treatments relysimply on preventing the ultimate emergence ofadult beetles which, coupled with the preventionof egg-laying, ensures that the infestation willeventually be eradicated, although the boringwithin the wood may be appreciably extendedbefore this is achieved.

One particular difficulty with insecticidalpreservatives is to achieve adequate permanence.Fixation invariably means a loss of repellent orcontact action so that insecticidal preservativeswith long life generally rely solely on astomachpoisoning action so that wood must beingested before the intestinal enzyme systemsrelease the toxicant, unfortunately allowingsome tasting damage before the borer iseventually killed. In most cases this tastingdamage is insignificant, representing a problem

only when the danger is from very large numbersof adult insects as with termite attack in tropicalareas and gribble attack marine conditions. Inmany cases the eradication of the borerinfestation is more important than preservationagainst further attack, particularly with Lyctidbeetles which attack the sapwood of a limitednumber of hardwoods but only within a shorttime after felling and conversion. Heattreatments, sometimes forming part of a kiln-drying cycle, have been widely used to achieveeradication although they give no protection andthere is a normal danger of reinfestation.

Heat and fumigation treatments have alsobeen used against Wood wasps with much,greater success; the wasps lay eggs only throughthe bark, so that reinfestation of sawnwood isimpossible. The Australian quarantineregulations were originally introduced to preventwasp larvae in imported sawnwood fromemerging, mating and infesting valuable newconifer plantations, and both heat andfumigation treatments were approved asmethods for ensuring that imported wood wasfree from infestation. While these are examplesof eradication problems encountered with newwood, it will be appreciated that remedial in-situpreservation treatment is necessary whereinfestations have become established in, forexample, buildings and boats; whilst theformulations typically used for these treatmentsare described in Chapter 4, the techniquesinvolved are highly specialized and are describedby the author in more detail in the bookRemedial Treatment of Buildings.

Natural control

Even very active infestations of wood-boringinsects have been known to die out naturallywithout the need for remedial preservationtreatment. In many cases this control can beattributed to the action of parasites andpredators—these are described more fully inAppendix B. The best known predator is


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Korynetes coeruleus, a handsome metallic-bluebeetle that is often found in association withheavy infestations of the Death Watch beetle.Amongst the parasites, mites are most frequentlyfound, particularly in Powder Post and CommonFurniture beetle infestations, both of which alsosupport minute ant-like parasites. Whilst thesepredators and parasites can eliminate infestationsnaturally, they cannot be realistically harnessed asa method of either preservation or eveneradication. The only possible method ofbiological control that has yet been devised isbased on the observation that the bacteriumBacillus thuringiensis is frequently found inassociation with termite colonies in decline, and ithas been found that similar decline can beinduced in otherwise healthy colonies when it isintroduced. Bacteria are very resistant toinhospitable conditions and it is therefore possibleto envisage a deposit of this bacterium on woodwhich would become active only through directcontact with a suitable host insect.

Dampness generally results in fungal decay inwood and this encourages the development ofinsect borer infestations; a general-purposepreservative should not be fungicidal alone butshould also be insecticidal. In the same way aninsecticidal preservative should preferablycontain fungicides to reduce the danger that afungal development will encourage unusualborer activity. There are few situations where asingle-action preservative can be clearly justified,perhaps fungicidal properties alone beingessential in wet mining conditions whilst aninsecticide alone might be justified for theprotection of furnishings. In the case of freshlyfelled green wood a fungicide is frequentlyapplied to avoid sapstain damage, although intropical conditions and insecticide is oftenadded, as the stain infection is introduced bypinhole borers which carry spores of theAmbrosia fungus which infects the walls of thegalleries and ultimately becomes the food forhatching larvae. Clearly this fungal infection isdependent on high moisture content and

protection is required only for the comparativelyshort period until the wood is too dry to supportthe staining fungi. In this type of situation astomach insecticide is generally unsuitable astasting damage can occur—before dying aninsect may penetrate through the superficialprotective treatment, allowing deep staindevelopment to occur although the surface mightappear to be entirely clean.

Marine preservation

Marine borer preservatives generally function inthe same manner as those intended to giveprotection against wood-boring insects anddecay fungi. Preservatives against crustaceanborers operate in precisely the same way as thoseagainst insect borers, functioning as repellants,contact toxicants or stomach poisons. Indeed,one feature is the excellent activity of contactinsecticides such as the organochlorine andpyrethroid compounds against crustaceanborers, presumably because the insecta andcrustacea are closely related, both being in thephylum Arthropoda.

Surprisingly little use is made of thisobservation and these contact insecticides arerarely used in commercial marine borerpreservatives. This may be due to the fact thatthese toxicants are virtually inactive against themolluscan borers such as Teredo species whichbehave in an entirely different manner. The larvaeof these borers are minute and capable of littlemovement so that they are distributed in the seain a random manner similar to fungal spores inthe atmosphere. If they settle on a suitable woodsubstrate they metamorphose and ultimatelydevelop into a borer concealed within the wood,but it is not clear whether the borer derives muchor even any nourishment from the wood which itremoves to provide a gallery for itsaccommodation. The main requirement is for apreservative treatment which will prevent themetamorphosis of the settling larva and this isoften achieved with preservatives that are


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considered to be predominantly fungicidal inaction, such as creosote. In contrast, creosotegives little protection against the crustaceanborers such as Limnoria species which are able tocause significant damage before they are affectedso that eventually the treated layer crumblesaway, perhaps exposing untreated wood beneath.

All toxic marine preservatives must becompletely resistant to leaching to givereasonable life to treated wood, although withthe exception of the contact insecticides thisnecessarily means that some tasting damagemust occur before attacking crustacean borersare affected. In practice the tasting damage iskept to a minimum by using very high loadingsof stomach insecticides which will ensure deathbefore significant damage can occur. Non-toxicpreservatives have also been considered,particularly silica deposits in wood, an idea thatoriginates from the observation that manynaturally durable woods possess high silicacontents. Trials have not been entirely successful,as mentioned earlier in connection withpreservative systems against insect borers, but itis probable that the silica must be deposited in aparticular crystalline form if the desiredprotection is to be achieved.

Decorative coatings

Although they are generally applied for aestheticreasons, decorative systems such as paint andvarnish coatings are often claimed to havepreservative value. It was explained earlier inthis chapter that there are two points ofweakness with coating systems. Minorimperfections due to discontinuous application,damage or very thin coatings on sharp edges,and inadequate paint applied on concealedsurfaces in contact with damp brickwork andmasonry, may permit limited absorption ofwater which is then trapped beneath the coatingwhere it accumulates. There is a danger thatfungal decay will eventually develop, althoughthis is perhaps less likely than preferential

wetting; wood is hydrophilic so it prefers to becovered with water rather than with a non-polaroil such as a normal coating system, so thataccumulation of water beneath a coating resultsin loss of adhesion between the coating and thewood. Failure through preferential wettingnormally becomes apparent first at joints andthen spreads with the development of opacityunder varnish systems. Failures of this typeinvariably occur and it is necessary to regularlystrip the coating system to the bare wood and toapply a completely new system in order tomaintain the appearance, a very expensivemaintenance liability.

It is frequently claimed by paint and varnishmanufacturers that these failures result from theuse of a poor paint system or carelessapplication. In Britain it is considered that apaint system should consist of an initial primercontaining excess oil which will penetrate thepores, blocking them as it dries and thus ‘killingthe suction.’ This primer is then followed by anundercoat with a very high pigment contentwhich is intended to fill irregularities in thesurface and to achieve opacity. The finish coatpossesses a high varnish or binder content inorder to give a hard durable surface. In the caseof varnish coatings only a single composition isgenerally employed but it is often thinned withsolvent for the initial priming coat. This isintended to achieve improved penetration of thepores to establish good adhesion. In fact thedilution with the solvent simply reduces theamount of varnish that is applied in the primercoat. Whilst the dilute varnish has a reducedviscosity this does not actually affect theviscosity of the varnish medium, which iscontrolled by its molecular size, so that theinactive diluent solvent tends to penetrate andleaves the varnish components on the surface, sothat there is no practical advantage withthinning. In other European countries such asGermany a priming oil is often used in place of apigmented primer. This priming oil consists of asolution of drying oil, essentially similar to the


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thinned varnish used in a British varnish coatingsystem and subject to the same criticisms.

Although it is true that good quality paintsand varnishes combined with carefulworkmanship achieve the best results, none ofthe normal systems is currently able to resistpreferential wetting. This is particularlyapparent under varnish, where it results in thedevelopment of dark stains and opacity. It isgenerally accepted that these failures areassociated with the presence of water under thecoating. Some paint manufacturers try to reducethe permeability of their coating systems tomake them very resistant to liquid waterpenetration, but diffusion of water vapour withseasonal changes of atmospheric relativehumidity is still able to occur. Unless the wood isunusually stable some swelling and shrinkage isinevitable with gain and loss of water vapour.This movement is virtually confined to the cross-sectional dimensions, the longitudinaldimensions remaining stable. Some paintmanufacturers have attempted to produce moreflexible coating systems able to withstand thiscross-grain movement but the resulting systemsare still unable to resist the shear stress whereend-grain is in contact with side-grain at a framejoint. This stressing invariably cracks paint andvarnish coating systems, allowing liquid water topenetrate by capillarity, although subsequentevaporation is prevented because most of thewood surface remains protected by the paint andthe water simply accumulates, introducing adanger of decay and preferential wetting failuredespite the conscientious application of anexcellent paint system.

Some other paint manufacturers attribute thefault to condensation beneath the paint coatingthrough the redistribution of moisture in thewood through daily thermal gradient changes.Some coatings are therefore designed to allowwater accumulations to disperse by evaporationthrough the coating system. In fact any coatingsystem that permits moisture vapour to disperseoutwards also necessarily possesses poor

resistance to seasonal changes in atmosphericrelative humidity and is usually particularlysusceptible to stress cracking through movement.

Water repellants

Despite extensive efforts over many years thesurface-coating industry has completely failed tosolve the problem of coating external woodjoinery (millwork) such as windows, doors andframes. In the United Kingdom and several othercountries there are now several recommendationsand specifications which require external joineryto be constructed from naturally durable wood orwood which is adequately preserved to avoidfungal decay, but preferential wetting failureremains a serious problem.

Water-repellent treatments have beenproposed to prevent migration of waterabsorbed into joints damaged by movementcracking of the coating system. These treatmentscertainly delay failure but they seldom representa complete solution to the problem as many ofthem, particularly those based on waxes andresins, are just as susceptible to preferentialwetting failure as the paint systems which theyare designed to protect. Indeed, many water-repellent formulations are essentially similar tothe thinned varnish or priming oil systems thathave already been described, and if theseformulations are based on drying oils they canintroduce a further problem as they mayencourage the development of stain fungi underthe paint or varnish system. Water introduced byany means, even condensation redistribution ofthe moisture within the wood, will eventuallyaccumulate beneath the coating system when theweather is cold and stain fungi can then develop,causing the darkening that is characteristic of theprogressive weathering of a varnish system.

The staining fungi are similar or identical tothose that develop on freshly felled sawnwoodbut some species, particularly Aureobasidiumpullulans, do not confine their activities to thewood but are also able to attack the coating


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medium. The inclusion of normal wood-preservative components in a water-repellenttreatment is not sufficient to give resistance tothese particular fungi which can cause thedevelopment of bore holes through the coatingsystem, eventually resulting in black dust,actually sporophores, on the surface. These holescan permit further water penetration through thecoating system, thus encouraging the ultimatedevelopment of wood-destroying Basidiomyceteattack as well as preferential wetting failure.Even if wood is treated with a toxicant that isparticularly active against these fungi and thusfree from staining under the coating, the resultmay not be entirely satisfactory as spores cansettle on the external surface and bore inwards.

In the British Isles these problems have alwaysbeen common on varnish but were virtuallyunknown in the past on paint coatings,apparently because old lead primer andundercoat systems were able to resist these fungi.Lead paints are not widely used nowadaysbecause of their toxicity but it is possible toachieve similar protection by the use of zincpigments. It will be appreciated that pigmentscannot be used in clear varnish systems butsuitable zinc compounds are available, such asresinates. Even then the result may not beentirely satisfactory as stain may develop in thewood beneath the varnish.

There is really a need for an entirely newpriming system which can perform a variety offunctions. It must be a wood preservative withactivity against both the wood-destroyingBasidiomycete fungi and also the staining fungiand superficial moulds, perhaps involving theincorporation of a single toxicant with a verywide spectrum of activity or multiple toxicants.In addition, the primer should act as a waterrepellant to give protection against rainfall orother sources of moisture before the finishingcoats are applied. It is not sufficient for thetreatment to be water-repellent in the sense thatit relies on a contact angle action alone butshould perhaps be best described as a water-

proofer as it must also block the pores. This poreblocking will reduce changes in moisture contentof the wood arising through fluctuations inatmospheric relative humidity, but pore blockingis also necessary to form a foundation on whichto apply the undercoat system. Finally, theprimer system should achieve permanentbonding to the wood substrate so that the entirecoating system is resistant to damage bypreferential wetting.

It has been suggested in the past that thesevarious functions can best be achieved byincorporating toxicants into a conventionalpigmented primer paint but this is entirelyunrealistic; the loading of a primer paint onwood is very low and it is impossible toincorporate an adequate amount of carriersolvent in order to achieve the penetration anddistribution of the preservative components. Analternative is to add the pigment and bindingcomponents in a primer to a normal organicsolvent wood preservative, but penetration ofthe toxicants can be achieved only if the systempossesses low viscosity and this necessarilyresults in difficulty in maintaining the pigmentsin suspension. Finally, neither of these systemsachieves any control of preferential wetting.

A more realistic technique is to achieve thedesired resistance to preferential wetting in apenetrating solvent system, perhaps applied bysome advantageous technique such as aconventional vacuum/pressure or double vacuumsystem, and then to add other compatiblecomponents in order to achieve the additionaldesired functions. Some long-chain alcohols havebeen suggested as a means for achievingresistance to preferential wetting but, althoughthey perform reasonably well in short-termlaboratory trials, they slowly hydrolyse andseparate from the wood and they possess nolong-term advantages over conventional waxwater repellants. Excellent results are achievedwith certain organometallic compounds,including those based upon group IV/IVb such assilicon and tin. Organotin compounds such as


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tri-n-butyltin oxide are used as preservatives butif they are required to give resistance topreferential wetting much higher loadings arenecessary and some practical problems arise suchas volatile losses which may introduce a toxichazard. Quaternary ammonium compounds canalso give good resistance to preferential wettingbut the quantity required is critical; if excesscompound is applied, dry adhesion ofsubsequent paint films is seriously affected.

Clearly the development of advantageouspriming treatments of these types must be aproprietary development but, although suchprocesses were first proposed in 1968, there havebeen no realistic commercial developments sinceand it must be assumed that the coating industryhas little interest in achieving permanent paintcoatings, perhaps because most of its marketarises through the maintenance of defectivepaintwork! It must not be supposed that thesepriming systems can function only underpigmented paint as pore blocking can be achievedusing resins, a universal system which can be usedunder both paint and varnish coatings. If thesesystems can successfully overcome preferentialwetting so that the coating system permanentlyadheres to the wood, maintenance becomessimply a matter of washing or sanding the surfaceand applying a further gloss coat to maintain theappearance. As labour costs increase throughoutthe world it becomes more important to producedurable systems than to rely on regularmaintenance to achieve reliability.

Decorative preservatives

Surface coatings such as paint and varnish arenot essential for the decorative use of wood.Bare wood can be very attractive and perfectlysatisfactory in service provided species is selectedwhich possesses natural resistance to decay andlow movement to give resistance to distortionand splitting. Alternatively non-durable speciescan be used if appropriate preservative treatmentis applied to achieve these requirements.

However, the wood will suffer from loss ofcolour, principally through leaching andoxidation of extractives and the development ofsurface-staining fungi, these factors generallycombining to give the grey shades that developwhen wood is naturally weathered. This naturalcolouration tends to be patchy related toexposure to rainfall with protected areas undereaves and sills retaining their colour (Fig. 3.1)but the patchiness can be reduced by the use of awater-repellent treatment containing a toxicantthat is effective against staining fungi, althoughit will be appreciated that this treatment must beparticularly resistant to weathering if it is toachieve a reasonable life. In fact there are fewcomponents that can achieve the desiredresistance to weathering and it is more normal touse a formulation which also contains pigmentsand binders which give the desired persistentcolouration but also leave the natural grain ofthe wood apparent. Whilst pigment and coatingbuild on the surface of the wood is extremelylow compared with conventional coatingsystems, a high degree of permanence can beachieved through the deep penetration.

FIGURE 3.1 Stain on western red cedar cladding isconfined to areas exposed to rainfall and is thus lesssevere under a window sill. (Penarth ResearchInternational Limited)


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Although these systems were originallydesigned as a means for introducing an artificialpigment system in order to maintain the naturalcolour of wood, a complete range of alternativecolours is now available and the systems havebecome generally known as architecturalfinishes. The life of these decorative treatmentsdepends largely on their resistance to preferentialwetting whilst their colour retention depends notonly on the pigments but also on the presence oftoxicants which will resist stain development.

In the physical sense, water absorption intowood occurs by capillarity. There is a tensionwithin the water surface and if the water wetsthe walls of a pore this tension will draw thewater into the pore as shown in Figs 3.2 and3.3. When water wets the surface in this way itis said to have a very small contact angle a.However if this contact angle exceeds 90° theforce tending to move the water along the porebecomes reversed and as the contact angleincreases further the surface tension tends torepel water from a pore. A water-repellenttreatment therefore consists of a coating on thewalls of the pores which results in a very largecontact angle so that the porous surface canresist water penetration provided that thepressure is not excessive. As the pores remain

open with a water-repellent treatment of thistype, trapped water, perhaps introduced bycondensation, can disperse by evaporation and itis still possible for the wood beneath thetreatment to be affected by changes in theatmospheric relative humidity but only bydiffusion along the treated pores. Thus a deepertreatment will result in slower diffusion so thatdouble vacuum or pressure impregnation willinvariably give better resistance to seasonalfluctuations than superficial brush, spray orimmersion treatments. Pore blocking will alwaysachieve better control over diffusion from theatmosphere and better resistance to seasonalchanges in atmospheric relative humidity butdispersion of trapped water will also berestricted.

Wood stabilization

A water-repellent treatment may be effective inreducing absorption of liquid water and deeppenetration and pore blocking both reduce theinfluence of fluctuations in atmospheric relativehumidity. If stabilization of the wood is required,both pore blocking and deep impregnation areessential. Resin impregnation alone can beeffective as it isolates individual wood fibresfrom atmospheric changes and physically

FIGURE 3.2 Capillary absorption resulting from thesurface tension within the fluid and the contact anglea between the fluid and the solid surface.

FIGURE 3.3 Water repellency; compared with Fig 3.2the only change is the contact angle a between thefluid and the solid surface.


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restrains movement. The effectiveness of resin-impregnation treatment is readily apparent inwooden cutlery handles which are now usuallyimpregnated to enable then to resist wettingduring washing. There is also increasing interestin the use of resin-impregnated floor blocks; oneprocess, developed in Finland as a peaceful usefor atomic energy, involves the impregnation ofthe wood with monomers which are then curedby exposure to radiation.

Waxes have also been used as impregnantsbut, while they are quite effective, they are notwidely used on their own as they tend to severelyaffect coating and glue adhesion, although theycan be used advantageously at low loadings inconjunction with resins. Polyethylene glycolwaxes are also used to stabilize wood but in arather different way. They are water soluble andgenerally applied by protracted diffusion intowet wood. When diffusion is complete thetreated wood is dried but shrinkage is preventedby baulking by the resin, so that the woodremains in its swollen dimensions. Unfortunatelythese treatments tend to be hygroscopic, giving atacky surface unsuitable for the application ofdecorative paint or varnish coatings. In fact anhumectant or hygroscopic agent alone can beused to achieve stability by ensuring that wood isunable to dry and maintaining it in its swollenstate. Whilst this type of treatment wasdeveloped many years ago it has not been widelyused because of the decoration and handlingdifficulties, but such processes are now beingseriously considered in conjunction withchemically reactive surface coatings to overcomethese disadvantages.

The wetting of wood as well as the movementwith changes in moisture content below the fibresaturation point can be largely attributed to thepresence of hydroxyl groups on the cellulosechains, as explained in Chapter 2. Stabilizationcan be achieved by replacing the hydroxylgroups with other terminal groups or by cross-linking the hydroxyl groups on adjacent cellulosechains. Various systems have been proposed such

as acetylation or the use of ethylene oxide, i-cyanates and organometal compounds but allthese systems suffer from the disadvantage thatthey are completely effective only if the wood isentirely impregnated. It will be explained later inthis chapter that complete impregnation ispossible only in a limited number of verypermeable woods such as birch and it is doubtfulat the present time whether many of thesetreatments are of greater value than the selectionof wood of naturally low movement. However ifwoods of low movement are to be used it isessential to appreciate that they will suffer fromnon-reversible shrinkage during initial seasoningand they must therefore be thoroughly driedbefore they are machined or put into service.

Fire

One disadvantage of wood is flammability. Firewill occur only when combustible material issubjected to sufficient heat in the presence ofoxygen. In the absence of any one of these threecomponents ignition cannot occur. Within abuilding it is necessary to withstand firepenetration so that an accidental fire remainsisolated for a sufficient period to permit theoccupants to escape and to give reasonable timefor the fire service to arrive and prevent furtherdamage. Fire resistance is most important wherea building is divided into relatively smallcompartments, but in large open spaces such ascorridors, stair wells and roof spaces it is moreimportant to prevent the rapid spread of flameacross surfaces. Fire resistance is best achieved ina building by using an adequate thickness ofwood in construction and avoiding any minorimperfection which will permit the fire topenetrate through a partition or fire barrier.Clearly, doors should be tight fitting but onesolution is to insert a special strip in the edges ofthe doors or the door frames which is composedof an intumescent material which swells whensubjected to fire and thus seals the gaps. The fireresistance of a structure is also improved if the


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wood is not combustible, and this appliesequally to preventing the spread of flame in largespaces in buildings.

Fire retard ants

While the provision of satisfactory fire resistanceis principally a matter of design, charring ratevaries with wood properties, such as density, andwood selection is therefore a significant factor,although charring rate cannot be greatlyinfluenced by treatments. In contrast, fire-retardant treatments can be highly effective inreducing surface spread of flame. An efficientfire retardant acts in several different ways.When the combustible surface is exposed to firethe treatment should prevent flaming and shouldpreferably reduce the rate of degrade andcharring. It should reduce ignitability andprevent after-glow, the sustained combustionwhen heat is removed. If it achieves theserequirements it must also be inexpensive, readilyapplied and permanent with no adverseproperties; it must not corrode metal fittings,interact with finish systems or possessunacceptable toxicity.

There are two principal fire-retardant systemsin extensive use at the present time. Intumescentcoatings act on exposure to high temperatures byfoaming into an insulating layer and reducingthe rate of temperature rise in the protectedwood. This foaming action, which must becompleted below wood charring temperature,generally follows a sequence consisting of filmsoftening, bubble formation and ultimate settinginto a rigid foam with good adhesion to thesurface. Ideally the bubbles should contain inertor preferably fire-retardant gases. Intumescentcoatings are really paints and can achieve adecorative function. They can also be applied asa remedial treatment to an existing structure.The use of intumescent strips to seal doors in firebarriers has already been described; these aresimilar in action to intumescent coatings but aredesigned to foam to a greater extent in order to

achieve complete sealing of relatively large gaps.Non-intumescent fire-retardant coatings are alsoavailable. In principle, they consist of heavyinsulating coatings containing componentsdesigned to inhibit flaming, perhaps by thegeneration of water vapour and other fire-retardant gases when exposed to heat.

Impregnation treatments are widely used asalternatives to these coating systems. Inprinciple, these impregnation treatments aresimilar in effect to coatings and designed toinhibit flaming as well as insulating the wood toprevent deep charring. While their ability toinhibit flaming is as good as or better than thatof coating systems, the insulating action isachieved by sacrificing the surface layers ofwood and allowing them to char to a limitedextent to provide an insulating layer. Unlike thesurface coatings which have no further effectonce the surface has been physically destroyed,deep impregnation treatments continue tocontrol the rate of charring and prevent flamingand after-glow, even after prolonged exposure tosevere fire conditions. A surface coating can beapplied virtually universally and will achieve thedesired result provided that a sufficient build isobtained, but impregnation fire retardants canfunction reliably only if adequate retention anddistribution is obtained. The reliability of suchtreatments depends as with wood preservativeson the use of a wood in which adequatepenetration can be achieved. Despite thesedisadvantages impregnation fire-retardanttreatments are widely used because structuralwood can be pretreated at very low costcompared with that of applying fire-retardantcoatings.

Impregnation fire retardants are generallyapplied by the conventional full-cell vacuum/pressure systems that are described later in thischapter. Generally, these fire-retardant systemsconsist of aqueous solutions of inorganic saltssuch as ammonium phosphates, ammoniumborates and zinc chloride. All these salts arewater soluble and treatments are therefore


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leachable if exposed to the weather. Where fireretardancy is required in external situations itwould appear that resistance to leaching can beobtained by coating with paint or varnish butmost fire-retardant treatments of this type arehygroscopic and encourage rapid failure ofcoating systems through preferential wetting.Polymer fire retardants have been developed thatgive non-hygroscopic systems which are resistantto leaching and which can reduce naturalmovement in wood by up to 40%. Whilstformulations of this type are clearly ideal wherefire-retardant treatment is required for woodthat will be exposed to the weather, they areunfortunately rather expensive and relativelydifficult to apply; a two-component system isusually involved which is reacted within thewood by heat application after impregnation.

3.2 Application techniques

Having decided on the active components thatare to be employed it is comparatively simple toprepare a preservative formulation but muchmore difficult to achieve the desired retentionand penetration within the treated wood. Themost obvious limiting factors are often ignored.There must be sufficient space within the woodto accommodate the desired volume ofpreservative formulation. This means that thewood must have adequate porosity but it mustalso have a low moisture content beforetreatment as this porous space is otherwiseoccupied by water.

Fluid penetration

A liquid is absorbed into a porous solid bycapillarity, a function of the surface tension ofthe liquid and the angle of contact between theliquid and solid surface as explained earlier inthis chapter and illustrated in Fig. 3.2. Thecapillarity force F which causes liquid flow intopores is given by the formula:

F=f p π cos α where f is the surface tension, d is the diameterof the pore and α is the angle of contact. Thepressure P developed by this force depends onthe cross-section of the pore and is given by theformula:

where r is the radius of the pore. Rate of flow Ris given by Poiseuille’s formula:

where v is the coefficient of viscosity of the fluid. Itis apparent from this formula that the rate ofpenetration depends particularly on the porediameter which is a feature of the wood species butit is also clear that a preservation formulation canachieve the most rapid penetration if the viscosityand contact angle are kept to a minimum, with thesurface tension as high as possible. It is alsoapparent form this formula that the penetrationrate depends on the ratio of the surface tension f tothe viscosity v. As the temperature increases thesurface tension decreases but the viscositydecreases to an even greater extent so that thisratio increases with temperature. For example therate of penetration of water increases by about25% over a temperature increase of 10°C (18°F).One other interpretation of this formula is that, ifthe pore diameter is halved, the treatment periodmust be increased eight times to achieve the samedepth of penetration.

Treatment time

Protracted treatment may be unacceptable foreconomic or technical reasons. For example, if apreservative has a fixation reaction it is probablethat, during a protracted treatment period, theactive components will be fixed near the surface ofthe wood and only the carrier solvent will continue

Application techniques

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to penetrate. Even where a fixation reaction isabsent there may still be a tendency for the activecomponents to be deposited at the surface throughthe preservative solution, migrating outwardsduring solvent drying. The uniform distribution ofpreservative components within treated wood isdifficult to achieve and the advantageousperformance of a proprietary preservative, basedon conventional toxicants, is frequently related tothe special solvent and fixation systems that areinvolved. One method that has been suggested forimproving penetration is the use of surfactants, orwetting agents, in order to reduce the angle ofcontact, but their use often results in a substantialreduction in the surface tension which actuallyreduces the rate of penetration. If such additionsare contemplated it is essential that they should beevaluated by simple experiments on woodenblocks. Finally, the coefficient of viscosity in thisformulation relates to the total preservativesolution but in organic solvent systems,particularly those incorporating resins, themolecular size of some components may be largerelative to the pore size so that they are unable topenetrate and tend to be filtered or screened out ofthe solution at the surface of the wood. If theseparticular components are included in order toachieve a measure of pore blocking this screeningmay be an advantage, but pore blocking will alsoobstruct the further penetration of the preservativesolution.

Treatment temperature

The viscosity of some organic preservatives such ascreosote increases very rapidly as the temperatureis reduced and heating is thus essential if areasonable penetration rate is to be achieved.However, heating the preservative may have onlylimited value as the preservative surface in contactwith the wood will tend to be cooled by the wooditself and, as it is this particular surface that isresponsible for the penetration, the temperature ofthe wood will tend to be of more importance thanthe temperature of the preservative. The wood will

be heated by immersion in very hot preservativebut this heating process is very slow due to the lowthermal conductivity of wood. In cold climatespenetration may be achieved more rapidly byslowly heating wood in storage prior to treatment,rather than by using preservative at a very hightemperature which may not achieve the same resulteven if a protracted treatment process is used.

Treatment pressure

It will be appreciated that the formula refers topenetration rate resulting from capillarity when thewood surface remains in contact with thepreservative, as in immersion treatments, butpreservative is frequently applied under pressure inorder to increase penetration rate. The applicationof pressure can have a substantial influence overpenetration rate in relatively porous woods,essentially those that are not classified as resistantto penetration, but unfortunately the application ofpressure has little influence over penetration intothe resistant species which are so difficult to treat.Reference to the formula above which gives thepressure P generated by capillarity shows that it isvery large if the pore diameter is very small so thata superimposed pressure has little significance. Ifresistant woods are to be treated, it is far better toemploy a non-pressure method with a preservativeformulation designed to possess maximum surfacetension, minimum viscosity and minimum contactangle, although with any preservative it will beappreciated that the maximum penetration ratewill be achieved if both the wood and thepreservative are heated prior to treatment.

Superficial treatments

Various superficial methods are used incommercial practice for the application of woodpreservatives. The use of brush and spraymethods is perhaps best known to the ordinaryhouseholder, both for the preservative treatmentof new wood and for remedial treatments,particularly against established borer infestations


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such as woodworm or Common Furniture beetle.Brush application is virtually useless. It hasalready been explained that efficient woodpreservatives must possess low viscosity and thisnecessarily means that only a relatively smallvolume of preservative is held within the bristlesof the brush. It is therefore very difficult toachieve a significant loading of preservative onthe surface of the wood. Indeed, in carrying outremedial treatment it is sometimes necessary toapply the preservative to the undersides of raftersand joists, and any householder who hasattempted this operation by brush will know thatthe preservative runs down the handle, treatingtheir own elbow far more efficiently than thewood surface! The superficial coatings that resultfrom brush application cannot achieve thetreatment of open joints or natural splits in thewood, and the superficial nature of the coatingensures that the preservative is lost extremelyrapidly by leaching when exposed to the weatherand by volatilization in a protected environmentwithin a building. Brush application is perfectlysatisfactory when it is necessary to apply asuperficial coating of high viscosity fluid such aspaint or varnish, but even then the loading on thewood surface is only about 25% of that whichcan be achieved by a simple airless sprayapplication.

Spray application

Penetration from a superficial treatment dependsentirely on the loading of preservative that can beachieved on the surface, and the reliabletreatment of cracks and open joints is entirelydependent on fluid flow which occurs only whenheavy loadings are achieved. The heaviest realisticloading is achieved by flood spray, the spraynozzle being moved slowly across the surface ofthe wood so that saturation is achieved with runsdown the surface but no wasteful drips (Fig. 3.4).It is often argued that about four brush coats areequivalent to a single flood spray. It is true thatabout the same volume of preservative is used but

the results are very different. Solvent is lostbetween the brush coats so that the flood effectnever develops and the treatment lacks the flowinto cracks and open joints, leaving thepreservative toxicants concentrated on the surfaceof the wood where they are particularlysusceptible to loss by leaching and volatilisation.

Different proprietary organic-solventpreservatives often contain the same toxicants buttheir efficacy may vary enormously through thedifferent distributions of these toxicants that areobtained through formulation variations.Generally, a preservative of low viscosity and lowvolatility achieves the greatest penetration butresidual odour must be avoided, particularly withremedial treatments in buildings, and it is necessaryto adopt the balance of properties that isconsidered to be the most efficient and mostacceptable. The best preservatives aim to achievemaximum penetration with the toxicant evenlydistributed throughout the penetrated zone, but inpractice there is frequently a tendency for toxicantsto migrate towards the surface as the carriersolvent disperses by evaporation. Naturally, woodporosity has a profound influence over bothpenetration and loading, but the surface texturehas special significance in the case of spraytreatments; a rough sawn surface allows a fargreater loading of a low viscosity preservative toadhere than does a smooth surface. Generally,preservatives for spray application are formulatedon non-polar organic solvents of low viscositywhich penetrate far more efficiently into dry woodthan do polar or aqueous systems.

It can be shown experimentally that aconscientiously applied flood spray is equivalentto application by 10–15 seconds immersion. It istherefore frequently argued that a spraytreatment is as efficient as a brief dip treatment,and this concept has resulted in the developmentof spray or deluging machines for thepreservation of new wood. In some cases actualsprays are abandoned in favour of a curtain ofpreservative through which the wood passes.Such processes can certainly achieve the same


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results as brief dip treatments on the side-grainof wood but generally preservative loadings atthe end-grain are far lower, and these systemsshould never be employed where thorough end-grain treatment is essential.

Spray pressures and jet sizes

These comments on spray application refer tonon-atomized systems which involve thespraying of the preservative fluid as relativelylarge droplets through a fan or cone nozzle. Innormal pressure-spray apparatus this necessarilyentails the use of low pressures and relativelylarge jet sizes. Atomization through the use of anair-entrained system such as a paint spray, orthrough the use of excessive pressures and smalljet sizes with pneumatic spraying equipment,results in the rapid loss of carrier solvent beforethe preservative reaches the wood surface,unpleasant spraying conditions, lack of

penetration and limited persistence of thepreservative. Mist coating systems are air-entrained or atomized by means of highpressures and result in only superficialtreatments; they are sometimes used for anti-stain treatment of freshly converted green woodbut they are much less reliable than simpleimmersion treatments.

Thixotropic systems

The loadings of flood spray or brief diptreatments depend particularly on the nature ofthe wood surface but also on the viscosity of thepreservative. Higher viscosities result inincreased loadings, but unfortunately, improvedpenetration may not be achieved as it tends to beinhibited by this viscosity increase. One way toimprove spray and dip preservative performanceis to formulate using a thickening agent whichwill enable high loadings of preservative to cling

FIGURE 3.4 Preservation treatment applied by spray tunnel or deluging. This equipment has been largelyreplaced by double vacuum impregnation plants. (Cuprinol Limited)


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to the surface of the wood but will not obstructthe release of low viscosity preservative solutioninto the wood, the thickening agent itselfremaining on the surface. One example is thebodied mayonnaise-type formulation which hasbeen developed particularly to achievepenetration in remedial treatment. Thisformulation consists typically of a water-in-oilemulsion in which the emulsion and watersystem are not essential but are simply a meansfor obtaining the viscous structure necessary toenable high loadings of preservative to cling tothe surface of the wood. The emulsion breaks atthe point of contact with the wood, allowing thelow viscosity organic solvent preservative tocome into contact with the wood and penetrate.Generally, an organic solvent of low volatility isemployed in order to avoid evaporation lossesduring the considerable time that can elapsebefore the preservative deposit is totallyabsorbed. There is therefore no restriction on thetime required to permit penetration, althoughmanufacturers of preservatives of this type oftenappear to be unaware of the basic mechanics ofthe system—if the preservative penetrates veryextensively the toxicants must be present at veryhigh concentrations in order to achieveretentions that will be effective.

Immersion treatments

Although these bodied systems are realistic for usein remedial treatment the same principles cannotbe readily applied to pretreatment preservatives.The essential feature of the bodied systems is thatheavy loadings of preservative are able to remainon the surface until adequate penetration hasbeen achieved, but this necessarily means thatthere will be difficulties in handling the treatedwood during the protracted penetration period. Incommercial immersion treatments, it is usuallyconsidered more realistic to use simple low-viscosity preservative solutions which willpenetrate as quickly as possible, extending theimmersion period to achieve the desired

penetration. Immersion for a period up to about10 minutes is usually described as dipping,whereas longer immersion treatments are knownas steeping. Sometimes one hour or more isneeded to achieve the necessary penetration,although a low-viscosity preservative may achievein a few minutes or hours the same depth ofpenetration as a high viscosity preservativeapplied for several days. The advantages ofimmersion treatments are numerous; they aresimple, involve low capital coast, and achieveexcellent preservative penetration and loadingprovided that adequate time is available.Immersion treatments are unsuitable for use withrapid-fixing preservatives such as the copper-chromium-arsenic water-borne preservatives andare most suitable for applying low viscosityorganic-solvent systems to dry wood.

Diffusion

Diffusion treatment relies on an immersion orspray treatment to load the surface of the woodwith a preservative that will subsequently diffuseslowly, to achieve the desired distribution. Thebest-known diffusion treatment, Timborising,involves the use of a highly soluble borate whichis usually applied as a hot solution in order toachieve the required concentration. Freshly sawngreen wood with a moisture content in excess of50% is immersed briefly in the preservative andthen close-stacked and wrapped to prevent lossof water by evaporation. A storage period ofseveral weeks or even months for pieces of thicksection is required to distribute the boratethroughout the wood. This treatment achievesbetter penetration in very impermeableEuropean whitewood or spruce than any otherpreservative treatment, apparently because theradial penetration pathways are still open ingreen wood, whereas they are closed in the drywood that is typically used for pressureimpregnation treatments. Fluoride preservativesare also sometimes applied by diffusion, andsome formulations containing bifluorides are


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said to be suitable for diffusion treatment intodry wood. In fact only the hydrogen fluoride gasdiffuses deeply in dry conditions but, whilst itcan be readily detected by a reagent when a pieceof treated wood is freshly cut, it is progressivelylost by volatilization and also by leaching if thewood is exposed to wet conditions.

Osmose process

In the Osmose diffusion process, originallydeveloped for the non-pressure treatment oftransmission poles, freshly cut logs are peeled,brushed with a preservative paste and thencovered with waterproof paper before beingstacked for about three months to allow thesalt to diffuse. A variety of preservatives isnow used with the Osmose method but theoriginal Osmolit paste was a mixture of sodiumfluoride, dinitrophenol and chromates, the latterbeing included to improve fixation. If thereis an insect borer danger, arsenic is frequentlyincorporated in preservatives that are appliedby this method. Very similar diffusion principlesare involved in the bandages that can be appliedat the ground line, to transmission poles inservice, in order to improve protection in thiszone where the decay hazard is most severe(Fig. 3.5). Bandages are often based on fluoride

salt pastes which are designed to diffuse into thepole when wet. Tar-oil formulations are alsowidely used, although they are able to diffuseonly when the pole is dry or pretreated with anorganic preservative such as creosote.

Boucherie process

In the Boucherie process (Fig. 3.6), preservative isintroduced into the sap and is required to diffuse inorder to treat the neighbouring zones which are notdirectly accessible. A cap is attached to the butt of alog immediately after felling and the preservative isintroduced under low pressure from a header tank;in this way the sap is displaced and replaced by thepreservative solution. Copper sulphate wasemployed when the process was originallydeveloped by Boucherie but it does not fix withinthe wood and has now been entirely replaced bymultisalt preservatives. Some of the preservativesthat are so widely used in vacuum/pressureimpregnation, such as the copper-chromium-arsenic(CCA) formulations, fix too rapidly to be applied bythis or other diffusion methods, and the more slowlyfixing copper-chromium-boron (CCB) and fluorine-chromium-arsenic-phenol (FCAP) formulations aremore suitable. In some modern versions of theBoucherie process the design of the caps has beenconsiderably improved by the incorporation ofinflated cuffs which permit much higher pressures tobe employed, giving more rapid treatment andbetter control.

Gewecke or Saug-Kappe process

In the Gewecke or Saug-Kappe process a conicalcap is fitted on the top end of the log and a

FIGURE 3.5 Pole bandage. The Wolmanit TS impreg-nated bandage is protected by a weatherproof cover.(Dr Wolman GmbH)

FIGURE 3.6 Improved Boucherie cap with separatelyinflated cuff to seal the pole butt.


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vacuum is applied to remove the sap and inducepreservative flow from a reservoir at the buttend. Although this process was developed manyyears earlier and improved in 1940, it was notuntil 1950 that Gewecke introduced it intoGermany and several years later into Denmark.Normally, the logs are peeled and then fittedwith the suction cap at the upper end beforebeing placed in an open tank of preservative.The original Boucherie process would treat a 12m (40 ft) log in 10–14 days but the Gewecheprocess reduced this time to 1 week and in theimproved Gewecke process, with the opentreatment tank replaced by a pressure cylinder,the treatment time is reduced to only 20 or 30hours. In these later processes it is normal torecirculate the sap in the treatment solution,adding preservative at intervals to maintain thenecessary concentration, a system thatconsiderably reduces wastage. Originally, theGewecke process was applied using Basilit UA orpreferably Basilit UAS, a more soluble version,although the process is now applied using manyother preservatives including the CCAformulation K33.

Hot-and-cold process

In developing countries there is clearly a need forwood preservation in order to improve theeconomic conditions by ensuring the mostefficient utilization of wood and labourresources, but it is equally important to ensurethat the process is simple with a low capital cost,even if this results in treatments that are lessreliable than those that are considered necessaryin more highly developed countries. Dip and sapdisplacement treatments are currently ofparticular interest in developing countries but analternative is the hot-and-cold treatment process.In its simplest form this involves the immersionof wood in cold preservative followed by slowheating which expands the trapped air. When thebubbles of expanding air have ceased thepreservative is allowed to cool, thus causing the

remaining trapped air to contract so that thepreservative is drawn into the wood. Aqueouspreservatives can be applied in this way but ifhigh-boiling organic preservatives are used it ispossible to heat the wood beyond the boilingpoint of water, eventually filling the wood withwater vapour. Cooling then results incondensation and the development of an almostcomplete vacuum which ensures excellentpenetration, provided the moisture content ofthe wood was not too high at thecommencement of the treatment process.

This simple form of the hot-and-cold processis widely used by farmers for the butt treatmentof fence posts; these are placed in an open drumcontaining creosote and a fire is lit underneathfor heating. If the drum is too full and the firetoo hot there is a danger of considerable foamingwhen the air expands, or even boiling of trappedwater, perhaps causing the creosote to overflowand the fire to burn out of control. A moresophisticated version of the process involves theuse of separate hot and cold storage tanks for thepreservative, with transfer pumps to convey it toan immersion tank, although it is more commonto have two separate immersion tanks,transferring the wood between them. In all casesthe principle remains the same; air is expandedduring immersion of the wood in hotpreservative and subsequent immersion in coldpreservative causes the air to contract, drawingthe preservative into the wood.

Gugel process

In Germany the hot-and-cold method oftreatment is known as the Gugel process. Insome systems the preservative is replaced in thehot stage by a high-boiling oil such as wastelubricating oil but as there is no appreciablepenetration during the hot stage this does notsignificantly affect the resulting treatment. InAustralia the hot stage sometimes consists ofprolonged steaming and is followed by transferto a tank of cold preservative, a process that has


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been fairly widely used with 3% borax solutionto give protection of hardwood against Lyctuspowder post beetle attack.

Pressure and vacuum treatments

In all treatment methods involving the use ofpressure of vacuum it is necessary to place thewood in a pressure vessel, usually known as acylinder or autoclave (Fig. 3.7). The wood isloaded through a door at one end, usually onrailway bogies but sometimes on wheeled baskets,or occasionally loose. In large plants with bogies,it is essential that the wood should be chaineddown to prevent floating when the cylinder isflooded with preservative as it is unrealistic topack the cylinder sufficiently tightly for the woodto be restrained. The impregnation process will inany case cause swelling which may cause the loadto jam in the cylinder and it is also necessary to

leave a small space at the top of the cylinder topermit recovered air to accumulate. Originallythe cylinder doors were fitted with bolted flangesbut these were very slow to operate and manyquick-release door designs are now availablewhich can appreciably improve plant utilization.Loaded bogies are moved into the emptycylinder across a railway bridge which is thenremoved to permit the door to be closed. Oneadvantage of a bogie system is that the cylindercan be emptied and refilled in a very short periodusing extra sets of bogies which can be loadedwhilst another charge of wood is being treatedwithin the cylinder (Fig. 3.8).

Although it is normal practice to usetreatment cycles which will achieve a reasonablydry surface when the charge is removed from thecylinder, some dripping may still occur and it isusual to allow the charge to stand over a drip-collection sump before the bogies are unloaded.

FIGURE 3.7 Typical pressure impregnation plant with bogies for the charge, quick-locking doors, pump houseand storage tanks for two different preservatives. (Hickson’s Timber Products Limited)


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With water-borne preservatives it is necessary toair- or kiln-dry wood after treatment in order toavoid problems with handling wet wood; whilstthe toxicity of these preservatives is importantthe corrosion of tools and fittings is often a moreserious problem.

Rails and bogies waste considerable space atthe bottom of a treatment cylinder, perhaps asmuch as 25% of the volume, and the curvedvertical arms which support the load may wasteas much again so that the maximum load may beonly 50% of the total cylinder volume. This

means that excessive preservative is required tofill these waste spaces during treatment and it isalso necessary to use a greater area of steel witha greater thickness to withstand the pressure. Inaddition, greater energy is needed to move theunnecessarily large volume of preservative andexcessive energy is also required to achieveappropriate pressures and vacuums.

One alternative, which is used in a Swedishdesign known as the 5-T plant (Fig. 3.9), is touse baskets constructed from perforated steel,closely conforming to the internal dimensions of

FIGURE 3.8 Layout of a conventional pressure impregnation plant showing the cylinder fitted with quick-locking doors and rails for the feed bogies, the storage tank, preservative mixing tank and pipework.(Hickson’s Timber Products Limited)


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the cylinder and fitted with small ball-bearingrollers running on the inside of the cylinder. Inthis way the maximum use is made of theavailable cylinder space. Unfortunately, therigidity of the baskets is critical and it isunrealistic to use this design for cylinderdiameters in excess of about 65 cm (2 ft 2 in), yeta cylinder of this diameter using baskets gives acapacity equivalent to that of a normal 1 m (3 ft3 in) diameter cylinder using rail-loading bogies.Baskets are pulled from the cylinder onto atrough for loading, although generally thetrough is mounted on a trolley running on rails

across the end of the cylinder, so that severalextra baskets are available for loading orunloading whilst a charge is in the cylinder. If itis required to increase the capacity it is moreeconomic to install extra cylinders, served fromthe same system of rails and troughs, than toinstall a new large diameter cylinder (Fig. 3.10).

In order to simplify installation the 5-T plantsare completely self-contained, consisting of acylinder mounted on top of a storage tank fittedwith the necessary pumps. A treatmentinstallation consisting of two, three or even moreof these small cylinders is much less expensive to

FIGURE 3.9 A small plant, the 5-T, in which the rail bogies are replaced by baskets to increase cylindercapacity. Baskets are hauled out onto trolleys; a loaded basket can be placed in the cylinder whilst a secondbasket is being unloaded and reloaded. A trolley system can feed several cylinders. (Anticimexbolagen andCementone-Beaver Limited)


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purchase, install and operate than a largeconventional cylinder capable of a similarthroughout but there is a limit on the size of thepieces of wood that can be economicallyaccommodated in these relatively small cylinders.

Pressure and vacuum units

Wood impregnation in cylinders can be achievedusing a variety of treatment cycles but beforediscussing these in detail it is necessary toconsider the units of pressure and vacuum whichare used to describe them. Firstly, it must beremembered that the atmosphere is at a pressureof 1 atmosphere (atm). Drawing a vacuum is anattempt to decrease this pressure to 0 atm. Onemethod to describe both pressure and vacuum isto consider that a complete vacuum has zeroabsolute pressure so that the atmosphere is at anabsolute pressure of 1 atm, and any extra

pressure applied on top of atmospheric pressureis additional. Thus the application of 5 atm willresult in an absolute pressure of 6 atm, whilst thedrawing of a complete vacuum will result in anabsolute pressure of 0 atm. This book is intendedto be practical and, while it is necessary tointerpret some of the more complex treatmentcycles in terms of absolute pressure, it is far moreconvenient to consider the actual plantrequirements so that cycles will be quoted interms of the pressure in atm that must be appliedand the efficiency of the vacuum as a percentagethat must be drawn. Whilst some perfectionistswill object to the use of atm as the pressure unitand percentage as the vacuum unit it must beclearly understood that these are, in fact the onlyuniversal units that are widely understood byscientists, technologists and plant operators.

Atmospheric pressure is sometimes describedas 1 bar (b), a unit of pressure that gives rise to

FIGURE 3.10 A Gorivac plant with a rectangular treatment vessel. Large rectangular loads can be treated suchas packaged wood and completed joinery items, but only vacuum and relatively low pressures can be used.(Gorivaerk A/S)


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the familiar millibar (mb) used bymeteorologists. Atmospheric pressure is alsofrequently derived directly from the height of amercury barometer and described as 760 mm Hgor 30 in Hg. In the metric system pressure isexpressed in terms of dynes (dyn) or Newtons(N) per unit area, and for all practical purposesit can be asumed that 1 atm is equivalent to 100kN/m2 or 1 000 000 dyn/cm2. Whilst the currentmetric standards demand that we should useunits involving Newtons, they are still notwidely understood and traditional units are stillin use at many commercial plants; thus 1 atmbecomes, for practical purposes, 1 kg/cm2 or 15lb/in2. The torr has also been fairly widelyadopted as a unit of low pressure, particularlyvacuum expressed on the absolute scale. A torr is1 mm Hg, so that complete vacuum is 0 torrwhilst atmospheric pressure is 760 torr. In viewof the maze of units that are used at present toexpress pressure and vacuum, the need toconfine our descriptions to very simple units, theatmosphere for pressure and the percentage forvacuum, becomes clearly apparent.

Full-cell impregnation

In a full-cell process the aim is to achieve thecomplete impregnation of the porous spaceswithin the wood in the hope that a proportion ofthe preservative will penetrate the surroundingcell walls or that they will at least be protected bythe very high loadings of preservative aroundthem. In the empty-cell process the initialimpregnation treatment is basically similar butthis is followed by a recovery process designed toempty the porous spaces whilst leaving anadequate coating of preservative on the cell walls.

Bethell process

In the traditional full-cell process a sequence ofvacuum and pressure is employed to achievecomplete impregnation of all the porous spaceswithin the wood. This impregnation process is

currently known as the Bethell method, althoughit was actually first developed by Breant, andBethell was responsible only for its adaptation tocreosote treatments. In the normal commercialprocess the wood is introduced into the cylinderand vacuum drawn of 90% or more, the timevarying from 15 minutes to several hoursdepending upon the permeability and cross-section of the wood involved. This vacuum, whichremoves most of the air from the porous spaceswithin the wood, is maintained whilst the cylinderis flooded with preservative; water-bornepreservatives are generally used at ambienttemperatures and warmed only to preventfreezing, crystallization or sludging in coldclimates but creosote is usually applied at 60–80°C (140–176F) to reduce the viscosity andimprove penetration. When the cylinder is full thevacuum is released and the preservative starts toflow into the porous spaces in the wood under theinfluence of atmospheric pressure (Fig. 3.11).

In order to encourage penetration a pressureis then aplied, typically 7–14 atm, and maintainedfor as long as is necessary to achieve the desiredpenetration and retention, typically 1–5 hoursbut occasionally several days, dependingon permeability and cross-section. Sometimes

FIGURE 3.11 Bethell full-cell cycle (F=flood; D=drain).


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treatment is specified ‘to refusal’, indicating thatthe pressure must be be maintained until gaugesfitted to the plant indicate that there is nofurther absorption. With some species of wood,particularly Eucalypts in Australia, much higherpressures are used, but not for softwoods as theymay suffer physical damage known as collapseor washboarding if excessive pressures areapplied. With some very permeable species ofwood the atmospheric pressure on release of thevacuum is sufficient to ensure the necessarypenetration or only a relatively low pressure of 1or 2 atm is necessary; a process involving avacuum without a superimposed pressure stageis described as a vacuum process whilst oneinvolving a superimposed pressure of less than 5atm is a low pressure process. After thenecessary period the pressure is released and thepreservative is removed from the treatmentcylinder. Typically, a final vacuum is then drawnin an attempt to remove excess preservative andavoid subsequent bleeding in service.

In theory, this final vacuum is intended toencourage the expansion of any residual trappedair within the wood, forcing excess preservative tothe surface where it can drip clear, but in practicethe process often leads to excessive surface depositsof high viscosity preservatives such as creosote. Amore important function of the final vacuum isperhaps to relieve the compressed state of thewood, thus allowing any excess preservative to beproperly absorbed. Whatever the true mechanism,avoidance of bleeding can be achieved withcreosote only if heating is maintained throughoutthe treatment process so that the viscosity of thepreservative remains relatively low.

With creosote treatment the nett retention,defined as the loading of preservative thatremains after completion of the entire cycle,varies from 80 to 250 kg/m3 (5–15.6 lb/ft3) insoft-woods, depending on the species, cross-section and the proportion of resistantheartwood and permeable sapwood. In the caseof a water-borne salt preservative the nettretention depends on the concentration of the

preservative in the solution, but typically 4 to 28kg/m3 (0.25 to 1.75 lb/ft3) is achieved, dependingon the preservative involved and the purpose forwhich the treatment is intended. The Bethell full-cell process is normally used for the applicationof water-borne preservatives and also forcreosote where exceptionally high nett retentionsare required in wood for use in extreme hazardsituations such as for marine piles. Full-cellimpregnation without the use of a superimposedpressure is also normally used in the laboratoryfor the impregnation of standard test blocks forbiocidal evaluation.

Empty-cell impregnation

In empty-cell processes, wood is impregnated withpreservative under high pressure on top of airtrapped within the wood. This trapped air is laterpermitted to expand, ejecting preservative from theporous spaces but leaving the cell wallsimpregnated or coated with preservative. Withempty-cell processes it is far easier to achievetreatments that are free from bleeding in service,but empty-cell can be used only when the necessaryretentions can be achieved despite the recovery ofpreservative from the spaces within the wood.

Rüping process

There are two empty-cell processes in common use,both originally designed for use with creosote. Theearliest empty-cell process was developed byWassermann but it is usually named after Rüpingwho first developed the process commercially.After the cylinder has been loaded and sealed an airpressure is applied, usually 1.7 to 4.0 atm for aperiod of 10 to 60 minutes depending on thepermeability and sizes of the pieces of wood in thecharge. The cylinder is then flooded withpreservative, usually creosote, without releasingthe pressure which is then increased up to perhaps14 atm, about 10 atm above the original airpressure, and this pressure is maintained until therequired gross absorption of preservative is


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obtained as indicated by the plant gauges. Thepressure is then released and the preservativeremoved from the cylinder, permitting the airtrapped within the wood to expand and ejectpreservative from the porous spaces (Fig. 3.12).

In practice a vacuum of about 60% is drawnduring this stage to encourage expansion of thetrapped air and to ensure that, despite therelatively high viscosity of the preservative, thereare no pockets of trapped air at a pressure inexcess of atmospheric. If the pressure is notreleased in this way there is a danger that theremaining pressurized air will cause continuingbleeding of preservative at the surface of thewood, but the final vacuum will reduce thepressure of trapped air to below atmospheric, sothat excess preservative will move inwards whenthe vacuum is released, giving a particularlyclean surface. This final vacuum was notincorporated in the original Rüping process butit is now always used—whatever the empty-cellprocess it is essential to ensure that any trappedair is under vacuum at the completion of theprocess in order to avoid subsequent bleeding.

The required gross absorption during thepressure stage is generally defined for individual

species of wood, taking account of theirpermeabilities so that a gross absorptionrequirement is really a means to ensure adequatepenetration. When the pressure is released andthe vacuum recovery period completed, asubstantial proportion of the preservative willhave been removed from the open porous spaceswithin the wood so that the net retention ofpreservative may be as low as 40% of theretention from a full-cell process whilstachieving almost as good penetration. Forexample, in transmission poles penetration isessential, but in most temperate areas a full-cellprocess is unnecessary with creosote as it willachieve an unnecessarily high retention.Typically, a retention of perhaps 250kg/m3

(15.6lb/ft3) will be achieved with a full-cellprocess, but with a Rüping empty-cell processthe penetration will be virtually the same butwith a retention of only about 110 kg/m3 (6.87lb/ft3). Preservative usage is thus substantiallyreduced but this nett retention is still adequate toprevent the fungal degradation at the ground linethat represents the principal hazard, and theempty-cell process can also achieve freedomfrom surface bleeding. However, it must beappreciated that good penetration coupled withhigh recovery and low nett retention can beachieved only with preservatives of relativelylow viscosity and this necessarily means thatcreosote can be used only at high temperatures.In addition, creosote will not satisfactorily coator penetrate the cell walls if the wood has amoisture content in excess of about 20%.

The Rüping and other empty-cell processesare generally employed for creosote treatments,although they can also be used with water-bornepreservatives possessing slow fixation reactions,particularly those that fix only when acomponent is lost such as the ammonia-basedpreservatives, which fix as a result of the pHchange that occurs when the ammoniavolatilizes. Empty-cell processes are alsoparticularly suitable for the application of lowviscosity organic-solvent preservatives, achieving

FIGURE 3.12 Rüping empty-cell cycle (F=flood; D=drain).


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excellent distribution combined with limitedconsumption of preservative, although withthese low viscosity systems it is unnecessary touse high pressures to achieve the requiredpenetration; a description will be given later of adouble vacuum process which is a normalempty-cell process operating with very lowpressure differentials.

While Rüping is the most widely used empty-cell process, particularly for the treatment oftransmission poles with creosote in Europe, thereare a number of other empty-cell processes ofimportance. The double Rüping process wasused on the German railways from about 1909,involving a normal Rüping cycle except thatduring the impregnation stage a short period ofpressure was followed by a vacuum withoutemptying the cylinder, followed by a return topressure and the completion of a normal Rüpingcycle. The advantages of this modified processare not clear. The additional vacuum wouldappear to reduce the effect of the initial pressure,perhaps thus improving penetration comparedwith a normal Rüping cycle but also increasingthe nett retention. The process would also seemto have an unnecessarily high energy demand,arising from the application of an initially highair pressure which is later effectively reduced bythe application of a vacuum involving theexpenditure of further energy. In theory itwould seem to be more sensible to reduce theinitial pressure alone, but this is effectively theLowry empty-cell process that was developed inthe United States as an alternative to theRüping process.

Lowry process

In the Lowry process (Fig. 3.13) there is noinitial air pressure and the preservative istherefore impregnated on top of air at normalatmospheric pressure. A more intense finalvacuum is desirable, perhaps as high as 90%,in order to achieve the maximum recovery butin this respect the Lowry process is never as efficient

as the Rüping process; the final nett retention istypically about 60% of the gross absorptioncompared with as low as 40% with the Rüpingprocess using a low viscosity preservative. Lowrytreatment results in less bleeding than theRüping process because any air trapped at theend of the treatment cycle is at a lower pressure.In addition, a Lowry treatment plant is lesselaborate than a Rüping plant as there is no needfor a separate air pressure pump. This was at onetime considered to be an important economicfactor but it is less significant today as many airvacuum pumps can also function as pressurepumps, so that an initial air pressure can beachieved simply at the cost of additional pipe-work and valves.

Nordheim process

The Nordheim process was an adaptation of theLowry process which attempted to achievefurther operating economies. During theimpregnation stage the pressure was raised tobetween 2 and 7atm and the cylinder valves werethen sealed, avoiding the necessity forcontinuous pumping to maintain the pressure.

In fact the pressure reduced steadily as thepreservative penetrated into the wood or

FIGURE 3.13 Lowry empty-cell cycle (F=flood; D=drain).


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through leaks in the plant, giving erratic resultsso that the process was eventually abandoned.

Energy considerations

It is unfortunate that impregnation processes areoften developed by wood technologists withchemical or biological training who usuallyignore energy considerations when designingtreatment cycles or preparing plant performancespecifications. During the impregnation stage itis important to pressurize using a preservativefeed pump as this ensures that the level ismaintained while the preservative is beingabsorbed into the wood. A pressure pump of thistype need have only low capacity in view of theslow rate of penetration of preservative intowood, and the relative ease with which a fluidcan be pressurized in a short period due to itsnon-compressibility. However, a high-pressurepump of low capacity is quite unsuitable fortransferring preservative between the storagetank and pressure cylinder. The pressure pumpcan be increased in capacity but this alsoincreases the power consumption whilstmaintaining the pressure and it may be moreeconomic to provide a second high-capacitypump to achieve rapid fluid transfer.

During the impregnation stage it is essentialthat the cylinder should be filled withpreservative without any air being left at the topas the compression of this trapped air willabsorb considerable energy, delaying thepressurization of the cylinder and increasing thecost of operation without achieving anyadvantage. Pressurizing air or drawing a vacuumin air also requires considerable energy and it istherefore essential to ensure that the cylinder isloaded with the maximum charge that can beaccommodated, in order to ensure minimum airspace. One possibility is to flood withpreservative before drawing a vacuum but,whilst the vacuum can certainly be achievedmore quickly, capillary forces between thepreservative and the wood prevent the full effect

of the vacuum from being transferred to spaceswithin the wood; a 90% vacuum above thepreservative may represent a 60% vacuum orless within the wood. In addition, simplehydrostatic forces are significant in a largecylinder so that the effective vacuum within thewood is considerably reduced at the bottom ofthe cylinder where wood is subjected to thehydrostatic pressure arising through the depthbelow the preservative surface. Clearly, anintense vacuum cannot be achieved within woodwhilst a cylinder is flooded with preservative.

Many water-borne preservatives are corrosivebut in some commercial and pilot plants directcontact between preservative and pumps isavoided by employing only air pumps. Usuallyboth the treatment cylinder and the storage tankare pressurized so that fluid transfer is achievedby applying vacuum or pressure. A single airpump can be used in this way for all operationsbut the compressibility of air ensures that theplant can be pressurized only rather slowly andat the expense of considerable energy. Whilstthese pumping operations represent perhaps themajor operating cost and an area whereconsiderable economies can be achieved byintelligent plant operation, heat energy is equallyimportant in all plants where preservativeheating is necessary. Lagging is an obviousprecaution but must clearly be extended to allthe surfaces of the cylinder, storage tanks andpipework. One interesting concept is to fit thecylinder and storage tanks with water jacketswhich have a high thermal capacity and thus actas a reservoir of heat energy, enabling off-peakelectricity, for example, to be used. Theimportant factor is the temperature of thepreservative at the zone where it is penetratinginto the wood so that heated preservative canachieve very little effect if the wood is cold.Current practice is to extend the treatment cycleand allow sufficient time for the woodtemperature to increase but a more efficientmethod is to retain wood for a day or two in aheated store before treatment.


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High and low pressure

Several variations of these basic treatment cyclesare used commercially. The Australian Eucalyptsare very impermeable and are often treated at veryhigh pressures to achieve the required penetration;in fact increasing pressure is rather less effectivethan increasing the treatment time. In contrast.South African pine Pinus patula is extremelypermeable and only very low pressures arenecessary to achieve complete penetration in ashort treatment time.

Double vacuum process

In double vacuum treatments an initial vacuum isfollowed by impregnation under atmosphericpressure with a final vacuum to achieve a degreeof recovery and reduce bleeding. Theimpregnation pressure depends, as in all otherprocesses, on the difference between the pressureapplied to the preservative during theimpregnation stage and the initial air pressureimmediately prior to this stage. In this cycle theinitial air pressure depends on the intensity of theinitial vacuum but the degree of recovery dependson the extent to which the final vacuum exceedsthe intial vacuum, or the degree of expansion oftrapped air that can be achieved when the finalvacuum is applied. Double vacuum is identical toany other empty-cell process but the impregnationpressure is very low and the process is suitableonly for the application of low-viscosityorganicsolvent preservatives, in situations whereonly limited penetration is necessary (Fig. 3.14).

Double vacuum is now extensively used,particularly in Europe, for the treatment ofexternal joinery (millwork) such as window anddoor frames but it is reliable only if the treatedwood is reasonably permeable. It is most oftenused for the treatment of European redwood orScots pine Pinus sylvestris as the sapwood of thiscan be readily treated and the heartwoodpossesses reasonable natural durability. Whilstredwood is most widely used for external joineryin Europe, whitewood, principally spruce Picea

abies, is preferred in many cases as it is nowmore readily available through extensiveplantings. This whitewood possesses very lowpermeability and only limited penetration can beachieved by double vacuum treatment, even withpreservatives of very low viscosity. Modifiedtreatment cycles with a superimposedimpregnation pressure of 1 or 2 atm are oftenused, a process that is almost identical to the lowpressure process that has been used for manyyears in South Africa, but this higher pressuredoes not significantly improve the penetrationrate and longer impregnation times are muchmore effective.

Toxicant concentrations

These double vacuum and low pressure processesfor external joinery are generally operated undera variety of proprietary names, the first onesbeing Gorivac in Denmark and Vac-Vac in theNetherlands and the United Kingdom. It isfrequently claimed that they provide more reliabletreatment of external joinery than the immersionprocesses which they have largely replaced, butthis comment requires some explanation. It is truethat short dip will achieve only limited

FIGURE 3.14 Double vacuum cycle (F=flood; D=drain).


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penetration but prolonged immersion can achieveboth deep penetration and high preservativeretentions. However, the required treatment timemay be too long and the high retentions may beuneconomic. The advantage of an empty-cellprocess is that it can achieve the requiredpenetration at reduced retentions incomparatively short treatment times, thusachieving both reliability and economy. However,it must be clearly appreciated that this does notmean that greater reliability is achieved with aparticular preservative if it is applied by a doublevacuum empty-cell process, in place of aprolonged immersion treatment which is virtuallyfull-cell, as it is clearly necessary to take accountof the nett retentions of preservative within thetreated zone in order to calculate the nettretention of the active toxicants that will beachieved. Thus, toxicant concentration informulated preservatives must be higher whenapplied by empty-cell processes than whenapplied by full-cell processes in order to achievethe required toxicant retentions, a factor that isoften entirely ignored, and the advantage of anempty-cell process when using a formulatedpreservative is that it actually achieves economiesin the use of carrier solvent without affectingtoxicant retentions.

Solvent recovery

Whenever organic-solvent preservatives are used,the cost of the non-functional carrier solventtends to increase the cost of treatment. Therehave been many attempts to develop processeswhich can recover the carrier solvent for re-use. Inthis sense the various empty-cell processes arelargely ignored, although they are certainly themost effective and the most economic means toachieve significant solvent recoveries, and theyare the only processes which can achieverecoveries of the relatively non-volatile high-flashpetroleum solvents that are so widely used.

For example, it may be required to treat somesawnwood (lumber) with pentachlorophenol at a

retention of 6.4 kg/m3 (0.4 lb/ft3). A pressureimpregnation process on the wood concernedcan achieve a gross absorption of about 200 kg/m3 (12.5 lb/ft3) and as this is effectively the nettretention when a full-cell treatment is employedit is evident that a 3.2% pentachlorophenolsolution is required. The nett retention ofpentachlorophenol will be 6.4 kg/m3 (0.4 lb/ft3)as required but the nett retention of solvent willbe 193.6 kg/m3 (12.1 lb/ft3). Alternatively, anempty-cell process can be employed whichachieves a recovery of about 50% with aformulation of reasonably low viscosity, so thatthe nett retention of preservative will be only100 kg/m3 (6.25 lb/ft3) and the formulationconcentration must be increased to 6.4%pentachlorophenol in order to achieve therequired retention of 6.4 kg/m3 (0.4 lb/ft3), butwith this process the wood contains only 93.6kg/m3 (5.58 lb/ft3) of solvent.

The use of an empty-cell process can thereforesubstantially reduce solvent consumptionwithout affecting nett retentions of toxicant,provided the formulation concentrations takeaccount of the degree of recovery that can beachieved, yet the normal Rüping, Lowry anddouble vacuum processes are seldom consideredas means for achieving substantial solventrecoveries. Instead, the emphasis is onalternative processes that utilize relativelyvolatile solvents which can be employed in theirliquid form for a conventional impregnationtreatment and later recovered as a gas.

Cellon or Drilon process

In the Cellon process, known in Europe as theDrilon process, the toxicants are applied in aliquified petroleum gas (LPG), normally butane,by conventional full-cell impregnation. Ifpentachlorophenol is used a concentration of 2to 4% is necessary, depending on the nettretention required, together with auxiliary co-solvents because of the limited solubility ofpentachlorophenol in butane. These co-solvents


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are either low-boiling such as i-propyl ether sothat they can be recovered with the LPG, orhigh-boiling such as polyalkylene glycols andretained within the wood. The persistent high-boiling solvents are preferred as they ensurebetter distribution of the pentachlorophenolwithin the treated wood and improve resistanceto losses through leaching or volatilization.

A vacuum is applied initially to the loadedcylinder and this is followed by purging with aninert gas to remove oxygen. A normal full-cellimpregnation cycle follows consisting of avacuum, flooding with the preservative solutionand the application of an impregnationpressure, usually achieved in this particularprocess by heating the solution with steam coilsin order to generate a pressure of 7 to 10 atm.The solution possesses very low viscosity,particularly when heated in this way, andpenetration is very rapid. When the requiredgross absorption has been achieved thepreservative solution is removed from thecylinder and a vacuum is drawn in order tovolatilize any solvent remaining within thewood. Unfortunately this stage of the process isadiabatic, meaning that the wood is cooledduring the initial evaporation stages, reducingthe volatility of the remaining solvent. Whilethe generation of pressure, by the temperatureincrease method, significantly improves therecovery rate it is still necessary to apply avacuum for a period of 1 to 3 hours beforegiving a final purge with inert gas and openingthe cylinder.

The energy required to maintain the vacuumduring the recovery period is expensive and theadiabatic restriction represents a severelimitation which cannot be overcome because ofthe good thermal insulation properties of thetreated wood and the surrounding vacuumconditions. The process seems unlikely to be veryeconomic when these disadvantages are coupledwith the time and cost involved in the purgingprocess and the high cost of insuring a pressureplant operating on such low-flash solvents.

Dow process

In the Dow process the butane carrier solvent isreplaced by non-flammable methylene chloridewhich avoids the need for an inert gas purgeand considerably reduces insurance costs. Inaddition, the use of methylene chloride enablesthe vacuum recovery process to be replaced byrecovery in steam so that the adiabaticlimitations of the Cellon process are largelyavoided. The steam is subsequently condensed,separating the methylene chloride for re-use.However, it is not clear that any significanteconomic improvement is achieved with thisprocess as the heat energy requirement forsteam generation is clearly large. The principaladvantage of the Dow process therefore lies inthe use of a non-flammable solvent.

Pressure-stroke process

It is clear that these various impregnationprocesses all have their advantages anddisadvantages, and continuous attempts arebeing made to develop more efficient and moreeconomic processes. Before attempting todevelop a new process it is advisable to considerthe numerous systems that have been devised inthe past and which are now used to only alimited extent or are completely forgotten. Thepressure-stroke method is a development of theBethell full-cell impregnation method in whichthe cylinder is first flooded with preservative.When the preservative appears in the overflowpipe the top valve of the cylinder is closed butthe pump continues to operate, rapidlyincreasing the pressure. The system is designedwith a complex change-over valve so that, afteronly 3–4 seconds of this pressure stroke, avacuum can be applied which, within 3 minutes,reaches the vapour pressure of the preservativesolution, rapidly expanding air trapped withinthe wood. After about 15 minutes, loss of airfrom the wood ceases and a pressure is appliedin the usual way (Fig. 3.15).


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This technique has been largely developed inconjunction with a particular proprietary plant,the 5-T manufactured in Sweden, and the rapidchanges between vacuum and pressure areachieved by using a single pump which runscontinuously and which generates the vacuumwhen required by diverting the flow through aventuri device. The method also relies on the factthat the cylinder remains flooded withpreservative at all stages; this avoids the heavyenergy demands which occur when producing apressure or vacuum when air is present, althoughdrawing a vacuum in a flooded cylinder reducesthe vacuum intensity within the wood aspreviously explained.

Pressure and penetration

The pressure-stroke method was developedfollowing observations on the rate of flow ofpreservative into softwoods. If the impregnationpressure is steadily increased the rate of flowincreases in proportion until a critical pressure isreached when there is a sudden increase inresistance to flow. It is sometimes suggested thatthis resistance develops when the increasing flowof preservative through a bordered pit achieves arate at which the torus is displaced, effectively

closing the pits. It is also suggested that thiscritical situation develops at far lower pressures inspruce than in pine, thus explaining why onlylimited penetration can be achieved throughpressure impregnation of spruce. Whatever thetrue explanation it is clear that pressure increasesare likely to be of limited benefit and thatprolonging the impregnation time is likely to be amore reliable way to achieve penetration intoimpermeable woods. However, it has also beenobserved that if a very high pressure is suddenlyapplied, a very high flow occurs initially, followedby a decrease to the normal expected flow,presumably as the torus becomes displaced.

Oscillating pressure process

The pressure-stroke method is designed totake advantage of this flow characteristic bygenerating pressures very rapidly and byintroducing two pressure stages. The oscillatingpressure method (OPM) further develops thisidea. The cylinder is flooded with a preservativeand then subjected to a pressure of about7.5 atm alternating with a vacuum of about95%. Usually the cycles increase progressivelyin length from about 1 to 7 minutes. Thenumber of cycles varies with the wood,permeable woods usually requiring about 40cycles and impermeable wood treated in largecross-sections up to 400 cycles. The oscillatingpressure method was developed in Swedenand introduced commercially by Boliden inabout 1950. In the original version of theprocess the loading of the cylinder was followedby pre-steaming at a pressure of about oneatmosphere, the duration depending on thecross-sectional dimensions of the wood. Thispre-steaming was introduced to make the woodmore flexible and more amenable to theoscillating pressure method of treatment. Anormal oscillating pressure cycle was employed,the total treatment time being 1 hour forevery (25 mm) (1 in) of thickness or radius.The process was used originally with S25 salt

FIGURE 3.15 Pressure-stroke cycle (F=flood; D=drain).


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preservative and later with K33. One adaptationof the process is the alternating pressure method(APM), in which the vacuum is omitted and thepressure fluctuates between atmospheric and 10–13 atm (Fig. 3.16).

Poulain process

In transmission poles the most serious decayhazard arises through the action of fungi at theground line. Any preservation process must takethis hazard into account, despite the fact thatdecay risk is much less on the freely ventilatedportion of the pole above the ground. In thePoulain process the poles are first treated to giveadequate durability to this exposed portion andthen a second treatment is applied to giveadditional protection to the butt. With thisprocess as originally developed, the poles arefirst impregnated with a light creosote using theRüping method then, after the cylinder has beendrained, it is rotated into a vertical position andfilled with a heavy oil to a depth of about 2 m(6.5 ft) and the air above pressurized to forcethis oil into the butts. The Poulain method hasbeen used in the Netherlands but with the initialRüping treatment with creosote replaced by salttreatment, originally by Kyanising but later

using Wolman salts—two preservative systemsthat are described in the next chapter.

Another system, developed by Kuntz inHungary before World War II, involvedimpregnating the poles with a low retention ofcreosote of lignite tar-oil, then, rotating thecylinder so that the poles were vertical with thebutts upwards. The cylinder was next floodedwith cold heavy creosote, leaving the buttsprotruding for about 2 m (6.5 ft). A hot, lightcreosote oil was then introduced on top of thecold oil and a pressure applied; this caused thelight oil to penetrate rapidly into the butts butthe heavy oil to penetrate only slowly into theremaining part of the pole, typically giving apenetration of 80 kg/m3 (5 lb/ft3) for the butt butonly 40 kg/m3 (2.5 lb/ft3) for the rest of the pole.

Boulton process

One problem is the limited penetration that canbe achieved when wood is wet as, even withwater-borne preservatives, there must besufficient space within the wood to accommodatethe required absorption of preservative solutionand this means that preservative should never beapplied when wood has a moisture content inexcess of the fibre saturation point of about 30%.With creosote and other preservatives that arelargely immiscible with water, a much lowermoisture content is desirable to ensurepenetration of the cell wall, although in practice amaximum moisture content of about 25% isusually specified. If the moisture content is higherand kiln-seasoning is unrealistic, as withtransmission poles to be treated during the wintermonths, it is possible to remove water during thetreatment process. Creosote is generally heated toreduce its viscosity and, in the Boulton process,this hot creosote is used to boil off the water.Generally, the creosote is heated to about 60°C(140°F) and a vacuum applied to induce boiling.When foaming ceases, pressure is applied as in anormal Bethell full-cell process. When Boultonoriginally introduced the process, boiling under

FIGURE 3.16 Oscillating pressure cycle (F=flood;D=drain).


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vacuum was used to avoid the necessity forheating the creosote above 100°C (248T); somecreosotes at that time had very high phenolcontents which were appreciably volatile at thattemperature, particularly in steam, and it was alsofeared that high temperatures would damage thewood. Damage does not occur and sometreatment plant operators are now using 120°C(248°F), boiling off water during a normalRüping or Lowry empty-cell process without theneed for an additional vacuum stage.

The use of a very hot creosote also helps toreduce bleeding as the viscosity of the preservativeis low, achieving both good penetration andrecovery, although it must be appreciated thatbleeding can be avoided completely only if thecycle is designed to ensure that any trapped airhas a pressure below atmospheric at the end ofthe process so that any movement of preservativewill be inwards rather than outwards. In view ofthe volatile losses that can occur from creosoteduring Boultonising a modified technique wasdevised by Rutgerswerke in which the cylinder isfirst flooded with a separate hot oil for the water-removal stage, if necessary with the application ofa vacuum to induce boiling. This heating oil isthen removed and creosote impregnated using anormal Rüping cycle.

Preservative bleeding

The normal Bethell, Rüping and Lowryimpregnation processes have been used for manyyears with considerable success, provided thatappropriate precautions are taken to ensure thatthe moisture content of wood is sufficiently lowand that the wood to be treated is sufficientlypermeable. In recent years the most seriousproblem has been bleeding, largely because of thecomplete lack of appreciation that trapped airmust have a pressure of less than atmospheric atthe completion of an empty-cell treatment cycle.

The most serious bleeding is associated withthe Rüping process which involves an initial airpressure followed by impregnation with

preservative. In the original process the pressurein the trapped air was relied on to eject excesspreservative from the wood but this expulsionwould continue, particularly with creosote ofrelatively high viscosity, for a considerableperiod after the wood was removed from thetreatment cylinder. In many yards, poles werestored for 6 to 12 months to permit the creosoteto drip free so that the poles were relatively cleanin service. In the Lowry process, in which thepreservative is impregnated on top of air atatmospheric pressure, the bleeding has alwaysbeen less as preservative recovery is achievedthrough the application of a final vacuum. Theaddition of this final vacuum to the Rüpingprocess considerably reduces but does notcompletely prevent bleeding, unless the plant isoperating at very high temperatures so that thecreosote has a low viscosity and the trapped airpressure can be totally relieved during thevacuum period. In theory, the best technique toavoid bleeding is to use a Lowry process with anextended intense final vacuum or alternatively amodified process involving a limited initialvacuum, perhaps 5 to 10%, with a very intensefinal vacuum to achieve the required recovery.

These methods to prevent bleeding can besuccessful only if the preservative possesses lowviscosity and with creosote this necessarily meansheating. This heating is wasted if the wood is coldor wet as the creosote will be cooled at the criticalzone where it is advancing into the wood. Perhapsin the future treatment yards will introducestorage sheds in which wood can be warmed forseveral days before treatment.

A method to reduce bleeding has recentlybeen introduced in Italy. A normal Rüpingprocess is used to apply creosote but an extendedperiod at atmospheric pressure is introduced,after completion of the pressure impregnationstage and before the preservative is removedfrom the cylinder. This allows the trapped air toexpand and a considerable degree of recovery tobe achieved without the necessity formaintaining expensive pumping, the final


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vacuum after draining more efficiently relievingthe pressure of the air trapped within the wood.However, this process can significantly improverecovery and resistance to bleeding only if asubstantial time is allowed for the atmosphericpressure stage. Whilst it is true that expensivevacuum pumping is reduced in this way, theextra time involved may considerably decreaseplant utilization so that perhaps only twocharges can be treated per day instead of three.

Pressure cycle stages

All vacuum and pressure impregnation processesare basically similar and involve five essentialstages. The first stage is to load the cylinder andthe second is to adjust the pressure of the airtrapped within the wood by applying a vacuumor pressure as appropriate. The third stage is toflood the cylinder with preservative and applythe impregnation pressure. The fourth stageinvolves draining the cylinder and adjusting thepressure of the air within the wood, usually bythe application of a vacuum. The and final stageis to return the treated wood to atmosphericpressure then remove it from the cylinder.

Penetration depends on the difference betweenthe initial air pressure and the impregnationpressure, stages two and three respectively, so thatit can be improved by reducing the initial airpressure or by increasing the impregnationpressure. The degree of recovery depends on therelationship between the initial and the final airpressures, stages two and four respectively, and thetendency to bleed depends on how these pressuresrelate to atmospheric pressure. In all casespenetration into relatively impermeable woods canbe achieved more readily by increasingimpregnation time rather than impregnationpressure; pressures in excess of about 12.5 atm arelikely to cause collapse in many woods. In all casesthe gross absorption depends on the porosity of thewood and the penetration that is achieved, and thenett retention is the gross absorption after recoveryhas occurred in empty-cell processes. In all cases

wood must be prepared before treatment. The needto ensure a low moisture content has already beenemphasized; with round poles and piles it is alsonecessary to remove the bark and phloem, andperhaps to peel the pole to ensure a uniform shape.All woodworking should be carried out beforetreatment.

Impermeable woods

A considerable amount of thought and effort hasbeen devoted to improving the penetration ofpreservatives into impermeable woods. Althoughit is clear from theory and practical experiencethat penetration is not usually improvedsignificantly by the application of high pressuresto woods of small pore size, it is a fact thatincreased pressure remains the favoured methodfor improving penetration. Pressures in excess ofabout 12.5 atm are likely to lead to collapse insoftwoods but much higher pressures of 50 to 70atm have been used in Australia for the treatmentof relatively impermeable Eucalypt species.Krüzner suggested in 1906 that penetration couldbe improved by steaming wood for 3 hours at apressure of 10 atm, equivalent to a temperature ofabout 180°C (356°F). Whilst an improvement inpenetration could be achieved in this way it wasfar short of his expectations. Two years laterChateau and Merklen adapted the process,relying on high pressure steaming to heat thewood so that a subsequent vacuum stage resultedin the boiling off of trapped water. This process isstill used in North America to reduce the moisturecontent of wood for treatment.

The Taylor Colquitt process was firstproposed in 1865 as a means for reducingmoisture content by vapour drying but it wasBesemfelder who patented the process in 1910.The process relied on passing solvent vapourover the wood—certain water-immisciblesolvents such as trichloroethylene and benzeneperforming best as they were able to absorbwater vapour which was later readily removedby condensation. The process was fully


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developed in 1940 and the first plant becameoperational in 1945. In the Kresapin processintroduced in Austria in 1951, full-cellimpregnation with Neckal or Saprocresol wasfollowed by drying in the open. The treatmentswere apparently designed to prevent furtherwater absorption whilst still permittingevaporation to occur; the Kresapin process wasfollowed by normal impregnation with creosote.

Estrade process

The Estrade process, introduced in France andSwitzerland for the treatment of spruce involvespre-treatment in the cylinder with hot air. Thisprocess induces rapid drying of the external layersof the wood so that substantial splits develop,which encourage deep penetration ofpreservative. Whilst this system permits thepreservative to gain access to the tangentialpenetration pathways, which are significantlymore permeable than the normal radial pathways,the splitting can be a considerable disadvantagethrough the loss of strength that occurs,particularly in wood suffering from spiral grain.

Incising

Incising can achieve the same penetrationadvantages whilst actually relieving stresseswithin the wood and significantly reducing thetendency for splits to develop. Incising isachieved by drilling or forcing needles into thewood, or cutting slits with knives or circularsaws; needles are favoured in Germany butknives are generally used in North America andthe British Isles. Where slits are cut with knives itis obvious that the best penetration can beachieved by cutting across the grain, to giveaccess to the longitudinal pathways which aremost permeable, but this would result insubstantial and unacceptable loss of strength sothat longitudinal slits, to give access to thetangential pathways, are most realistic. Theseslits do not need to be continuous as the

tangential pathways give access in turn to thevery permeable longitudinal pathways.

Whilst incising was first introduced as ameans to improve penetration into round poles,it was adapted in England for sawn spruce usingsmall knives to give a relatively close pattern ofslits. When used for this purpose the knives arearranged to penetrate to a depth of about 6 mm(1/4 in) instead of the 15 mm (5/8 in) or morethat is normal for the treatment of poles forsevere decay hazard conditions. However, itmust be appreciated that incising enablestreatment to penetrate only to the depth of theincisions. Incising is realistic for Douglas fir,Pseudotsuga menziesii, where the treatment ofthe sapwood alone is required as the heartwoodis naturally durable, but the system is lessefficient with spruces such as Picea abies andPicea sitcbensis which possess non-durableheartwood. On the other hand, spruce polesoften fail through the development of checkspenetrating through the treated zone but incisingrelieves these stresses. Indeed, in woods that areparticularly susceptible to checking, a verticalsaw kerf is sometimes cut before treatment inorder to reduce this danger. One alternativemethod to reduce checking, in the case ofcreosote treatments, is to use a cycle that gives adegree of bleeding which will ensure that thesurface is coated, and thus sealed, against thewetting and drying conditions that induce thatmovement within the wood that causes checking.

High energy jets

Prior to the development of incising, even highpressure treatments with water-bornepreservatives were unable to effect significantpenetration into European whitewood orspruce—a species of increasing importance to theEuropean construction industry in view of thedeclining availability of European redwood orpine. Only the borate or fluoride diffusiontreatments of freshly felled green wood were ableto achieve consistent deep penetration but their


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usefulness is limited as both these treatments arelost in leaching conditions. Needles, drills andknives are not the only means for incising woodas high-energy liquid jets can be used to achievesimilar incising patterns. In fact, the preservativecan be used as the cutting fluid, achieving incisingand preservation in a single operation. Thissystem has been evaluated in the United States butenergy costs were found to be very high,apparently because an attempt was made toachieve normal gross absorptions ofpreservatives. It is much more realistic to achievelow gross absorptions of concentratedpreservatives but this is acceptable only withpreservative formulations that can subsequentlydiffuse to achieve reasonable distribution. Thedepth of the incision depends on the pressure andalso on the volume of preservative to be appliedto a unit area of wood, so that energy savings canbe achieved if the volume can be reduced by theuse of a concentrated preservative.

Cobra process

The Cobra process was developed principally as aremedial treatment for transmission poles at theground line, in order to control incipient decay andextend their service life. The soil is dug awayaround the pole and a hollow pin is driven in to adepth of about 3 cm (1¼ in). Usually a reservoir isthen attached and 2–3 g of preservative solution orpaste is forced through the needle. Generally theWolman type of salt is employed as this is able todiffuse throughout the pole. The process has alsobeen used for the treatment of railway sleepers inservice and is sometimes used for the treatment ofnew wood in special circumstances, such as fortreatment with bifluoride to give protection againstHouse Longhorn beetle. The HS-Presser is asimilar system for remedial treatment in buildings,particularly in roof structures. A hole of about 8mm (5/16 in) diameter and 220 mm (83A in) long isbored into the wood at an angle of about 30°. Ahollow needle is then inserted and fitted with acylinder containing about 0.7–1.5 litres (1.2–2.6

pints) of preservative solution such as 4% Wolmansalt or 10% Osmol WB4. The preservative isusually absorbed in a few days, even in relativelyimpermeable wood. The Springer-Presser is similarbut usually uses 10% Wolmanit or Hydrasilpacked in a container pressurized at about 20 atmso that penetration is achieved more rapidly.

Injectors

These injection processes have been simplified bythe introduction, originally in France, of smallplastic injector nozzles. These are hammeredinto a previously drilled hole, leaving only anipple on the surface of the wood. Preservative isthen injected at high pressure and the gunremoved, a ball valve maintaining the pressurewithin the wood. If necessary, furtherpreservative can be applied later. This system isideal for the remedial treatment of externaljoinery (millwork) such as window and doorframes, the preservative spreading for aconsiderable distance around the injection pointbut particularly along the grain. When treatmentis complete the nipple can be removed and thesmall hole stopped to conceal the injection point.

Ponding and water spraying

There have been many attempts to developsystems which will enable preservatives toachieve significant penetration into spruce butfew have been realistic. One system that deservesparticular mention is ponding; if spruce isfloated in water for a period of several weeks thepermeability of the sapwood is substantiallyincreased, but it must be appreciated that thewood must then be dried before it can be treated.Considerable success has also been achieved,principally in the Irish Republic, withwaterspray treatments which are more readilycontrolled and with the deliberate introductionof bacteria which apparently achieve theincreased permeability without affecting thestructural strength of the wood. The most


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serious criticism of this process, at least withregard to spruce, concerns the fact thatpenetration cannot be achieved in this way intospruce heartwood which is non-durable. On theother hand, in a transmission pole, the strengthis only marginally affected by the decay of theheartwood so that this is probably the mostrealistic method that has been introduced so farfor the utilization of spruce in place of pine.

Plywood and particle-board

Wood-based products such as plywood andchipboard must also be treated with preservativesif they are not naturally durable and if they are tobe exposed to a deterioration hazard. Plywood canbe treated by normal preservation methods in thefinished sheet but the glue-lines tend to obstructpenetration. A water-borne treatment increases themoisture content and if the wood veneers possessmedium or high movement the individual veneerswill swell across the grain, causing enormous stressso that the veneers may separate from the glue-line.The alternative is to treat the veneers beforeassembly. The simplest method is to spray themwith a water-borne preservative as they leave thepeeler. Borate preservatives such as Timbor areable to diffuse rapidly into the wet veneer and evenrapid fixing copper-chromium-arsenic salts can beapplied successfully in this way to veneers oflimited thickness.

A further alternative is to incorporate thepreservative in the glue-line, a system that isparticularly suitable for the application ofcertain organic insecticides such as Lindane,Dieldrin and Heptachlor (Chlordane) which areappreciably volatile and can become uniformlydistributed during the hot pressing stage. Theseinsecticidal glue-line treatments are perhaps themost suitable means for treating plywood so thatit conforms with the Australian quarantineregulations but slightly volatile fungicides suchas pentachlorophenol can also be applied in thisway. Boric acid is not normally considered to bevolatile but it becomes volatile in the presence of

the steam generated during the hot pressing andit is therefore a very suitable preservative for thisprocess, particularly as it also acts as anaccelerator for some adhesives.

The incorporation of preservative componentsin the adhesive is virtually the only realisticmethod for treating chipboard. Fibre-board ismore difficult to treat as an adhesive is notnormally used (see Wood in Construction by thepresent author, page 77). When a fibre-boardplant operates as a closed system so that thebackwater recirculates, it is possible to dose thebackwater with preservative, adjusting the dosageuntil the required retention is achieved in the finalfibre-board product. This procedure is unrealisticwith open systems where the backwater isdischarged but plants of this type are rare nowthat there are more severe controls onenvironmental pollution. Alternatively, apreservative solution can be sprayed either on thewet pulp immediately after straining to form theboard or on the completed board, a procedurethat is more suitable if the chosen toxiciants areappreciably volatile and likely to be lost duringhigh temperature processing. The high-densityfibre-boards or hardboards are often ‘tempered’by treatment with a drying oil. This temperedboard is the only fibre-board product whichshould be exposed to dampness in service andthus the only product that really requiresfungicidal treatment which can be readilyachieved by addition to the tempering oil.

Remedial treatments

This description of wood preservation applicationtechniques would be incomplete without areference to remedial treatment, or the applicationof preservatives to eradicate established borerattack or fungal decay. Remedial treatment shouldcommence when the damage is observed orsuspected. A considerable amount of knowledgeand experience of both structures and the wood-destroying insects and fungi is essential to reliablyinspect structures and diagnose wood deterioration


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problems. For example, both termites and HouseLonghorn beetles can cause very considerabledamage before there is any external evidence oftheir activity, and the Dry rot fungus can spreadthrough plaster, brickwork, masonry and concretein its search for nourishment and more distantpieces of wood within the structure. The inspectionmust detect the extent of the damage or,alternatively, suggest the areas which should beopened up to permit a more detailed inspection.

The first task for the treatment operatives isto expose the full extent of the damage in orderto decide whether the affected componentsshould be replaced with adequately preservedwood or whether treatment to eradicate thefungal infection or insect attack will besufficient. This exposure work is perhaps themost important aspect of conscientious remedialtreatment as it largely ensures that concealeddamage is detected. Preservative treatment thenfollows. Fungal decay can generally be attributedto a fault in the design, construction ormaintenance of the structure which permitswood to become wet, and this fault must becorrected as part of the treatment so that furtherexpertise related to damp-proofing processesmay be needed. It is not sufficient to treat onlythe wood that is visibly affected; adjacent woodmay already be infected by a fungus and, in thecase of an insect infestation, an attack in onewood component will clearly indicate thehazards faced by all others in the structure.

Remedial treatment must be both eradicantand preservative so that the preservativeformulations are often pre-treatmentformulations with additional eradicantcomponents. Spray treatment is normally used inconjunction with organic-solvent formulations,which are both more penetrating than water-borne types and also free from the staining thatoften occurs when, for example, a roof treatmentaccidentally soaks into a plaster ceiling.Occasionally holes are drilled into beams andother large wood components to permit deeptreatment by pressure injection; large beams in

ancient buildings are often in contact with dampmasonry at either end and therefore suffer frominternal decay through the absorption of waterby the permeable end-grain. In some cases simpleconical nozzles are used fitted to the spray gunbut, when pressure on the gun is released, thereis a tendency for the preservative to flow out ofthe injection hole. A more efficient methodinvolves the fitting of an injector system asdescribed earlier in this chapter. Where damagehas been caused by wood-borers such as DeathWatch beetle it is unnecessary to drill the woodas an injection route has already been providedby the borer galleries. It is unnecessary to injectall flight holes as it will be found that injectioninto one hole will result in flow from severalothers. Brickwork and masonry infected by theDry rot fungus are also drilled to permit propersterilization, usually with an aqueousformulation as the presence of fungus in the wallindicates dampness which might obstruct thespread of an organic-solvent preservative.

Whilst the stripping of all damaged woodcoupled with spray application of preservativesis the most reliable method for remedialtreatment, there are less sophisticated methodsused in many parts of the world. In Britain, theuse of contact insecticide smokes has beenrecommended as a method for eradication ofinsect borers such as Death Watch beetle. In fact,smokes do not penetrate into the wood but leavea deposit largely on the upper horizontalsurfaces which may kill emerging adult beetlesand thus prevent subsequent egg laying.Unfortunately the contact insecticides that areused such as Lindane, Dieldrin and DDT havelittle persistence when finely dispersed on thesurface of wood in this way and these treatmentsmust be repeated at annual intervals for perhaps8 to 12 years to permit all larvae within thewood to pupate, emerge as adult insects and bekilled by the insecticidal deposit. Even prolongedtreatment can fail to control an attack of DeathWatch beetle in the interior of large beams; thisinsect is always associated with fungal decay and

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beams can be hollowed out, as previouslydescribed, if their ends are built into dampmasonry. Toxic gases, particularly methylbromide, are also used to eradicate insectinfestations but they have distinct disadvantages.The structure must be enclosed within animpervious sheet during treatment and mustsubsequently be well ventilated to remove thetoxic gas. In addition, the gas can kill onlyinsects that are within the wood and thetreatment has no preservative action to preventsubsequent reinfestation.

Remedial treatment is extremely complex anda separate science or art. It requires considerableexperience in the identification of deteriorationhazards, not only those that are of significanteconomic importance as in wood preservation butalso the comparatively rare deteriorationproblems, described in detail in Appendices B andC, that naturally cause considerable concernwhen they occur. Knowledge of structures is alsoessential so that the causes of fungal decay can beclearly appreciated but other remedial treatmentsmay be needed. Remedial wood and relatedtreatments are considered in detail in RemedialTreatment of Buildings by the same author.

3.3 Evaluating preservative systems

Service records

The only completely reliable method forevaluating a preservative system is to observe itsperformance in actual service. Service recordsare therefore very valuable in confirming thereliability of an established preservation systembut alternative techniques are required torealistically evaluate a new preservative systemduring its development and before itscommercial introduction. All acceptableevaluation techniques attempt to reproduceconditions which have been observed in practiceto represent severe deterioration risks. There isthus a danger that these evaluation systems canbe too exacting, imposing unnecessarily severe

performance requirements on preservativeswhich may have been designed for use in far lessonerous conditions in actual service. Forexample, a preservative for the carcassing orframing wood of buildings may be required togive protection against fungal decay, resultingfrom occasional leaks or condensation, orperhaps give protection only against wood-boring insects, and such a preservative does notneed to be assessed for performance in severeground-contact conditions.

Performance classification

Preservatives are classified in this book into fourbasic groups, shown in detail in Table A.2 inAppendix A, in order to take account of thesevarious hazards. This classification system is notstandard but is similar in many respects to theNordic system, and the required retentionsquoted in Appendix A can be readily related tosimilar classification systems operatingthroughout the world. Class A refers to wood innormal ground-contact conditions such astransmission poles, fence posts, railway sleepers(ties), piles and structural foundations. It canalso be considered to refer to wood immersed infresh water as in river defence works and evencooling towers, although in the latter case thereis an increased risk of Soft rot attack, indicatedin the Danish system by a sub-classificationClass AS. Such a subdivision is unnecessary as itis now recognized that even ground-contactconditions introduce a risk of Soft rot damage.Class B refers to building and construction woodwhich is not in ground contact but which is stillsubject to a moderate risk of decay throughaccidental leaks or condensation. In manyrespects the risk is the same as for Class A exceptthat significant leaching conditions are notpresent. This class includes building carcassingand framing, as well as joinery (millwork) andcladding. Class M refers to preserved wood formarine conditions but applies only when there isa risk of attack by marine borers, particularly


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gribble Limnoria species. Class I applies onlywhen there is a risk of insect attack, particularlyby House Longhorn beetle, Hylotrupes bajulus,in temperate areas and Dry Wood termites in thetropics. Preservatives meeting this classificationgenerally conform with the Australianquarantine requirements which are designed toprevent the introduction of new wood-borers toAustralia. These four classes thus define the mostimportant deterioration risks that preservativesmust be capable of withstanding. There areseveral other service situations which need to beconsidered such as stain, Pinhole borer andPowder Post beetle control treatments that areused in the forests and mills, but these do notinvolve standard evaluation techniques.

A Class A preservative in normally assessedthroughout the world by the performance of thepreservative in actual ground contact in staketrials (Fig. 3.17). Generally, the stakes are

comparatively small in cross-section, typicallyabout 50×50 mm (2×2 in), in order to exaggeratenatural leaching and the deterioration damage. Itis usually considered that the performance of apreservative can be judged reasonably reliablyafter a period of about five years. Obviously thetime factor ensures that this system cannot beused during the development of a preservative.

Laboratory tests

Preservative development normally involvesexposure of relatively small blocks of wood tocultures of single fungi in laboratory conditionswith decay assessed by the weight loss after aperiod of perhaps 12 to 16 weeks. This principleis used throughout the world, the tests varyingonly in the medium on which the fungus iscultured; in the British, Dutch and Germansystems the fungus is cultured on malt agar

FIGURE 3.17 Simlangsdalen test field in Sweden, one of the sites used to assess Class A preservatives for theNordic approval scheme.

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whereas in the Nordic and American systems thefungus develops on a small untreated block ofwood resting on soil (Fig. 3.18). This techniqueis used for evaluation new toxic chemicals; thetest blocks are impregnated with differentsolution concentrations so that a toxic limit orthreshold can be established between theconcentrations at which the blocks just decayand those at which the blocks are just free fromdecay. In this way the preservative activity ofvarious toxicants can be compared andconcentrations proposed for their use. Whencomplete preservatives are assessed in this wayconsiderable care is required in diluting them inorder to define their safety factor, or the amountof dilution that can be tolerated before decayoccurs. Although these tests involve increasingdilutions of the preservative solution and resultsare therefore obtained as toxicant orformulation concentration, it is normal to takeaccount of absorptions and report the results asretentions in kilogrammes per cubic metre kg/m3

or pounds per cubic foot lb/ft3.

Test fungi

The choice of test fungi is also important; forexample, Poria species are tolerant to copperand should always be used when a preservativeformulation contains this element. Generally thetest fungi are defined in the appropriate nationalstandards. No attempt is made in this book todescribe these standards in detail as they arebeing continuously revised and it is alwaysadvisable to obtain the current standard fromthe appropriate national authorities.

Weathering resistance

Laboratory block tests can also be used to assessthe performance of a preservative afterweathering by leaching or volatilization. If aproduct has good weather resistance and a widespectrum of activity against the test fungi it canbe considered to be a realistic candidate for a fullstake trial, although if it is meant to meet onlyClass B requirements a block test may beconsidered adequate in many countries, as in the

FIGURE 3.18 Laboratory methods for assessing the efficacy of preservatives against Basidiomycetes: (1) aminiature soil and wood block technique suitable for rapid and inexpensive product development tests; (2) thesoil and wood block technique used in the American standard test; (3) the malt agar and wood block techniqueused in the British, German and Dutch standard tests; (4) the soil and wood block technique used in the Nordicstandard test. (Penarth Research International Limited)


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Nordic system and in the British system for theevaluation of preservatives for treating joinery(millwork).

Insect borers

If a preservative is also intended to giveresistance to insect borer attack, further tests arenecessary against the most appropriate species,these tests usually being carried out in thelaboratory. In Europe the House Longhornbeetle represents the greatest danger tostructural wood yet protection is also requiredagainst the Common Furniture beetle whichgenerally has a greater tolerance topreservatives. Performance against termites isalso frequently assessed, the subterraneantermite, Reticulitermes santonensis, usuallybeing considered the most important species inEurope.

New preservatives

Normally a new preservative is first assessedagainst a single Basidiomycete fungus such asthe Brown rot, Coniophora puteana, and if itproves effective at apparently economicretentions the test is then extended to furtherfungi such as White rot and Soft rot asappropriate to the intended use of thepreservative. Leaching and insect borer tests

follow so that, ultimately, comprehensiveinformation is available which clearlyestablishes whether the new preservative systemis likely to be reliable in service. Some tests areunrealistic in the laboratory such as the ground-contact stake test and assessment tests againstmarine borers; these tests are best carried out innatural conditions where there is known to be aparticular hazard. In all these assessments it isnormal to compare a new preservative systemwith a well-known established system, in orderto confirm that the tests are realistically severeand in order to provide direct comparisons forcommercial reasons. Unfortunately theestablishment of preservation reliabilityrepresents only a small part of the the time, costand effort that is required today to establish anew preservative system, health andenvironmental evaluation being much moredifficult. It is therefore not surprising that newpreservatives are rarely introduced; only thelargest companies and consortia can afford todevelop new products today, and most newsystems are simply adaptations, based as far aspossible on established knowledge andexperience. This is unfortunately a situationwhich encourages the retention of existingproducts, even if they would not be acceptableif submitted for approval today, a situation thatactively discourages the development of moreeffective and safer products.

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4.1 Preservative types

Many different toxicants and other unpleasantsubstances have been used as woodpreservatives. The various preservationsystems and their action mechanisms havebeen described in Chapter 3 but whenconsidering preservative selection forcommercial purposes it is first necessary todecide on the basic preservative type that ismost likely to achieve the desired function,and it is therefore necessary to establish areasonably simple and realistic classificationfor preservatives.

The normal system as adopted in BritishStandard 1282 involves three main types ofpreservative. Type TO (tar-oil) comprisesdistillates of coal-tar including creosote. TypeWB (water-borne) includes Wolman salts ofthe fluoride-chromium type and the copper-chromium formulations which currentlydominate this market. The boron diffusionprocess for green wood is also water-borne butis usually considered to be a special case, as isthe use of aqueous solutions of sodiumpentachlorophenate in sapstain control andaqueous emulsions of insecticides in Pinholeand Powder Post beetle control. Type OS(organic solvent) involves light petroleumsolutions of pentachlorophenol, naphthenatesof copper or zinc, chlorinated naphthalenes,organotin compounds and many other lessimportant compounds including contactinsecticides. In some areas such as

Scandinavia, many of the organic-solventformulations are decorative and intermediatebetween a preservative and a paint but oftenachieving only l imited preservativeeffectiveness.

Tar-oil preservatives are also organic and insome countries such as Denmark it is the practiceto include them with organic solventpreservatives when preparing national statisticsbut there are many other examples of confusingclassifications. Thus pentachlorophenol isnormally described as a type OS preservative asit is traditionally used in organic solvent carriersbut it can be reacted with sodium hydroxide toform sodium pentachlorophenate, which iswater-soluble, and concentrated organic solventsolutions can be dispersed in water, as emulsions.The increasing cost of organic solvents and theprogressive introduction of more stringent healthand safety restrictions have prompted moreextensive use of organic preservatives in watercarrier systems which should be correctlyclassified as type WB, but these developmentsalso mean that the distinction between organicsolvent and water carrier systems is now lessimportant and no longer an appropriate basis fora preservative classification system. Thetraditional classification into types TO, WB andOS has therefore been abandoned in this secondedition. Tar-oil systems are still considered as animportant group but the preservative toxicantsor biocides are now classified as inorganic,organic or organometallic compounds, withcarrier systems considered in a separate section.

4

Preservationchemicals

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In recent years the health andenvironmental dangers associated with woodpreservation have attracted particularattention. Restrictions on the use of existingpreservatives and the requirements forapproval of new preservatives have becomeincreasingly stringent, and are now causingserious difficulty to this industry. Thesechanges have not necessarily resulted inimproved safety to health and the environmentas the development of safer preservativesystems is now discouraged by the costsinvolved and it has been necessary, foreconomic necessity, to extend the life ofestablished preservative systems which wouldnot be acceptable if they were submitted forsafety approval today. These health andenvironmental safety problems are consideredin more detail at the end of Chapter 5.

4.2 Tar-oils

Coal-tar, a product of the distillation of coal,was originally used as a wood preservative butthe lighter creosote fraction was later preferredwith the introduction of pressure impregnation,as its lower viscosity improved penetration. Thedevelopment of creosote has been closelyassociated with the history of the woodpreservation industry as previously described inChapter 1. The word creosote was first used todescribe a tar-oil fraction prepared by thedestructive distillation of wood and in NorthAmerica this product is still known as wood-tarcreosote. It is now more usual in Europe todescribe wood-tar as Stockholm tar and theword creosote is now reserved virtuallyexclusively for the oil prepared by coal-tardistillation.

The nature of coal-tar varies considerablywith the coal type and the processing method.The tar was originally derived principally as aby-product of the manufacture of town gas usinghorizontal, vertical or inclined retorts with

intermittent or continuous operation at high orlow temperatures, about 1350°C (2460°F) or450–600°C (840–1110°F) respectively. The coal-tar was then distilled to separate the volatilecreosote from the residue or pitch, the creosoteboiling at 200–400°C (390–750°F) and the pitchboiling above 355°C (670°F); a light creosotehas a low residue and conversely a heavycreosote has a high residue.

Creosote availability

It is often said that, as creosote was derived fromtown gas production, it is no longer readilyavailable. It is certainly true that there is acurrent world shortage of creosote for woodpreservation but this arises through economicfactors and is certainly not the result of truescarcity. Coal-tar is still available in enormousquantities from the coke ovens which areassociated with the metal smelting industries,but difficulties are encountered in preparingcreosote. The old tar distilleries, based in urbanareas close to town gas plants, have been closedin many countries as their sources of tar havedisappeared and it is not economical to supplythem with tar from distant coke ovens. There isno justification for building new distilleries closeto the coke ovens while the prices of creosote forwood preservation and other purposes remainlow, and it is better to use the tar as a heavy fueloil in place of petroleum which is becomingincreasingly expensive. Poland, the lastsubstantial source of town-gas creosote, hasconverted to natural gas and coke ovens are nowthe only source. The price of creosote, depressedfor many years while the industry was able tocontinue to use very old plants with minimalcapital investment, is now increasing steadily butthis is essential if new coal-tar distilleries orrefineries are to be established.

It is interesting to consider the way in whichthe situation has developed in the United States,as illustrated in Fig. 4.1. Creosote wasoriginally imported into America from Europe as

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return cargo in petroleum tankers, andconsiderable shortages were experienced thereduring World War I, encouraging more extensiveuse of salt preservatives but also dilution of theavailable creosote with petroleum. The very

rapid expansion of the railways and thetransmission pole systems carrying power andtelephone lines created an enormous increase inthe demand for wood preservation after the warand which continued until after 1950, involving

Figure 4.1 Development of wood preservation in the United States.


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particularly creosote treatment. The availabilityof creosote was decreasing andpentachlorophenol was used increasingly as asubstitute for creosote, but ultimately the steelindustry established new distillation plants sothat the creosote products required by theindustry could be produced locally. Creosote andpentachlorophenol have retained a constantvolume of the wood preservation market but themarket has expanded rapidly since 1960 with theadditional volume satisfied entirely by water-borne salt preservatives and by the alternativeoxide formulations that are now becomingincreasingly popular. The increasing water borneshare of the market probably results more fromthe increasing cost of petroleum rather than anyincreasing resistance to the use of creosote.

Creosote composition

Clearly, changes in the sources of creosote haveresulted in significant changes in composition overthe years, particularly in the 19th century with theincreasing interest in coal-tar distillation as asource of valuable chemicals. Variousspecifications have been introduced during the lastcentury in an attempt to standardize creosote andensure reliability as a wood preservative. It is notthe purpose of this book to quote specifications indetail as they are amended progressively and it isfar better to consult the appropriate standardsorganizations when information is required toensure that only current specifications are used.

The wood-preserving properties of creosotedepend on many factors. Coal-tar contains taracids such as phenol, xylol and cresol, togetherwith tar bases such as pyridine, chinoline andacridine, as well as neutral or dead oil consisting ofnaphthalene, fluorene, anthracene, phenanthrene,etc. An increase in the high-boiling tar-acid contentincreases the viscosity whereas an increase in thenaphthalene content reduces the viscosity. It wassoon recognized with the introduction ofimpregnation methods that only the use of the low-viscosity creosote fraction can achieve adequate

penetration, but it was also appreciated that thefungicidal components in coal-tar are almostexclusively associated with this creosote fraction.

Creosote as a preservative

The tar acids have excellent fungicidal activityand early creosote specifications thereforeemphasized tar acid content. It was presumed thatthe dead oil possessed no significant preservativeproperties, and this type of specificationcontinued in use until about 1930. In fact, thewood-preserving properties of creosote depend onmany factors and, whilst the tar acids are goodfungicides, they are also the components whichare most susceptible to loss from wood byvolatilization and leaching. A high residue tendsto trap tar acids and protect them from loss but itis now appreciated that very high loadings ofother less toxic components such as naphthaleneare also very important, and high naphthalenecreosote has been found to perform best in marinesituations. In recent years health risks associatedwith the use of creosote have been more closelyscrutinized, the main risks being identified as thereported carcinogenic properties of polycyclicaromatic hydrocarbons, including thebenzopyrenes in creosote. The benzopyrenecontent can be limited by using only creosotedistilling at higher temperatures, a restriction thatdoes not affect impregnation grades but which isnow restricting the availability of lighter creosotefor surface application for maintenance offencing. In other respects the use of creosote doesnot present any unusual health hazards, providedthat plants are operated with proper care andpersons handling creosote and treated woodobserve normal personal hygiene precautions.

The creosote specifications have thereforedeveloped progressively, largely as a result ofextensive service experience, but the manyconflicting specifications in different countrieshave introduced unnecessary difficulties. It will beappreciated that the nature of creosote dependson the type of coal used and the nature of the

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distillation processes so that individual localspecifications have tended to be concerned largelywith the control of the available product. In 1936the International Advisory Office of WoodPreservation in The Hague arranged a conferencein Copenhagen to agree a specification for use byScandinavian purchasers. The following year asimilar conference was held in Budapest for thebenefit of other European consumers, thespecifications resulting from these conferencesvarying only slightly in distillation characteristicsand permissible tar acid content so that somecreosotes could comply with both specifications.In the United States the American WoodPreservers’ Association had developedspecifications, and in the United Kingdom theBritish Standards Institution had specificationsfor three types of creosote obtained from differentsources. Gradually the range of sources hasdecreased, particularly in recent years as naturalgas has replaced coal gas so that all tar is nowderived from the coke ovens in the smeltingindustries. The current specifications ensurereasonable wood-preserving activity combinedwith reasonable permanence, and are largelybased on comparatively recent research on theresidues found in wood after long service.

Clearly, long-term performance depends onthe heavier and more persistent components butpenetration, which depends on the lightercomponents, is also important. If pressureimpregnation is used for creosote treatment it istherefore to increase the temperature, in order toreduce the viscosity of the heavier componentsand improve their penetration. For example, ifthe temperature of a typical creosote is increasedfrom 40 to 80°C (100 to 175°F) the viscosity isreduced from 16 to 4 cS. It has been wellestablished in service that after a period of 15 to20 years in ground contact the failure rateincreases. While the failures remain very small innumber they are particularly significant inrailway sleepers (ties) and transmission poleswhere the system must be closed if replacementsare required. These failures are far less

significant with heavy oil, assuming similarpenetration, and the value of heating heavier oilto reduce viscosity is therefore clearly apparent.

Most specifications limit the residue becauseof the problems that occur through settlementduring storage, transport or treatment. Heavyoils tend to bleed more than light oils, althoughit will be appreciated from the comments inChapter 3 that this is largely the result of failureto relieve the trapped air pressure in empty-cellprocesses. In some countries such as the UnitedStates creosote continues to be used incombination with coal-tar as it is believed thatthe tar limits loss of the creosote byvolatilization and leaching, and adulterationwith petroleum distillates is also permitted incertain circumstances. These mixtures areunusual in other countries where specificationsgenerally require creosote to be used alone, withrestrictions on composition to ensure optimumpreservative activity and permanence.

Creosote does not perform simply as a toxicpreservative as the residue and other heaviercomponents tend to limit moisture contentchanges so that treated wood is more stable andresistant to splitting. Creosote thus possesses avariety of advantageous properties which are notall readily imitated by alternative preservatives.Formulated organic-solvent preservatives aregenerally more expensive, particularly if theycontain high resin and wax contents designed toachieve this moisture resistance. Water-bornepreservatives lack this property completely;attempts to incorporate wax emulsions have notbeen very successful.

It will be appreciated that creosote possessesmany advantages in contrast with the singledisadvantage that treatments are relatively dirty.In recent years it has been limited availability thathas restricted use rather than any doubtsregarding preservative efficacy, yet sufficient coal-tar is certainly available. In 1990 there wassufficient tar feedstock in west Europe for 1 000000 tonnes of creosote annually compared with alocal demand for wood preservation of about 200


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000 tonnes and an export demand of about 50000 tonnes, mainly for North America but alsofor the Nordic countries and the Middle East. In1985 half of the European wood preservationcreosote was used for railway sleepers (ties) and athird for transmission poles, with most of theremainder used for fence posts. In some countrieslocal variations ensure rather different marketdivisions; in the United Kingdom there is nosignificant market for creosote treatment ofrailway sleepers (ties) as wood has been almostentirely replaced by concrete.

Carbolineum or anthracene oil—Solignum

This section has concentrated so far on creosotefor application by pressure impregnation, but asubstantial amount is also applied superficially bybrush, spray or immersion treatment, particularlyon wood that is to be used for fences and othergarden or agricultural purposes. Only limitedpenetration can be achieved and it is thereforeessential to use a creosote with good resistance tovolatilization and leaching. There are separatespecifications in, for example, the UnitedKingdom, the United States and Germany forcreosote of this type which contains a greaterproportion of high boiling fractions than thecreosote oil used for impregnation. Generally, thiscreosote is known as carbolineum in continentalEurope and anthracene oil in the British Isles andNorth America. Coloured pigments aresometimes added; the decorative Solignumproducts introduced into Denmark and the BritishIsles shortly after World War I were originallybased on coal-tar fractions, although modernproducts contain biocides in petroleum solvents.

Fortified creosote—CarbolineumAvenarius

The preservative activity of anthracene oil orcarbolineum is generally similar to that ofimpregnation creosote and superficial treatments

must be particularly generous if effectivepreservation is to be achieved. Internal decay is notunusual with woods that are resistant topenetration and there have been many attempts toimprove reliability. Carbolineum Avenarius wasfirst developed in 1888 and involves thechlorination of carbolineum. In a sense this is theorigin of the chlorinated organic compounds thatare so widely used in formulated organic-solventwood preservatives and which are described laterin this chapter. Developments in formulatedorganic-solvent and water-borne preservativeshave also suggested several means for fortifyingcreosote, such as the addition ofpentachlorophenol to achieve similar enhancementof the fungicidal properties. Another approach hasbeen to enhance activity by increasing theconcentration of components in creosote whichseem to be particularly advantageous; one recentdevelopment has been the additon of sulphur.

Barol

Barol was developed by Nordlinger in about 1900and consists of a mixture of copper salts incarbolineum. Combinations of zinc salts andcreosote have also been widely employed.Originally the Burnett zinc chloride process wasfollowed by impregnation with creosote; in theUnited States a petroleum distillate calledBakensfield oil was used. This two-stage processwas superseded by a single full-cell impregnationprocess involving zinc chloride solution suspendedin creosote by continuous agitation. Rütgerswerkein Germany developed a mixture of this type withhigh tar-acid creosote and it was used by theAustrian, Prussian and Danish railways for sleeper(tie) preservation prior to about 1907 when it waslargely displaced by creosote alone.

Card process—Tetraset process

The Card process, which was widely used in theUnited States, involved a mixture of 20%creosote, 2.4% zinc chloride and water

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maintained in suspension by agitation alone.Despite the low creosote content the preservativeproperties were almost as good as for creosotealone and the leach resistance was far better thanfor zinc chloride solution alone. The Tetrasetprocess used in Poland was similar, but a moresophisticated process introduced by the Polishrailways shortly before World War II abandonedcontinuous agitation in favour of a stableemulsion, achieved by using a sulphonate soapand bone glue as a stabilizer; a similar processwas used in Croatia during World War II.

These proprietary processes have now beenabandoned but two-stage treatments are stillused. The Wolman company in Germany replacedthe zinc chloride with Triolith salt which isdescribed later, and creosote butt treatment ofpoles impregnated with water-borne preservativesis still widely used, particularly in the Netherlandswith the Poulain rotating cylinder method.Impregnation with a water-borne preservativefollowed by a non-preservative oil can achieve thereliability of a creosote treatment without itsdisadvantages; the use of petroleum oil in thisway will be described later.

Arsenical creosote

Some creosote such as the vertical retort creosoteused in Australia will accept small amounts ofarsenic compounds such as 0.46% arsenic trioxide.This addition, introduced in about 1965, hasproved very advantageous whenever insecticidalproperties are required such as when there is adanger of termite attack. Contact insecticides suchas Lindane and Dieldrin have also been used tofortify the insecticidal properties of creosote butthey also give additional protection in marinesituations against gribble attack which tends to bevery resistant to creosote treatment.

Lignite oil—Kreosotnatron

Creosote is normally derived from the fractionaldistillation of coal-tar produced by the

carbonization of bituminous coal, but this isonly one of the products of degradation ofvegetable matter under anaerobic conditions; thefull sequence is peat, lignite or brown coal,lignitous or soft coal, bituminous coal, steamcoal and anthracite. Bituminous coal is preferredfor the manufacture of creosote as it gives thehighest yield of coal-tar but in some areas,particularly in Germany, the most extensivedeposits of such vegetable matter are lignite orbrown coal. Lignite-oil is creosote derived fromlignite-tar which possesses a high paraffin andnaphthalene content as well as a substantialdegree of unsaturation which results in atendency for the tar to solidify when exposed tooxygen. Lignite-oil has a very high tar-acidcontent which was originally extracted withsodium hydroxide to give Kreosotnatron whichwas used as a wood preservative for pit props inthe lignite mines. Tar acids derived in this waywere also used to increase the tar acid content ofnormal creosote when this was considereddesirable. The high paraffin content of lignite-oilresults in only poor fungicidal activity in theabsence of the tar acids and it has usually beenused mixed with normal creosote to give goodpreservation and a clean and dry surface. Mostlignite-tar is now used for the manufacture ofwax and some resinous compounds.

Wood and peat tar—No-D-K

Peat tar has been used in parts of Russia far frommajor coal deposits. Peat tar is similar to the tarderived from the destructive distillation of wood.This wood tar can be separated by distillationinto various fractions, including wood tar oil orcreosote. Softwood tar, often known asStockholm tar, was at one time extensivelyproduced in Scandinavia and widely used as abrush-applied preservative for log houses andother wood buildings. The preservative actvity ismuch lower than that of coal-tar or anthraceneoil and the treatment was decorative rather thanpreservative. Wood-tar creosote was also used as


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a pressure impregnation wood preservative, itslow viscosity ensuring excellent penetrationcompared with coal-tar creosote. Hardwood taris darker in colour and has been used extensivelyin wood preservation only in the United Stateswhere it has been marketed as No-D-K.

Water gas tar

Water gas is formed by passing steam over hotcoke to produce a mixture of hydrogen and carbonmonoxide. Water gas can also be carburetted toincrease the calorific value by the injection of apetroleum distillate such as gas oil, but this alsoresults in the formation of water gas tar. Thisprocess is rare in Europe but more common in theUnited States. The lower viscosity tar-oil fractioncontains similar hydrocarbons to creosote, butwithout tar acids and with only traces of tar basesand some petroleum hydrocarbons of low activity.Water gas tar oil therefore has limited preservativevalue but it is often blended with creosote,particularly in the United States where creosote hasalways been comparatively scarce.

Créosite

Créosite was introduced in Belgium in about 1922.At that time the Belgian railways considered that ahigh naphthalene creosote was essential for goodpreservation. In contrast, Créosite was based onthe opinion that the naphthalene was virtuallynon-preservative and that the tar acids, tar basesand neutral hydrocarbons were more important.The naphthalene was therefore removed andreplaced by high-boiling hydrocarbons, derivedfrom asphaltic bitumen. The arguments justifyingthe preparation of Créosite sound like an attemptto justify the removal of naphthalene for someother purpose!

Creofixol—Cornelisol

Creofixol was another special creosote introducedin Belgium in about 1919. This was a normal

creosote but with a higher proportion of low-boiling fractions, giving a low-viscosity penetratingproduct which was particularly suitable forimmersion treatment at ordinary temperatures.Cornelisol was introduced in the Netherlands in1935 and was a creosote free from tar acids andfree from residue, giving a particularly attractivelight colour. This preservative has given excellentperformance as a treatment for transmission poles.

Transote

Transote was introduced in the United States in1917 as Reilly transparent penetrating creosote. Itconsisted of 25–30% refined colourless creosote in avolatile solvent, giving a low-viscosity penetratingproduct and a treatment that was colourless, freefrom bleeding and paintable. Laboratory testsindicated that the preservative activity was ratherless than might be expected from the creosotecontent, suggesting that the refining process hadremoved important components.

Fibrosithe

Fibrosithe was another American product,introduced in 1923 as a creosote formulation forapplication by brush, spray or immersion. Itconsisted of 70% creosote with 9.5% coal-tar togive improved surface sealing, 6% phenol toimprove the fungicidal properties and finally 8%light coal-tar naphtha and 6.5% benzene toreduce the viscosity and improve penetration. Itis perhaps worth mentioning at this stage thatlow viscosity diluents can significantly reducethe viscosity of a mixture in this way but they donot affect the molecular size of individualcomponents which may still tend to filter out onthe surface of the wood rather than penetrate.

Shale oil

Shale oil is derived from the distillation ofbituminous shale tar. Bituminous shale is availablein many countries including Sweden, Russia, France,

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Scotland and various parts of the United States. Thetar yield from shale is low and processing is justifiedonly when there is a shortage of competitive coal-taror petroleum products. In Estonia, three types ofshale tar have been used as alternatives toanthracene oil or carbolineum and as recently as1936 shale oil was employed as an alternative tocreosote for the impregnation of railway sleepers inEstonia and Lithuania. Tests indicate that shale oilhas lower preservative activity than coal-tarcreosote but in practice it has performed reliablywhen used at very high retentions in full-cellimpregnation processes, perhaps because thephysical barrier properties then become moresignificant than the toxic properties.

Petroleum distillates

The preservative properties of petroleum oils canbe attributed solely to their physical propertiesas they are virtually non-toxic. In the UnitedStates heavy oils are used as diluents forcreosote, a practice based on the concept thatloadings of creosote can be substantially reducedas in empty-cell treatments whilst still retainingadequate preservative properties, although highloadings achieve greater resistance toweathering. Shortages of creosote in the UnitedStates, particularly during World War II, haveled to an increase in the use of petroleum-basedpreservatives, particularly pentachlorophenol inheavy oil, which possess similar properties tocreosote. Organic-solvent formulations will bedescribed in detail later in this chapter but it isappropriate to mention this particularformulation at this point as it is directlycompetitive with creosote. The various toxicantsthat have been used to fortify creosote have alsobeen applied in heavy petroleum oils in anattempt to develop an alternative to creosote.Copper, zinc and arsenic compounds have beenadded, but a two-stage treatment involving awater-borne treatment to provide toxicity,followed by an oil treatment to protect againstleaching, gives the most reliable results.

4.3 Inorganic compounds

Water-borne preservatives consist of solutions ofinorganic ions, originally prepared as mixturesof salts but now often formulated from oxides inorder to avoid the unnecessary inactive ions suchas sodium and sulphate which are introduced inmultisalt formulations. Some inactive groups areimportant in fixation, so that the loss ofammonia from some formulations results infixation by a pH change. In salt formulations, aninactive ion may be varied to assist in achievingpenetration or the desired retention. Thussodium fluoride which has limited solubility canachieve an adequate concentration in the normalWolman-type salts used for pressureimpregnation, but it must be replaced by themore soluble and more expensive potassiumfluoride to give the higher concentrations thatare required when these salts are applied by non-pressure immersion, or when pressureimpregnation is used for the treatment of a woodof low permeability.

Multisalt systems—active oxide contents

Originally, simple salts were used aspreservatives but they were found to havevarious disadvantages; many of the most suitablesalts were poisonous, corrosive or leachable, orpossessed a rather narrow spectrum of activity.All these disadvantages have been overcome bythe development of multisalt systems which arenow being progressively replaced by oxidemixtures designed to achieve similarcombinations of functional ions. There has beenan enormous variety of developments and assimilar formulations are often known by severaldifferent names it is difficult to ensure that anydescription of water-borne wood preservatives iscomplete. This account is therefore concernedwith the major developments and the mostimportant or most interesting formulations.Where it is necessary to compare multisaltpreservatives the current American Wood


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Preservers’ Association system has been adopted.The most important comparisons are necessaryon the copper- chromium-arsenic (CCA)preservatives where the three toxicants areexpressed as oxides, the copper as CuO, thechromium as CrO3 and the arsenic as As2O5. Thepreservative activity of the formulation is thenapproximately indicated by the total active oxidecontent, whatever the ratio of the individualtoxic elements, although it will be appreciatedthat the insecticidal properties depend on thearsenic alone and that the ratios also influencethe fixation of the various toxic elements.

Mercuric chloride—Kyanising

Mercuric chloride or corrosive sublimate is verysoluble and the most important and most effectiveof the simple salt preservatives. It was firstproposed by Homberg in 1705 but the processwas patented by Kyan in 1832. Kyanisinginvolved the treatment of wood through simpleimmersion in a mercuric chloride solution,generally at a concentration of 0.66%. Thistreatment is very toxic to both insects and fungibut it is also absorbed onto the wood; if woodremains for a protracted period in the solutionmore mercuric chloride must be added tocompensate for this absorption. Mercuric chloridesolution is very corrosive and it is necessary toconstruct the treatment plant from wood,concrete or stone; this is the reason why mercuricchloride has never been applied using animpregnation plant. Mercuric chloride is also verypoisonous and very expensive but Kyanising isperhaps the most effective water-borne treatmentfor European white-wood; spruce transmissionpoles can be treated by immersion for ten days inmercuric chloride solution and there is still noalternative treatment that can achieve the samedegree of reliability! Unfortunately, mercuricchloride was banned as a wood preservative inGermany in 1935 at a time when the Wolmansalts dominated the water-borne salt preservativemarket, but these were insufficiently soluble for

use in place of mercuric chloride in existingKyanising tanks. A special soluble form ofWolman salt was therefore prepared by replacingsodium fluoride by the more expensive but verysoluble potassium fluoride. The progressivedevelopment of Wolman salts is described later.

Mixed Kyanising—Deep Kyanising—Chromel—Lignasan

In 1914 Bub developed Mixed Kyanising whichinvolved the use of a mixture of mercuric chlorideand copper sulphate with either zinc chloride orsodium fluoride. Deep Kyanising was developedby Kinberg in 1924 and involved steaming andthe use of resin solvents such as trichloroethyleneto improve penetration. These processes werefollowed by several attempts to reduce thecorrosive action of mercuric chloride. Bryan andRichardson developed a preservative consisting of1 part mercuric chloride and 2 parts potassiumdichromate, a large concentration of sodiumnitrite and some sodium carbonate. One productof this type, known as Chromel, gave very goodresults in stake tests. Although the dichromatewas originally incorporated to reduce corrosionso that this product could be used in a normalsteel impregnation plant, it was found that it alsoimproved fixation and the product has proved tobe very resistant to leaching. Mercury compoundsare only rarely used in preservation today.Organomercury compounds were used for manyyears for sapstain control. Originally ethylmercury acetate was used but it was soonreplaced by the phosphate, a compound marketedas Lignasan, and later by phenyl compounds. Theuse of these compounds has now been largelydiscontinued following pollution of streams andlakes through the use of the same compounds forslime control by the paper industry.

Fluorine compounds

Fluorine is historically one of the mostimportant wood preservative elements.

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Fluorides were originally proposed as woodpreservatives in Britain in 1861 in processesdesigned to precipitate calcium fluoride orfluorosilicate (silicofluoride) in wood but thesesystems were never developed commercially.The first practical fluoride process, developedin 1901 and patented in 1903, involveddissolving zinc in hydrofluoric acid. The loss ofexcess hydrogen fluoride after application ofthe solution to wood resulted in fixation. Asimilar process developed in France in 1907involved the treatment of wood with a mixedsolution of sodium fluoride and zinc chloride,followed by heating to achieve fixation by theprecipitation of zinc fluoride within the wood.The same year Wolman patented the use ofsodium and potassium fluorides for thepreservation of mine timbers. Sodiumfluorosilicate had first been proposed as a woodpreservative in 1904 but it had not been widelyused, despite its availability and low cost, onaccount of its low solubility and corrosiveproperties. In 1926 Wolman developedmixtures of sodium fluoride and sodiumfluorosilicate for the treatment of mine timbers,mixtures which were less expensive thansodium fluoride alone, particularly wheresodium fluorosilicate was available locally.

Sodium fluoride has been also used as acomponent in many multisalt preservatives. It isextremely fungicidal and also toxic to somewood-borers such as the House Longhorn beetle,Hylotrupes bajulus, although its insecticidalproperties are specific to only a few species.Sodium fluoride is non-corrosive and treatedwood is paintable, but the treatment is leachablewhen used alone. The solubility of sodiumfluoride is only 4–5% at ambient temperaturesbut the high toxicity enables adequate retentionsto be achieved by pressure impregnation.Potassium fluoride is more expensive but alsomore soluble and is used when higher solutionconcentrations are required, such as forimmersion treatments or impregnation of woodswith low permeability.

Wolman salts—Bellit—Basilit—Malenit

Formulations containing fluorine, chromium,arsenic and phenol components, known in theUnited States as FCAP preservatives, areusually known as Wolman salts. Theseformulations originated in Austria in 1907 witha Wolman patent for the use of fluorides for thepreservation of mine timbers. In 1909Malenkovic introduced a preservativeconsisting of a mixture of about 88% sodiumfluoride and 12% dinitrophenolanilin; thispreservative was first known as Bellit butrenamed Basilit in 1914. In about 1913Wolman introduced a mixture of sodiumfluoride and dinitrophenol which was known asSchwammschutz Rütgers (Rütgers fungicide) asat that time Wolman was associated withRütgerswerke. Antimony fluoride and zincfluoride were proposed as additives to reducethe corrosive properties of thedinitrophenolanilin and dinitrophenol, and amixture of sodium fluoride, zinc fluoride anddinitrophenol was introduced as Malenit in1921 by Malenkovic.

Flunax—Fluoxyth

A parallel development to these Wolman saltsinvolved a mixture of 84% sodium fluoride with8% xylenol saponified with 8% of 38% sodiumhydroxide solution. This product was known asFlunax or Fluoxyth, and it was usually appliedby full-cell impregnation at a concentration of1.5–3.5%. In 1923 Flunax was used by theGerman railways for the treatment of pinesleepers (ties), giving a life of 15–16 yearscompared with 27 years for normal creosoteimpregnation. Flunax can cause some chemicaldeterioration, particularly the development ofbrashness in beech sleepers, so it has not beenextensively used, although it was re-introducedduring World War II when the shortage ofdinitrophenol and chromates limited theproduction of Wolman salts.


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Thriolith—Triolith—Minolith—Thanalith—Tanalith

The true Wolman salts developed following aproposal in about 1913 for the addition ofchromates, phosphates or borates to thesodium fluoride and dinitrophenol mixtures ascorrosion inhibitors in place of the zincfluoride which was used in Malenit.Dichromates were found to be most suitableand the resulting product consisting of 85%sodium fluoride, 10% dinitrophenol and 5%sodium or potassium dichromate was veryextensively used under the name Triolith,originally Thriolith. This formulation was thestarting point for many later improvedformulations, the first being Minolith whichconsisted of Triolith with the addition ofretardant properties for wood used in mines.fungal decay but in 1922 it was mixed with anequal amount of sodium arsenate to improveinsecticidal activity, the resulting productbeing known as Tanalith, originally Thanalith.Several further patents, principally in theUnited States, covered similar mixtures butwith the dinitrophenol replaced bydinitrocresol or, later, sodiumpentachlorophenate.

Dichromate fixation—Triolith U—TanalithU—Wolmanit U—Wolmanit UA

Although dichromate was originallyintroduced to reduce the corrosive propertiesof dinitrophenol it was soon appreciated thatit considerably improved fixation and theresistance of the treatment to leaching. Inabout 1930 the dichromate content in Wolmansalts was increased to further improve fixationand these high dichromate versions of theformulations were identified by the suffix U.Preservatives of this type are sti l l used,generally with the following typicalformulations:

The figures in brackets represent the ratios ofactive components using the system introducedby the American Wood Preservers’ Association,and now widely adopted elsewhere, to simplifycomparisons between the ratios of the toxiccomponents in different formulations. FollowingWorld War II, Triolith U and Tanalith U wererenamed Wolmanit U and Wolmanit UArespectively, for use in Europe.

Trioxan U—Trioxan UA—Wolmanit Uhochl. and Wolmanit UA hochl.

In 1934 a proposal was made in Germany to banthe use of very poisonous mercuric chloride. TriolithU and Tanalith U were insufficiently soluble toprepare the relatively high solution concentrationsnecessary for application by immersion usingexisting Kyanising tanks, which had been used withmercuric chloride, and special soluble versions ofthese Wolman salts were developed by replacing thesodium fluoride with more soluble and moreexpensive potassium fluoride and, at the same time,reducing the dinitrophenol. These new formulationswere known as Trioxan U and Trioxan UA,although they were renamed after the World War IIas Wolmanit U hochl. and Wolmanit UA hochl.respectively, the suffix indicating that they werehighly soluble.

Other Wolman salts

It is useful at this point to consider the currentEuropean nomenclature which enables salts ofthe Wolman type of different manufacture tobe identified. The original Triolith became

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Triolith U with the addition of furtherdichromate to improve resistance to leachingand equivalent products are Basilit U, OsmolitU, Wolmanit U, etc. The addition of arsenic toTriolith gave Tanalith, really an alternativename for Triolith A so that Tanalith U isequivalent to Triolith UA, and similar productsare therefore Basil it UA, Osmolit UA,Wolmanit UA, etc. Readily-soluble productsfor use when high solution concentrations arenecessary, either for the impregnationtreatment of wood of low permeability or forimmersion treatments, were achieved byreplacing sodium fluoride by potassiumfluoride and by reducing the dinitrophenolcontent to originally give Trioxan U andTrioxan UA, the equivalent products in theBasilit and Osmolit range having the suffix‘leicht löslich’, meaning readily soluble, andthe equivalent Wolmanit products having thesuffix ‘hochl’, which is an abbreviation forhighly soluble. In the most recent developmentit has been found that the addition of acidgives improved fixation and this has resultedin products known as Wolmanit U (or UA)Reform, now UR (or UAR). In Wolmanit U thefluorine fixation is improved from 20% to80% whilst in Wolmanit UA the fluorinefixation is improved from 10% to 67% andthe arsenic fixation from 75% to 97%. BasilitUAF is similar to Wolmanit UAR.

FCA (or FCAP) salts

Termites represent a risk throughout most of theUnited States and only the Tanalith or arsenicaltypes of Wolman salts will give reliableprotection; they are usually described as fluor-chrome-arsenate-phenol (FCAP) preservatives inAmerica but fluorine-chromiumarsenic (FCA) inEurope. In the United States they are marketedas Osmosalts, Osmosar, Tanalith, Wolman SaltsFCAP and Wolman Salts FMP, all conformingwith the American Wood Preservers’ AssociationStandard P5 for FCAP preservatives:

AWPA FCAP salts

Fluoride 22% FHexavalent chromium 37% CrO3

Arsenic 25% As2O5

Dinitrophenol 16%

Sodium pentachlorophate may be used in place ofdinitrophenol

Care is necessary in the interpretation of theseAmerican specifications as the percentages arebased solely on the active components which arepresent. In this specification the fluorine,chromium and arsenic are generally present assodium or potassium salts so that allformulations contain inactive components inaddition. A typical commercial formulation willtherefore contain much lower concentration ofthese active components and will be described asonly X% active, depending on the proportion ofthese components in the product, so that theTanalith U shown in the earlier table would beonly about 47% active; the ratios of thecomponents are also different from thoserequired by the AWPA specification so that theEuropean Tanalith U formulation is not used inthe United States.

FCAP pole bandages

Wolman salts of the Triolith or U type are usedwhen fungal decay represents the principal hazardor when an arsenic content is unacceptable. TheTanalith or UA type salts are used where there issignificant insect hazard, particularly from HouseLonghorn beetles or termites. These formulationsare generally very effective but severe failureshave occurred where they have been used for thetreatment of coolingtower fill, the wooden slatsover which hot water runs, exposed to a currentof cooling air. This damage was not found to bedue not to leaching but to the development ofpreviously unknown resistant fungi which arenow known as Soft rots. Some ground-line decayis caused in transmission poles by the same fungi


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and as a result the Wolman or FCAP salts havebeen largely replaced for these purposes by thegreensalt or CCA preservatives which aredescribed later. Unfortunately greensalts fixextremely rapidly and cannot diffuse to asignificant extent before fixation so that, ifpermeable sapwood is coupled in a wood specieswith impermeable heartwood, the heartwood canbe treated more reliably by Wolman salts as theseare able to diffuse into it. This advantage ofWolman salts is particularly noticeable in speciesof low permeability such as spruce in which heartrot in CCA-treated poles may be more significantthan the superficial Soft rot in FCAP or Wolmansalt-treated poles. If it is required to treat, forexample, spruce poles which are both non-durable and relatively impermeable throughout,Wolman salts still provide one of the most reliabletreatments available, particularly if the highlysoluble salts are used at high concentrations.These highly soluble salts are also used inOsmose, Cobra and other bandage methods forin-situ ground-line treatments to extend theservice life of transmission poles.

Berrit—Fluoran OG

Many other fluoride compositions have beendeveloped, usually as imitations of the originalsalts. These compositions vary in the ratios of theactive components but their development was alsoinfluenced by materials availability in times ofshortage such as during World War II. The seriousshortage of chromium compounds throughoutEurope resulted in a progressive reduction in theirconcentration and ultimately their omission orreplacement by other corrosion inhibitors. In theNetherlands Triolith was used at first but replacedby Berritt and Fluoran OG, similar to Triolith andTanalith respectively but with the components indifferent proportions. Progressive changescontinued as individual components becamescarce. Eventually, zinc sulphate was used to partlyreplace the components in short supply, giving so-called Wolman salts containing about 3 parts

sodium fluoride, 3 parts zinc sulphate, 1 partsodium dichromate and a small amount ofdinitrophenol. These salts were still in use in theNetherlands in 1950.

Tanalith Un and Tanalith K

The chromium shortage during World War II wasthe most serious problem and in RumaniaTanalith Un was used which consisted of 30%sodium fluoride, 50% sodium arsenate, 10%dinitrophenol and a reduced chromium content of10% sodium dichromate. Tanalith K was similar,except that the sodium dichromate was replacedby 5% hexamethylene tetramine and thedinitrophenol by 15% sodium dinitrophenate.This formulation was widely used, although insome areas such as Croatia it often lacked thedinitrophenol content.

Basilit A57—NAF salt—Fluorising

Basilit A57 was used in Switzerland in World WarII, essentially Basilit UA but without thedichromate. A similar product had been introducedinto Denmark in about 1935 where it was knownas NAF, nitrophenol-arsenic-fluoride. Morerecently the process of fluorising has beenintroduced in Australia for the hot-and-coldtreatment of karri sleepers (ties) involving a similarformulation of dinitrophenol, arsenic and fluoride.There is virtually no fixation as these formulationslack chromate, but they have been used for theBoucherie treatment of poles where traditionalcopper sulphate is considered unacceptablebecause of the danger of development of copper-resistant fungi such as Poria species.

Bifluoride treatments

The so-called bifluorides are potassium, sodiumand ammonium hydrogen difluoride. Hydrogenfluoride is released when these bifluorides areapplied to wood and can diffuse, giving very deeppenetration in laboratory tests. Unfortunately this

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hydrogen fluoride is not fixed and can be readilylost by leaching or volatilization. In remedialtreatments this diffusion is particularly valuableand the bifluorides are still widely used in parts ofEurope for House Longhorn beetle eradication.The hazards associated with release of toxichydrogen fluoride do not appear to have beenproperly considered; reports published in Germanyas early as 1940 warned of these dangers, butbifluorides continue to be used. These reports alsopointed out that even sodium and potassiumfluoride treatments slowly release hydrogenfluoride. Whilst there may therefore be dangersassociated with the well-established Wolman FCAPsalts which contain fluorides, the bifluorides arecertainly an even more serious danger.

Improsol—Rentex

Improsol and Rentex are mixtures of fluorides,bifluorides and chromates, developed asimmersion treatments designed to achieve deeppenetration. Improsol originally consisted of amixture of potassium and ammonium bifluoridewith wetting agents, and was intended to beapplied by immersion at a concentration of 5 to10% to achieve an average retention of about 1kg/m3 (0.06 lb/ft3) salt. The treated wood wasthen required to be maintained for about fourweeks at a minimum moisture content of 20% topermit diffusion. This treatment had noresistance to loss by leaching and volatilizationof hydrogen fluoride. Very ambitious claimswere made for Improsol treatment and theformulation has been frequently modified toreduce corrosion and increase peristence.

Mykocid BS—Osmol WB4

Improsol has also been used for sapstain controlon freshly converted green wood. Mykocid BS is asimilar bifluoride product for sapstain control.These products achieved some penetration of thesapstain market following restrictions on the useof chlorophenols, such as sodium

pentachlorophenate, but they are comparativelyexpensive. Ammonium bifluoride is very effectiveagainst stain fungi but it stimulates thedevelopment of the mould Trichoderma viridewhich then covers the surface of the wood unlessa very high fluoride concentration is employed.The dangers associated with chlorophenols arediscussed later but it is difficult to understand theattitude of the Swedish safety authorities inpermitting the use of bifluorides whilst restrictingthe use of chlorophenols which are not known tohave caused any mammalian injury in theircountry. In contrast, regulations in the UnitedKingdom actually prohibit the use of bifluorideswhilst permitting the use of chlorophenols insuitably controlled circumstances, although thebifluoride restrictions were not actuallyintroduced to control wood-preservationtreatments. Osmol WB4, introduced in about1949, is another proprietary sapstain controlproduct based on bifluorides.

Fluorosilicates

Fluorosilicates (silicofluorides) are not asefficient as fluorides as wood preservatives butthey are widely available as industrial waste andare therefore a very attractive source of fluorinein many countries. Sodium fluorosilicate wasfirst proposed as a wood preservative as longago as 1904 but there is still research effortdevoted to developing improved systems inGermany, Yugoslavia and Australia. The mainproblems are relatively low activity associatedwith low solubility, and corrosion.

Fluorex V and Fluorex S—Sikkuid—Fluralsil—Hydrazil (Hydrarsil)—BasilitCFK

In 1926 Wolman developed a mixture of sodiumfluorosilicate with sodium fluoride for thetreatment of mining timbers in an attempt tocombine reasonable effectiveness with low cost, thefluorosilicate being available from the mineral


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extraction processes associated with the mining. Inthe United States sodium fluorosilicate was theprincipal component in Fluorex V, whereasmagnesium fluorosilicate was the principalcomponent in both Fluorex S and in the Germanproduct Sikkuid. Another German productFluralsil consisted of a mixture of sodiumfluorosilicate and zinc chloride which was designedto precipitate zinc fluorosilicate. It was originallyintroduced in 1909 as a wood preservative and forthe sterilization of walls infected by the Dry rotfungus, Serpula lacrymans, as an early remedialtreatment. Fluralsil was also used in Denmark inabout 1935, together with the NAF salt that hasbeen described earlier, as an alternative to coppersulphate in the Boucherie treatment oftransmission poles, as deterioration by copper-resistant fungi such as Poria species was causingconcern. Neither Fluralsil nor NAF containchromates or any other fixation components andboth can be readily lost in leaching conditions.Hydrazil, originally Hydrarsil, was a mixture ofzinc and mercury fluorosilicates which achievedgreater resistance to leaching, although it hasnow been abandoned because of the toxicity ofthe mercury content. Copper fluorosilicate isused in Basilit CFK, a product which is describedin detail later.

Zinc salts—Burnettising

Zinc has been used for wood preservation inseveral different forms. Zinc salts were used incombination with fluorides in multi-saltpreservatives, particularly as an alternative todinitrophenol to reduce corrosion and to enhancefungicidal activity and fixation. The use of zincchloride alone was originally proposed by Burnettin 1838 and for many years Burnettising andKyanising with mercuric chloride were the mostpopular salt treatments in Europe. Zinc chloridewas used until 1921 in the United States,particularly for railway sleepers (ties), and wasstill widely used in Russia as recently as 1948.This treatment has poor resistance to leaching but

it was found to give excellent performance whenused in combination with creosote, originally in adouble- but later a single-stage treatment asdescribed earlier in Section 4.2. The use of zincchloride followed by creosote economized on theuse of creosote but achieved virtually as goodprotection as higher retentions of creosote alone.This double treatment was used by the Danishrailways from 1889 to 1907 when economicfactors caused it to be abandoned in favour ofcreosote alone. The toxicity of the creosote isunnecessary and this treatment can therefore usenon-toxic petroleum oils, a useful alternative intimes of creosote shortage. In the Card process inthe United States and the Tetraset process inPoland the two-stage treatment is replaced by amixture of salt solution and creosote, maintainedin suspension by agitation or emulsifiers; theseprocesses were described in detail in Section 4.2.

Kulba salt—chromated zinc chloride(CZC)—copperized CZC (CCZC)

The excellent solubility of zinc chloride is helpfulin the preparation of the solution but also resultsin poor resistance to leaching. The corrosion offerrous fittings can usually be attributed to thepresence of hydrochloric acid. Although zincchloride is not so active as some other salts it isstill used as a component in several multisaltpreservatives. One of the simplest is Kulba saltwhich was introduced in Belgium during WorldWar II and which consists of a mixture of zincchloride and sodium hydroxide. Chromated zincchloride (CZC) was developed in the UnitedStates in about 1934 and achieved considerableresistance to leaching. It consists of zinc chlorideand sodium dichromate mixed according toratios defined in the American Wood Preservers’Association Standard P5 which requires thehexavalent chromium content to be 20%, asCrO3, and the zinc content to be 80%, as ZnO.In copperized CZC (CCZC) about 10% of thezinc chloride is replaced by cupric chloride, tobroaden the spectrum of activity.

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ZFD—ZFM

Zinc sulphate has also been used as a component inseveral multisalt preservatives. ZFD preservativewas used in Belgium in World War II and consistedof a mixture of zinc sulphate, sodium fluoride anddinitrophenol, whereas ZFM was zinc sulphateand magnesium fluorosilicate (silicofluoride). Inthe Netherlands zinc sulphate was added toTriolith when it was in short supply, resulting in aproduct that was still used in 1950, but whenTriolith was completely unavailable zinc sulphatewas used alone. Large quantities of zinc sulphateare available from the Witwatersrand miningoperations in the Transvaal and this was used fromabout 1918 in a mixture with Triolith as apreservative for the treatment of mining timbers; itwas normally prepared using 3% zinc sulphate and0.3% Triolith, and replaced zinc chloride whichhad been used previously. High loadings werenecessary to achieve an appreciable preservativeeffect, partly because zinc sulphate has poorresistance to leaching, and this formulation wassuitable only for application to permeable wood.

Zinc meta-arsenite (ZMA)—BolidenBIS—Boliden S—Boliden S25

Zinc meta-arsenite (ZMA) was developed as awood preservative by Curtin in the United Statesin 1928. It is applied to wood in an acetic acidsolution but, as the acid is lost by volatilization,a non-leachable precipitate of zinc meta-arseniteis formed. This process was extensively used inthe United States before World War II. Zinc wasalso used in a number of the early Boliden saltsdeveloped in Sweden. Boliden BIS was a zinc,arsenic and chromium salt mixture. Boliden Swas similar in composition but prepared as anoxide mixture. In Boliden S25 about 25% of thezinc was replaced by copper, but all thesemixtures were eventually superseded by K33, acopper-chromium-arsenic oxide preservative; thedevelopment of Boliden salts will be describedlater. In Australia a copper-chromiumzinc-

arsenic salt mixture is still used which is similarin composition to Boliden S25 but prepared fromsalts rather than oxides.

Copper salts

Copper is the most important component in mostof the relatively modern preservatives. Coppersulphate was originally proposed as a woodpreservative by Margary in 1837 and was usedextensively, usually as a 1% solution, although itcould not be applied in hard water which causedprecipitation. Copper sulphate possesses highfungicidal activity but is very corrosive to iron andsteel. Copper impregnation equipment wassometimes used but it was very expensive. Coppersulphate is very soluble but, while most of thetreatment is leachable, a proportion remains fixedwithin the wood. The most realistic use of coppersulphate was in transmission pole treatment by theBoucherie method—a cap was fitted to the butt ofa freshly felled log and the copper sulphate solutionwas introduced under low pressure from a headertank, gradually displacing the sap. This sap-displacement process was extensively used inFrance and later introduced into other countriessuch as Denmark and Finland; Boucherie andsimilar processes are still in use with variouspreservatives. The performance of copper sulphate-treated poles was reported to be poor in alkalinesoils but the failures were actually associated withsoils containing ammonia rather than withcarbonaceous soils in which the copper isconverted to the carbonate which is still fungicidal,but resistant to leaching. The main problem withall copper preservatives has been that some fungiare resistant, particularly the important Poriaspecies, which are able to detoxify the copper byformation of the oxalate. Copper-basedpreservatives must therefore contain other toxicfungicides if reliable preservation is to be achievedat reasonable retentions, although it is interestingto note that the arsenic incorporated as aninsecticide distinctly improves activity againstresistant fungi.


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Aczol—Viczsol

Aczol, known in Germany as Viczsol, anammoniacal solution of copper and zinc salts withphenol, was originally introduced in 1907 and wasprogressively improved during the following thirtyyears. After treatment, the loss of ammonia resultsin good fixation but it has been found in laboratorytests that the effectiveness of the product declinessteadily, as the fixation progressed, so that realisticassessments can only be made after a sufficientperiod to allow for complete fixation. Early testresults exaggerated effectiveness and there wereinitially some failures in service through the use ofinadequate retentions. It was also reported that thistreatment made wood rather brittle.

Chemonite—ammoniacal copper arsenite(ACA)—ammoniacal copper-zinc arsenite(ACZA)

In Chemonite the copper is used without zinc butarsenic is added to improve the insecticidalproperties. Chemonite originated in the UnitedStates in about 1925 when Dr Aaron Gordon ofthe University of California proposed that Parisgreen, copper aceto-arsenite, should be used as awood preservative. Chemonite, which was firstmarketed by the Diamond Match Company in1934 and from 1935 by the Chemonite WoodPreserving Company, was originally a mixture ofcopper, arsenic and ammonium acetates, but it isnow usually prepared as a 6% solution for woodpreservation by mixing 1.84% copper hydroxideand 1.3% arsenic trioxide with ammonia andsmall amounts of acetic acid and glycerol, althoughdifferent copper compounds are used at times. Thisformulation, in which the ratio of copper andarsenic is 49.8% CuO to 50.2% As2O5, is knownin America as ammoniacal copper arsenite (ACA).Half of the arsenic As2O5 content has beenreplaced since about 1983 by zinc as ZnO, aversion known as ammoniacal copper-zinc arsenite(ACZA) which has now replaced the original ACAformulation in all plants in the United States. The

copper and arsenic are fixed, mainly as oxides, asthe ammonia evaporates; this slow fixation processis very advantageous in the treatment ofimpermeable species and also enables Chemoniteto be applied by the Boucherie and similar sap-displacement processes. There is no corrosion ofsteel plant but the treatment corrodes copper.Chemonite is generally very reliable, although ithas been reported that it may be less effective inmarine situations.

CAA—ZAAMore recently, the Canadian Forests ProductsLaboratory has developed ammoniacalpreservatives which are described as copper-ammonia additive (CAA) and zinc-ammoniaadditive (ZAA), although it would seem moresensible to attribute these initials to copper-ammonia-arsenic, as the formulations containarsenic as an insecticide. These preservatives canachieve exceptional penetration and are thusconsiderably more reliable than the well-established copper-chromium-arsenic (CCA)preservatives for the treatment of impermeablespecies. However, difficulties have beenencountered in substantiating these claims and itseems more likely that the main advantage of theCAA and ZAA preservatives, as well as of theolder-established ACA (Chemonite) product, liesin their slow fixation and the protracted diffusionthat can occur, resulting in deep penetration insuitable circumstances. The use of arsenic in arecently developed product was surprising andDomtar in Canada soon developed similarproducts with the arsenic replaced by quaterneryammonium compounds; these formulations areknown as ammoniacal copper-quaternery (ACQ)or zinc-quaternery (AZQ) formulations.

Ammoniacal copper borate (CAB)—acidcopper borate (ACB) or acid zinc borate(AZB)Increasing resistance to the use of arsenic alsoprompted the development of ammoniacal copper

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borate systems in about 1965 but they have notbeen used commercially to a significant extent,apparently because the various less expensive andwell established arsenical systems have continuedin use in many countries, despite health fears.Fixation by loss of ammonia is not essential tocopper or zinc borate systems and much simplerformulations are possible comprising copper orzinc acetate with boric acid. Different ratios canbe used to form various borates, the zinc borateshaving well-established fungicidal and fireretardent properties; they are often used as activepigments in plastics and paints.

Another ammoniacal formulation involvescopper carboxylate but it is essentially a water-solubilized copper soap; this system will bedescribed more fully in Section 4.5.

Copper-chromium-arsenic (CCA)

One of the most important advances in woodpreservation was the development of the copper-chromium-arsenic preservatives which areknown in the United States as chromated copperarsenate (CCA). There were three independentdevelopment routes that ultimately yieldedbasically similar CCA products.

Acid copper chromate (ACC)—CelcureN—Celcure A—Celcure F

Celcure was developed by Gunn in Scotland in 1926but was then improved progressively up to 1942.The original Celcure product, now sometimesknown as Celcure N, is described in America as acidcopper chromate (ACC). It consisted of a mixture ofcopper sulphate, a dichromate and acetic acid, butthe acetic acid was replaced with boric acid andphosphates and zinc chloride were added to a fire-retardant version, Celcure F. The product wasvirtually free from corrosion and highly fixed, givingexcellent protection against all fungi except a fewcopper-resistant species, particularly Poria species,but its performance was unreliable against insectsand crustacean marine borers, even at high loadings.

Arsenic was therefore added to form the CCAproduct Celcure A which gives excellentperformance in all respects; this final product will bediscussed later in comparison with other CCAformulations.

Arsenic salts

A second route of CCA development, inScandinavia, involved particularly Bolidenproducts. These preservatives were originallydeveloped as a means of utilizing arsenic wastefrom the iron-ore industries in Sweden. Arsenic is apoison and this has always caused difficulties whenit is used for any purpose; the dangers associatedwith arsenic preservatives will be described later.Arsenic was first proposed as a wood preservativeby Baster in 1730 and there have been severalproposals since, principally for preservativesagainst insect attack. Some fungi are alsocontrolled by arsenic but others are tolerant andcan convert arsenic deposits to the toxic gas arsine.There is no significant danger of arsine productionin wood preservatives, provided that arsenic is usedonly as a component in formulations whichcontain other fungicides capable of controllingthese arsenic-tolerant organisms.

Boliden BIS

Arsenites, derived from arsenic trioxide As2O3, areless stable and less soluble than arsenates, derivedfrom arsenic pentoxide As2O5. Sodium, copper andzinc arsenites have been proposed as woodpreservatives, as well as sodium, copper, zinc andchromium arsenates, but all have now beensuperseded by modern multicomponentpreservatives. The first Boliden preservative systemwas developed before 1932 by Stålhane as a two-stage process; impregnation with sodium arsenitesolution was followed by a second impregnationwith zinc chloride solution, a doubledecomposition resulting in the precipitation of zincarsenite. The sodium arsenite was soon replaced bythe more soluble arsenate but the process was


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rather unreliable. Drying was necessary after thefirst stage to ensure sufficient free volume withinthe wood to accommodate the second-stageimpregnation. An empty-cell treatment wasconsidered for the first stage followed by a full-celltreatment for the second stage but the cost ofdouble treatment was excessive and the processwas abandoned in 1936, to be replaced by a single-stage process developed by Häger. In this processthe reducing properties of the wood were used tochange the valency state of the dichromate in theformulation, in order to achieve fixation of the zincand arsenic components—by formation ofchromates and by attachment to the woodstructure. This product was known as Boliden BISsalt and was still in use in Sweden in 1950. Itconsisted of a mixture of arsenic acid, sodiumarsenate, sodium dichromate and zinc sulphate, theproportions varying slightly over the years of use.Boliden BIS generally performed well in service butthere were some failures and the product wasconsidered to be unreliable.

Boliden S—Boliden S25

Häger therefore developed a new version of theformulation in which he replaced the salts byoxides, omitting virtually all the non-active partsof the formulation and achieving a more toxicproduct which was consequently less expensive totransport. This substitute for the Boliden BIS saltpreservative was known as Boliden S and wasprepared as a paste by mixing zinc oxide andchromium trioxide into arsenic acid (arsenicpentoxide solution). 25% of the zinc was replacedwith copper in Boliden S25 to further improve thepreservative activity but, although both thesepreservatives were given extensive trials in manyparts of the world, they were soon superseded byK33 in which all the zinc was replaced by copper.

Lahontuho K33—Boliden K33

K33 was first marketed in Finland in 1949 asLahontuho K33 and was introduced in Sweden asBoliden K33 the following year, eventually

becoming one of the most widely used CCApreservatives. It was normally manufactured as apaste by mixing copper oxide or carbonate andchromium trioxide with arsenic acid (arsenicpentoxide solution). It was sometimes produced asa concentrated dry powder to reduce transportcosts but the paste version was preferred as itavoided toxic dust problems during thepreparation of preservative solutions. In someareas close to production plants it was alsoproduced as a bulk solution, normally equivalentto 60% of the normal K33 formulation. K33 wasmarketed by several different companiesthroughout the world; it is known as LahontuhoK33 or Häger K33 in Finland, as Boliden K33 inSweden, and as Boliden CCA and Osmose K-33 inNorth America. There are also a number of otherpreservatives which possess similar toxiccomponent ratios such as Koppers CCA-B inNorth America, the latter title reflectingclassification of formulations of this type as CCAtype B in American Wood Preservers’ AssociationStandard P5, as explained later. CCA-Bformulations have been largely withdrawn inrecent years and replaced with lower arsenicsystems, mainly formulations conforming to CCA-C and British Standard BS 4072 requirements.

Falkamesan

In about 1930 Falck and Kamesan developed amixture of arsenates and dichromates. It will beappreciated from earlier comments on Wolmansalts that dichromates were originally incorporatedin preservatives as corrosion inhibitors but that ithad been later observed that they often improvedleach resistance. In this particular development itwas claimed that the dichromate was includedsolely to improve fixation. The ratio of arsenate asAs2O5 to dichromate as CrO3 was studied and theoptimum resistance to leaching was found to occurat a ratio of about 1:0.65 and this ratio wastherefore used in Falkamesan preservative. Thisproduct could be prepared as a mixture of sodiumarsenate and potassium dichromate, or

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alternatively arsenic pentoxide and chromiumtrioxide if it was required to increase the activeconcentration and reduce transport costs. Thepotassium dichromate could be displaced in theoryby less-expensive sodium dichromate but thesodium salt is hygroscopic and the potassium saltwas normally used to ensure a free-flowing powderformulation; these comments apply to allpreservatives of this type.

Ascu—Greensalts (CCA)—Erdalith

Falkamesan had been developed mainly as apreservative against termite attack but in about1933 copper sulphate was added to improveactivity against fungi, resulting in Ascu, the firsttrue copper-chromium-arsenic wood preservative.In its original form Ascu was prepared bydissolving one part arsenic pentoxide dihydratewith three parts copper sulphate pentahydrateand five parts potassium dichromate in 200 partswater, to give a preservative for application bynormal full-cell pressure impregnation. Thetreatment gives the wood a distinctly greenishcolouration and the preservative soon becameknown as greensalt, a general term that is nowapplied to all CCA products, although it has alsobeen adopted as a trademark in the United States.Ascu was marketed in the United States as theoriginal Greensalt, or Greensalt K, whichincorporated potassium dichromate and whichwas first used by Bell Telephones in extensiveexperiments in 1938 in an attempt to developclean and reliable pole preservation treatments.Greensalt S was identical but the potassiumdichromate was replaced by an equivalentamount of sodium dichromate; this product wasalso known as Erdalith. Greensalt O was analternative formulation, patented by McMahon in1948, and prepared by mixing 2.9 partschromium trioxide with 1.35 parts of alkalinecopper carbonate and one part arsenic pentoxidedihydrate, giving approximately the same ratiobetween copper and arsenic but with a slightincrease in chromium.

Celcure A—Tanalith C

Industrial expansion in the United Kingdomafter World War II was dependent on asubstantial expansion in power generationcapacity which involved the rapid constructionof new power stations, many equipped withnatural-draught water-cooling towers. In thesetowers water is sprayed over a stack (or fill) ofspecially shaped and positioned wood slats, toexpose the hot water to a rising draught ofcooling air, the flow being induced naturally bythe parabolic shape of the tower. The fill is thuscontinually exposed to warm water. In addition,there is often a second layer of slats known asthe mist eliminators higher up the tower abovethe sprays to prevent droplets of water frombeing carried out of the tower and causingwinter icing on neighbouring roads. These misteliminator slats generally possess a lowermoisture content than the fill. There is clearly asevere decay risk and cooling tower wood wasoriginally treated in the United Kingdom withthe ACC preservative Celcure or the FCAPpreservative Tanalith U. Deterioration of theTanalith fill slats developed rapidly but it wassoon discovered that this was not due toexcessive leaching, but to the presence ofpreviously unknown fungi which were namedSoft rots as they attacked the external surfaces ofthe wood, causing softening to a progressivelyincreasing depth. There were also reports ofground-line failure in Tanalith U transmissionpoles in South Africa, again apparently due tosoft rot attack. In contrast the Celcure fill slatswere in sound condition, although some of themist eliminator slats were decayed by copper-resistant Poria fungi. The British manufacturersof Celcure and Tanalith U were aware of thereports of the excellent performance of greensalttreatments in the United States and bothintroduced new treatments of this type. Celcurebecame Celcure A through the addition ofarsenic which improved effectiveness againstinsects as well as against copper-resistant fungi.


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Tanalith U was replaced with Tanalith C, anentirely new CCA preservative, developedfollowing extensive studies of the Bell Telephonetrials with greensalt preservatives based on theoriginal Ascu formulation.

CCA preservatives

CCA preservatives such as Celcure A and TanalithC have proved to be very reliable in service andthere remain only minor criticisms of thesesystems. Firstly, the moisture content of thetreated wood can fluctuate in service in the sameway as for untreated wood and there is atendency for checks and shakes to develop whichmay penetrate through the treatment if it isconfined to the relatively permeable sapwood.This problem would be insignificant if treatmentcould be restricted to only permeable species suchas Corsican pine or species such as Scots pine inwhich non-durable but easily treated sapwood isassociated with relatively durable heartwood.However, in species such as spruce which combineimpermeability with a lack of durability CCAtreatment is unrealistic, as it fixes too rapidly andis unable to improve its zone of protection bydiffusion so that splits may expose non-durableheartwood. Thirdly, some deterioration has beenobserved in hard-woods, even when treated athigh retentions, apparently through poormicrodistribution of the toxic components whichdo not fully protect the cell walls although theymay be present in large amounts in the pores orvessels. Finally, there are criticisms of the toxicityof CCA preservatives, particularly in relation totheir arsenic content, but these criticisms arecommon to other preservative systems and theywill be discussed later.

There are now many copper-chromium-arsenic(CCA) wood preservatives, although they alloriginate from the three development routes thathave been described. Within the CCA group thecopper, chromium and arsenic components arepresent in varying proportions and incorporatedvariously as oxides or salts. To add to this

confusion the standard specifications define theseproducts in terms of an arbitrary selection ofoxides or salts, whatever the actual compoundthat is incorporated in the individual formulation.Thus the copper content is expressed as CuSO4–5H2O in British Standard 4072 but as CuO in theAmerican Wood Preservers’ Association StandardP5, whatever the nature of the compoundincorporated in the actual formulation. It mightappear to be logical to describe preservatives interms of their toxic elements alone, a system thathas been used in Scandinavia and which certainlyenables the ratios of the toxic elements to bereadily compared, but it appears that if the activeelements are described in terms of their equivalentoxide content, the total preservative activity ofthe formulation can be judged from its total activeoxide content, thus enabling both the ratios of theactive components to be compared and theprobable preservative activity of an individualformulation to be assessed without difficulty.

For many years the American WoodPreservers’ Association specifications definedcopper-chromium-arsenic wood preservatives interms of their equivalent salt contents as BritishStandard 4072 but this system was abandoned in1969 and preservatives have since been definedin terms of their equivalent active oxidecontents:

AWPA CCA Type A Type B Type C

Hexavalent chromium 65.5 35.3 47.5 CrO3

Copper 18.1 19.6 18.5 CuOArsenic 16.4 45.1 34.0 As205

According to the specification the hexavalentchromium can be incorporated as potassium orsodium dichromate, or as chromium trioxide.The bivalent copper can be incorporated ascupric sulphate, basic carbonate, oxide orhydroxide. The pentavalent arsenic can beincorporated as sodium arsenate orpyroarsenate, arsenic acid or arsenic pentoxide.

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Mixtures of salts involve significantconcentrations of inactive ions such as sodiumand sulphate and these reduce the effectiveconcentration of the preservative, which isexpressed as a percentage indicating the activeoxide content. For example, a BS 4072 type 1preservative such as Celcure A has an activeoxide content of 61.1% whereas a type 2preservative such as Tanalith C has an activeoxide content of 59.05%. There are variousproducts that comply with this British Standardbut, as they are not actually manufactured fromthe salts listed in the specification, their activitymay not be 100%. Some have an effectiveactivity of less than 100% but K33 paste wouldhave an effective activity of 125% according tothis British Standard if it complied with thestandard in terms of the ratios of the activeelements. In contrast, K33 paste has an activecontent of 75.4% according to the AWPAsystem as this is the concentration of activeoxide actually present in the formulation, theinactive component consisting solely of water.

Greensalt—Langwood—Boliden CCA—Koppers CCA-B—Osmose K-33—Crom-AR-Cu (CAC)—Wolman CCA—Wolmanac CCA

In the American specification, CCA type Arepresents the original Ascu type formulationsand includes Greensalt and Langwood whichpossess a high chromium content. CCA type Bwas introduced solely for K33 type formulationsand includes Boliden CCA, Koppers CCA-B andOsmose K-33. CCA type C includes productssuch as Crom-Ar-Cu (CAC), Wolman CCA andWolmanac CCA are similar to BS 4072preservatives. The American specificationtolerates some variation from the nominal ratiosdefined above and this permits productsconforming with both types 1 and 2 in theBritish specification to be formulated to meetalso the American CCA type C specification.

It will be appreciated that considerable care

must be taken in interpreting retention figuresin different countries. A British Standardretention of 10 kg/m3 (0.6 lb/ft3) means that thisretention must be achieved in terms of the saltmixture defined in the specification, a systemthat was originally used in America and whichis still widely used in some other countries. ThisBritish Standard preservative has an activeoxide content of about 60% so that a retentionof 10 kg/m3 is equivalent to a retention of 6 kg/m3 in terms of the AWPA specification. In theNordic countries and many other areas it isconventional to define retentions in terms of thepreservative as supplied, so that a K33retention of 10 kg/m3, based on paste with anactive oxide content of 75.4%, would beequivalent to 7.54 kg/m3 in terms of the AWPAspecification; K33 was also available as a drypowder product with a higher active oxidecontent, and as a solution with a K33 contentof 60% and active oxide content of only about45%. In order to avoid confusion it isrecommended that all specifications shouldexpress retentions in terms of active oxidecontent as in the AWPA system as this alsoindicates the relative effectiveness of theproduct, and the contents should be clearlystated on all literature and labels.

Unfortunately, this system would notcompletely avoid confusion. In the BritishStandard retentions are expressed as overallretentions although only the sapwood is treatedin species such as Scots pine in which theheartwood is relatively impermeable. Thissystem has now been abandoned in the Nordicspecifications, with overall retentions nowreplaced by sapwood retentions which can berealistically controlled and readily checked byanalysis. European redwood or Scots pine fromthe Baltic area currently contains an average ofabout 50% sapwood so that a sapwoodretention of 10 kg/ m3 is equivalent to an overallretention of about 5 kg/m3, although it must beappreciated that the overall rtention will varydepending on the percentage of heartwood.


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Tanalith C (Tancas C)—Tanalith CCA—Tanalith CT106—Tanalith Plus

Tanalith C with an active oxide content of 59%is known as Tancas C in some areas such asFinland. Tanalith C (CT106) is a concentratedversion of this product containing about 62%active oxides, but another concentrated productTanalith CCA contains about 72% active oxideso that it can be used at the same retentions asK33. Tanalith Plus is the name applied when anemulsion additive is used in an attempt toimprove the moisture resistance of the treatedwood. The emulsion additive is prepared bydissolving waxes and a surfactant in a solvent,and it is then added at about 2% to a normalTanalith C treatment solution. The emulsiontends to be unstable, through the low pH and theoxidizing properties of the hexavalent chromiumin the preservative.

Celcure A—Celcure AP

Another interesting development was Celcure AP,a paste version of Celcure A which avoids toxicdust problems during solution preparation. Thisformulation is manufactured from cupric oxide,copper sulphate, sodium dichromate and arsenicpentoxide, a small amount of water being addedto adjust the total active oxide content to 59.7%.The oxide contents are closely similar to those forCelcure A and this product therefore conformswith British Standard 4072 type 1 without theneed for any concentration adjustment.

Celcure A—Celcure AN—Tanalith C—Tanalith CA—Tanalith NCA

The situation in New Zealand illustrates the furtherdevelopments that are possible. The original Celcurecopper-chromium (ACC) preservative was used at firstbut was found to be unreliable against insects andcopper-tolerant fungi. Boric acid was added but, whilethis improved the insecticidal activity, it wassignificantly leachable and did not markedly improve

the performance against the copper-tolerant fungi. Thissituation coincided with the problems in water-coolingtowers in the United Kingdom, which resulted in theaddition of arsenic and the development of Celcure Awhich was approved in New Zealand in 1959; thesimilar competitive product Tanalith C was introducedat about the same time. In 1961 the arsenic content inTanalith C was increased to give Tanalith CA so thatlower retentions could be used for the treatment ofRadiata pine in buildings where the principaldeterioration hazard was considered to be insectattack, Common Furniture beetle being the mostwidespread hazard but termites representing the mostsevere deterioration problem in some areas. In 1966Tanalith NCA was introduced; in this the coppercontent was increased at the expense of the chromiumcontent so that a lower retention could be used for thetreatment of wood in ground contact and where thereis also a risk of insect infestation. Celcure AN, a high-copper formulation similar to Tanalith NCA, wasapproved in 1967, although it was not introduced until1969. Boliden S25 was also used in New Zealand,although it was replaced in 1967 by Boliden K33.These various CCA preservatives are compared inTable 4.1.

The main hazard in buildings is insect borerattack and it will be seen that the approvedretentions therefore relate to the arsenic content,apparently designed to achieve a retention of about0.9 kg/m3 As2O5. For wood in ground-contact suchas poles, piles and fence posts, the main risk is fungaldecay and it will be seen that the approvedretentions, which are based on experimental resultsin laboratory tests and long-term stake trials, relateto the total active oxide content of eachformulation, although the Tanalith CA retentionis slightly higher than might be expected from thisrelationship. The use of the high arsenicformulations cannot be recommended as theexcess arsenic is not fixed and, when used atlower retentions where insect attack is the mainnormal hazard, the fungicidal protection may notbe adequate if accidental wetting should occur andthere is then a danger that arsenic-resistant fungimay generate toxic arsine gas.

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After-glow suppression—3.S CCA—3.0CCA—Celcure AG—Tanalith AG

Normal CCA preservatives used for fence posttreatments in Australia have been found to sufferfrom after-glow or continuing charring followinggrass fires. The Government CSIRO laboratorieshave developed and patented a modificationinvolving the addition of zinc oxide and phosphoricacid to a normal CCA formulation, to give type 3.SCCA based on salts, or type 3.O CCA based onoxides and containing 7.32% Cu, 10.94% Cr,10.28% As, 3.93% Zn, and 4.61% P; Celcure AGand Tanalith AG are commercial formulations ofthis type. These modifications are not new—Gunnadded zinc and phosphorus to the original Celcure(ACC) formulation to form Celcure F, the fireretardant version.

CCA and ACC fixation

There have been various explanations of thefixation of copper-chromium and copper-chromium-arsenic preservatives. In pine it isprobable that some of the copper reacts with or

condenses on cellulosic wood components,probably forming a copper-cellulose complex.The remaining copper reacts with dichromate toproduce mixed copper chromates. Excessdichromate is reduced from the hexavalent totrivalent state and then reacts with any arsenicpresent or, if arsenic is absent, it is absorbed ontothe wood. Arsenic is fixed principally bytrivalent chromium, probably as CrAsO4,although some arsenic may be absorbed onto thewood elements. At high arsenic concentrationssome may be precipitated as copper arsenate. Atlow solution concentrations, absorption by thewood elements may be the major fixationprocess. The influence on fixation, of changes inthe toxic component ratios, has been examined.The ratio of CrO3 to As2O5 must exceed 1.5 toensure arsenic fixation but if the ratio exceeds 2the excess chromium is wasted. The ratio ofCrO3 to CuO should be at least 2. In order toensure maximum fixation the toxic elements in aCCA preservative should be present at ratios ofabout 41–50% CrO3, about 17% CuO andabout 42–33% As2O5. These optimum ratios are

TABLE 4.1 Comparison of CCA preservatives


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met in CCA-C and BS 4072 formulations; as aresult CCA-A formulations are declining in useand CCA-B formulations such as K33, with theirexcessive arsenic contents, have been withdrawnin most countries.

The copper-chromium-arsenic or CCAformulations are the most reliable general-purpose preservatives currently available,providing excellent protection against all typesof fungal and wood-borer deterioration. Therapid fixation remains a problem as it limits thefurther diffusion of the preservative when it isused to treat relatively impermeable species andit is still frequently claimed that the WolmanFCAP preservatives, with their slower fixation,are more reliable in these circumstances. In fact,the FCAP preservatives are not so well fixed, andthe fluoride components are slowly lost ashydrogen fluoride. Some of the formulationswhich were described earlier, and which rely onammonia or acetic acid loss for fixation, cancombine all the advantages of the ACC and CCAformulations, with deeper penetration inimpermeable woods and more uniform micro-distribution in hardwoods.

Arsenic toxicity

The arsenic content in CCA, FCAP, CAA, ZAA,ACA, ACZA and similar formulations is aserious problem. It is unfortunately true thatcattle have been poisoned as a result of lickingtreated transmission poles and fence posts butthis normally occurs only in areas where there isa natural salt deficiency and the danger can becompletely avoided by providing proper saltlicks and using only highly fixed preservatives.Arsenic preservatives are banned in buildings insome countries such as Finland, yet in othersthey are happily used even for the treatment ofplayground equipment. In Switzerland, arsenicpreservatives are banned for the treatment oftransmission poles as they may introduceenvironmental pollution. In most countries thesehazards are considered to be insignificant with

CCA preservatives in view of their excellentfixation (although fixation is only reliable withCCA-C and BS 4072 formulations), and themain fears are related to the possiblevolatilization of arsenic when treated wood isdestroyed by burning.

Yet another fear is the danger of arsinepoisoning. In about 1890 several fatalities occurredin homes and these were eventually attributed toarsine poisoning. Wallpaper, decorated witharsenical dyes, had been attacked by the fungusScopulariopsis brevicaulis, at that time known asPenicillium brevicaule, it is now known that otherfungi can generate arsine in this way. There is nodanger of arsine poisoning from preserved woodprovided that the fungicidal components are ableto control the fungi that generate arsine. Ifpreservation is required against insect attack alone,as for the treatment of timber in accordance withthe Australian Quarantine Regulations or forFurniture Beetle control in New Zealand, there isclearly a temptation to use just a simple arsenicalpreservative, and there is then a danger of arsinepoisoning if fungi are able to develop. Manyinsects are dependent on or encouraged by thepresence of fungal attack, and fungicidalcomponents in multicomponent formulationstherefore assist in their control. This factor iscompletely ignored in the Australian and NewZealand situations where the minimum retentionsof approved preservatives are based solely on anarsenic retention of about 0.97 kg/m3 As2O5.

More recently there have been fears in theUnited States concerning the carcinogenic dangersassociated with the arsenic contents in woodpreservatives. There have been extensive enquiriesand the current evidence suggests that thecarcinogenic properties are largely associatedwith arsenic trioxide As2O3 and arsenites ratherthan with the arsenic pentoxide As2O5 and thearsenates that are used in modern preservativeformulations. The carcinogenic dangersassociated with arsenical wood preservatives areslight, but arsenic pentoxide is prepared from thetrioxide and increasing controls on the latter

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compound are likely to introduce manufacturingdifficulties, perhaps leading to scarcity andincreased cost. The dangers associated with fixed-arsenic wood preservatives are often exaggerated,sometimes by manufactures of competitiveproducts; the enquiries in the United States wereprompted by the cement and concrete industrywhich feared the competition of pressure-treatedwood foundations in domestic construction!

Arsenic dangers always attract the mostattention but chromium may represent thegreatest hazard in wood preservation. If properlyformulated preservatives are used in a competentand responsible manner the dangers are veryslight and this is confirmed by the very low levelof illness or injury in this long-establishedindustry but it must be recognized that dangersexist. Tropical conditions discourage the use ofprotective clothing and operatives then frequentlysuffer from chrome ulcers which are painful anddifficult to heal, but it is interesting to note thatthere are no arsenic problems, and the injuriessustained are an indication of poor plant hygieneand control rather than serious criticism of thepreservative formulations involved.

Copper-chromium-boron—Wolmanit CB(Ahic CB)

There have been various criticisms of copper-chromium-arsenic or CCA preservatives,particularly with regard to the possible dangersassociated with their arsenic content and theproblems that arise in the treatment of relativelyimpermeable woods through their rapid fixation.There have been various attempts to overcomethese difficulties, the best known being thereplacement of arsenic by boron to give acopper-chromium-boron or CCB formulation.Combinations of chromium and boron were firstproposed by Wolman in 1913 and the earlydevelopment of Celcure included copperformulations in which borates were used in placeof dichromates. It is often suggested that theseideas were first combined in the CCB

preservative Ahic CB, subsequently renamedWolmanit CB, which was first marketed inGermany in 1960, although the CCBpreservatives were actually first developed byKamesan in India during World War II. Thetoxic components in Wolmanit CB are atconcentrations equivalent to 10.8% CuO,26.4% CrO3 and 25.5% H3BO3.

The development of CCB preservatives wasseverely criticized particularly by manufacturesof CCA preservatives, as it was evident fromlaboratory experiments that the boron waslargely unfixed, although this disadvantage isoffset in actual service by the deeper diffusion ofboron that is achieved in resistant heartwood—avery important factor where heartwood is non-durable, as in spruce. Trials on poles in servicehave given very good results and it has beenfound that CCB preservatives performparticularly well on impermeable species such asspruce; good penetration is achieved through thecontinuing diffusion of the borate but leaching inservice is limited by the low permeability.However, the Nordic-recommended retentionsbased on stake trails suggest that on Scots pinethe performance in ground contact depends onthe cupric oxide and chromium trioxide contentsalone, the borate content making no significantcontribution in this test situation, although thetest stakes are relatively small and the leachingof unfixed components is exaggerated comparedwith service performance in larger section poles.

Tanalith CBC—Celcure M

Other manufacturers produce CCB productssuch as Tanalith CBC and Celcure M, but theytend to be relatively expensive because of thehigh retentions that are demanded by someapproval authorities, and they are usuallypromoted in markets such as Switzerland wherethere is resistance to the use of CCApreservatives because of the arsenic content.Slower fixation enables CCB preservatives to beapplied by the Boucherie sap displacement


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method, and the development of Wolmanit CBstimulated considerable improvements in thedesign of the caps that are used for this process.

Celcure N—Tancas CC

As the boron content in CCB preservatives doesnot significantly contribute to ground contactperformance, at least in the Nordic small-staketrials, the best technique to avoid the criticisms ofCCA preservatives might be to omit the arsenicand return to the old copper-chromium (ACC)formulation now usually known as CelcureN.Tancas CC has been introduced in this way as analternative to Tancas C (CCA), and such changesare certainly realistic in situations where there is nosignificant risk of insect borer attack or resistantfungi such as Poria species. The original Celcureformulation is still extensively used with completesuccess in the Netherlands, Sweden and parts ofthe United States; it is particularly suitable foragricultural purposes as it is completely free fromthe livestock poisoning dangers associated withpreservatives containing arsenic.

Copper and zinc borates

Another obvious solution is to develop boratefixation systems. Ammoniacal copper boratesystems were first proposed in about 1965 and werefurther developed (by the author’s laboratory) inabout 1978. They have never been extensivelyadopted, probably mainly for commercial reasons;although the ammonia liberated during the finalvacuum stages of impregnation and duringsubsequent slow fixation is unpleasant and difficultto control, a problem associated with allpreservative systems which fix by ammoniavolatilization. An alternative simpler system (alsodeveloped in the author’s laboratory) uses mixturesof copper or zinc with boric acid and fixes throughthe volatilization of a small amount of acetic acid;the ratios can be adjusted to form several alternativeborates but the best resistance to leaching isachieved using an excess of copper or zinc.

Boliden P50

Boliden P50 was developed in Sweden as anarsenic-free replacement for Boliden K33 with thearsenic pentoxide replaceded partly byphosphorus pentoxide and partly by an increasein the cupric oxide content. The currentlyrecommended Nordic retention suggests that itsperformance depends upon the total oxidecontent, including the phosphorus pentoxide.Whilst this preservative can be expected toperform well in normal ground-contactconditions it is not clear that it has any advantageover simple copper-chromium formulations; theremust be doubts about its activity against borersand it is known that phosphates actuallyencourage the development of some stain fungi.

Cuprinol Tryck (KPN)

Cuprinol Tryck, originally Cuprinol KPN, wasdeveloped by Häger as an alternative to bothCCA products and his earlier KP Cuprinolsystem which will be described later. This is anammoniacal formulation; copper carbonate isdissolved in caprylic acid and ammonia is addedto form cuprammonium caprylate.

Basilit CFK

Basilit CFK is another ammoniacal formulation inwhich the toxic elements are chromium, fluorineand copper as indicated in the name. Thisformulation consists of a mixture of very solublecopper hexafluorosilicate, ammonium dichromateand a small amount of diammonium hydrogenphosphate. The Nordic retention recommendationssuggest that only the cupric oxide and chromiumtrioxide contents contribute to the preservativeaction in ground-contact conditions and anycontribution from the fluoride is insignificant. Thispreservative thus possesses the same disadvantagesas other copper and copper-chromium systems,such as low activity against insect borers andcopper-resistant fungi, but it achieves more

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efficient penetration than the rapid-fixing copper-chromium (ACC) and copper-chromium-arsenic(CCA) systems.

BFCA salts

The BFCA salts developed in Australia containboron, fluorine, chromium and arsenic. Theywere originally introduced in 1955 but thecurrent formulation dates from about 1963.Wood is treated by immersion in concentratedsolutions of the salt mixture, which can thenpenetrate by slow diffusion.

Diffusion treatments in plywood andpoles

While referring to Australia it is worth mentioningseveral special diffusion techniques. Plywood canbe protected against insect attack by addingsodium arsenite to the alkaline phenolic adhesive;sodium arsenate or arsenic pentoxide can be used,and may be easier to add as they are more soluble,but they are much more expensive. Adequateprotection is obtained at a retention of 0.8 kg/m3

As2O3 and veneers up to 2.5 mm are completelypenetrated. However, this treatment cannot berecommended as it gives no significant protectionagainst fungal decay and, if the plywood is wettedthrough any cause, there is a danger that arsenic-resistant fungi will develop which may generatetoxic arsine gas. Copper hexafluorosilicate, thevery soluble compound used in Basilit CFK, can beused at high concentrations for injection into polesas a remedial treatment to control progressiveheart rot—the initial rot ensures that gooddistribution is achieved whilst the treatment willensure that no further damage occurs. Thiscompound is relatively inexpensive as sodiumhexafluorosilicate is available in Australia as awaste from processing phosphate rock for thefertilizer industry.

Double diffusion

It will be appreciated from the earlier descriptionof the decay of treated wood in cooling towers

that this deterioration represented a serious andcontinuing problem. The Soft rot damage wasinitially superficial but progressively increased indepth on the relatively thin-section fill slatswhilst, in the Celcure-treated towers, copper-tolerant fungi were attacking structural membersand the mist eliminators. The Soft-rotted zonewas very permeable and was used withconsiderable success in conjunction with adouble diffusion remedial treatment processdesigned to halt further decay. The towers wereoperated in order to ensure that the fill wasthoroughly wetted and it was then flood-sprayedwith copper sulphate solution which diffusedinto the water in the Soft-rotted layer. Aftersufficient time had been allowed for deepdiffusion to occur a second application, ofsodium chromate, was made to precipitate thecopper as copper chromate.

Another double-diffusion treatment that isworth mentioning and which was originallydeveloped in the United States during World WarII is currently in use in Papua New Guinea as it isa relatively simple treatment, suitable for use indeveloping countries. Dry wood is immersed in a3% copper sulphate solution at about 88°C(190°F) for a period of seven hours and then thesolution, still containing the wood, is allowed tocool for about sixteen hours. The wood isremoved, any excess copper sulphate on thesurface is rinsed away, and the wood is thenimmersed for 48 hours in a 10% sodium arsenatesolution. The process was modified by using amixture of sodium arsenate and dichromate forthe second treatment to improve fixation. It isalso possible to treat in the same way usingsodium fluoride followed by copper sulphate.

Boron compounds

Perhaps the earliest record of the use of boron inwood preservation is in the chromium-boronpreservative developed by Wolman in 1913.Twenty years later boron was proposed as acomponent in Celcure, principally as a replacement


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for dichromate in the flame-retardant formulationCelcure F.Borates possess fungicidal, insecticidaland flame-retardant properties, and their use inmulticomponent preservatives such as CCB and thevarious copper and zinc borate systems has alreadybeen described. As borates combine bothinsecticidal and fungicidal properties they areefficient preservatives even when used alone,although they are not considered suitable for usewhen treated timber will be subjected to leachingor ground contact conditions. This restriction isbased on the observation that the preferred boratesare soluble, but it ignores the fact that borates canachieve exceptional penetration and, that when thesodium ions are neutralized by atmospheric carbondioxide, the final boric acid deposit possesses verylow solubility at normal temperatures.

Timbor

Borates can be applied by normal impregnationmethods but their greatest value is in diffusiontreatments. Freshly felled and converted greenwood with a moisture content in excess of 50% istreated with the borate preservative by immersionor spray, and the treated wood is then close-stacked and wrapped or placed in storage roomsto prevent moisture evaporation, thus allowingthe borate treatment to diffuse deeply. Boric acidand sodium tetraborate (borax) are insufficientlysoluble but much higher concentrations can beachieved if a solution is prepared using 1 partboric acid to 1.54 parts sodium tetraboratedecahydrate; this mixed solution is dried toproduce Polybor, known as Timbor when used asa wood preservative and correspondingapproximately to disodium octaboratetetrahydrate Na2B8O13–4H2O which has a boroncontent equivalent to 117.3% H3BO3.

Diffusol

Despite this high boric acid equivalent andexcellent solubility it is still necessary to heat thesolutions to maintain the required concentrations,

which vary with the thickness of the wood to betreated, so that the retention is uniform whateverthe cross-section area. Thus in 25 mm (1 in) thickwood a minimum solution concentration of 20%H3BO3 is required and a minimum temperature of40°C (104°F) is therefore necessary, whilst at 75mm (3 in) thickness the required concentrationincreases to 40% and the temperature to 57°C(135°F). Timbor diffusion has been widely usedthroughout the world but the minimum storageperiod of 4 weeks per 25 mm (1 in) thicknessnecessarily involves considerable capital cost andhigh interest rates have discouraged the use of thesystem in many countries, although it remainsextremely attractive for the treatment of non-durable tropical hardwoods in developingcountries. Diffusol is a thickened borate treatmentwhich can achieve adequate surface loadingswithout heating.

Timbor rods—Boracol 20—Boracol 40—Trimethyl borate (TMB)

Sodium octaborate tetrahydrate is also available asTimbor rods which can be inserted in drilled holes inwood at risk such as window frames, joist and beamends in damp external walls, and even poles andsleepers (ties), producing a preservative solution ifthe wood becomes wet. Boracol 20 and 40 areconcentrated borate solutions containing 20 and40% disodium octaborate tetrahydrate respectively;they are generally used as alternatives to Timborrods by injection into drillings in wood componentssubject to severe fungal decay hazard, usually asremedial treatments. Trimethyl borate (TMB) is avery volatile compound which can be applied as avapour-phase treatment to achieve deep penetrationin woods which are impermeable to normal liquidtreatments; subsequent steaming hydrolyses theTMB to deposit boric acid in the wood.

Borester 7

Borates are particularly useful as treatments forhardwoods that are susceptible to Lyctid beetle

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attack as they are effective at very lowconcentrations, applied by immersion or spray;boric acid solution is frequently used for thispurpose in Australia. Borates are also extremelyeffective in the control of stain fungi, althoughthey are most effective as alkaline high-pHtreatments so that sodium tetraborate is morereliable than boric acid or the highly solublemixture. However, they are relatively inefficientagainst superficial moulds such as Penicilliumand Trichoderma species so that they must beused in combination with other toxicants, suchas sodium pentachlorophenate, as described laterin this chapter. Borate esters enable boric acid tobe used in organic-solvent formulations;hexylene glycol biborate, Borester 7, is mostextensively used in this way.

Chlorophenates

Sodium pentachlorophenate is well known aloneor in combination with borates as a sapstaincontrol treatment but it can also be used as acomponent in wood preservative formulations. Itsuse as an alternative to dinitrophenol in WolmanFCAP salts has already been mentioned but it alsoforms the basis of several two-stage treatments inwhich copper or zinc pentachlorophenates aredeposited. For example, copper sulphate andsodium pentachlorophenate solutions can beadded to fibre-board to precipitate copperpentachlorophenate, an organometalliccompound which will be considered later andwhich is also the active component in KP-Cuprinol and several other preservatives.

There are hundreds of different water-bornesimple salt and multicomponent preservativescurrently on the market throughout the world orwhich are historically significant. It is impossibleto mention them all in a brief description and itwould, indeed, serve little purpose as there arecontinuous changes. This description is thereforerestricted to a few of the most importantformulations in an attempt to define the generalprinciples involved. Multicomponent water-borne

preservatives are reliable and economic. They canachieve excellent fixation and provide permanent,clean and safe treatment of wood. Their maindisadvantages are their failure to control changesin the moisture content in wood so that there is adanger that checks and shakes may developthrough the preserved zone if the penetration islimited. Some of the components used,particularly arsenic, are very toxic but excellentfixation ensures that treated wood is entirely safewith properly designed formulations. There areseveral other minor criticisms of water-bornepreservatives which should be mentioned. Theelectrical insulation value of treated wood isimportant for transmission poles and railwaysleepers (ties). With CCA preservatives wood hasa relatively low electrical resistance when freshlytreated but this increases steadily with drying; theresistance is never as high as with creosotetreatment but with oxide preservatives it isgenerally the same as for untreated wood,although with salt preservatives it may be less andperhaps too low for track signalling systems insome sleeper (tie) treatments.

4.4 Organic compounds

Historically, the most important organicpreservatives were the fractions obtained by thedistillation of tar, principally from coal;preservatives of this type have already beendescribed in detail in Section 4.2. It was observedthat changes in the composition of these distillatesaffected preservative performance, apparentlythrough changes in the concentrations ofindividual components. At first, changes incomposition were caused mainly by processing, toremove compounds that were useful for otherpurposes, but there were later attempts to controlprocessing to achieve the most reliable creosotewood preservative. There were subsequentlyattempts to fortify tar-oil products, either byprocessing such as by the chlorination used inCarbolineum Avenarius, developed about 1888,


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or by the addition of toxicants, such as the coppersalts added by Nordlinger about 1900, and thearsenic compounds that are today added tocreosote in Australia to enhance insecticidalproperties. Attempts to replace creosote withpetroleum distillates of a similar physical naturewere unsuccessful as it was soon discovered thatthey lacked preservative properties, and toxicantsneeded to be added; pentachlorophenol in heavyoil has been extensively used in the United Statesas an alternative to creosote.

The addition of toxicants to heavy petroleumoil was originally an attempt to find alternativesto creosote at times of scarcity. While the newformulations were designed to possess all theadvantages of creosote in terms of toxicity andeven the limited volatility that is so valuable instabilizing the moisture content in treated wood,they also retained the disadvantages such as thedirty appearance and the tendency to bleed,largely as a result of the selection of acomparatively crude and inexpensive solvent.True organic wood preservatives consist of asolution of toxicants in volatile solvents and giveperfectly clean treatment if the solvent iscompletely volatile and the toxicant is colourless.The preservative action depends solely on thepersistent toxic deposit and the solvent has nopreservative action, however expensive it may be.The use of such solvents can be justified onlywhen water-borne preservatives are unacceptable.The main problem with the use of water is theswelling that it causes as this is unacceptable forworked joinery (millwork); even if the treatedwood is dried as there is still a danger ofpermanent distortion. For such purposes the extracost of an organic solvent can be justified andworked joinery now constitutes the main marketfor organic solvent-based preservatives.

Organic solvents—proprietary advantages

Whilst the organic solvent may not contributedirectly to the preservative properties it iscertainly of considerable importance. Non-polar

light petroleum distillates have low viscosities andare able to penetrate rapidly into dry wood sothat they are particularly suitable for use inpreservative formulations that are designed forsuperficial application by brush, spray orimmersion. However, it is not sufficient to achievedeep penetration by the preservative formulationas the subsequent volatilization of the solventmay cause the toxicants to return to the surfacewhere they will be particularly susceptible tolosses by volatilization and leaching. Co-solventsare frequently used, sometimes to speed up thesolution of the toxicants during manufacture, butoften to provide high-viscosity residues or tailswhich will retain the toxicants whilst the carriersolvent volatilizes, ensuring proper distribution.In some cases the use of co-solvents distinctlyreduces the apparent toxicity of an activecomponent, perhaps due to deeper distribution ora protective action which also ensures greaterpersistence and a longer effective life. Whilstmany products may be based on the sameconcentration of a particular toxicant, theirperformance may vary widely and it must not beassumed that apparently similar proprietoryproducts achieve similar performance.

Nitrated compounds

Nitration may increase the fungicidal activity ofcompounds such as phenol, cresol, xylenol,naphthol and anthranol. Various nitratedcompounds were proposed as wood preservativesin the 19th century but few were adoptedcommercially as tests indicated that high retentionsof, for example, dinitrophenol or dinitro-o-cresolwere required when they were used alone.

Raco—Antinonnin—Antingermin—Mykantin

Raco consisted principally of nitrated phenolswhilst Antinonnin, which was introduced in 1892but still in use in 1913, consisted of a mixture ofpotassium dinitro-o-cresolate, soft soap and

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water. In Antingermin copper was used incombination with dinitro-o-cresol. However, themost important use of nitrated organiccompounds was in combination with salts in theWolman-type preservatives as described in Section4.3. Whilst there have been many recent attemptsto develop in-situ groundline treatments to extendthe life of treated poles, it is interesting to notethat a sodium dinitro-phenate formulation knownas Mykantin paste was developed by Falck forthis purpose as long ago as 1912.

Chlorinated compounds—chlorophenols

There were many attempts to enhance thepreservative activity of creosote by chlorination,stimulating interest in chlorinated phenols,although they were not seriously considered aswood preservatives until much later whenHatfield in the United States completed anassessment of their activity. He first investigatedtetrachlorophenol and later pentachlorophenol,whereas Iwanowski and Turski at the same timeinvestigated di- and trichlorophenols. By 1935Hatfield was able to review the properties of thecomplete range of chlorophenols and chloroo-phenylphenols, as well as their sodium salts. Itwas found that higher chlorination generallyincreased the fungicidal activity but also themelting point, giving more active and morepersistent preservatives. Following completion ofthese studies, pentachlorophenol was introducedfor exterior joinery (millwork) preservation inabout 1936, and within a few years a 5% solutionof pentachlorophenol was widely accepted as thestandard organic-solvent formulation.

Pentachlorophenol solutions have beenprepared using a wide range of solvents. A heavypersistent solvent is often selected where thepreserved wood will be exposed to groundcontact or severe weathering, as this will protectthe pentachlorophenol from leaching and willtend to stabilize the wood as with a creosotetreatment. Volatile solvents are employed whenthe preservative is intended for internal use or

where the treatment must be paintable. Generally,co-solvents must be added, perhaps in order toobtain the necessary solvency power but also asanti-blooming agents to prevent the migration ofthe pentachlorophenol to the surface as the lightcarrier-solvent volatilizes. These anti-bloomingagents are normally non-volatile such as dibutylphthalate or trixylyl phosphate but paintability isimproved if solid co-solvents are used such asrosin esters. The non-volatile co-solvent content isparticularly important as it has a profoundinfluence over the life of the preservationtreatment as it largely determines the distributionof the pentachlorophenol and perhaps physicallyprotects it from volatilization.

Phenylphenol

The water-soluble sodium, and occasionallypotassium, salts of the chlorophenols are alsoextensively used, principally as sapstain controltreatments for freshly felled green wood but alsofor masonry sterilization associated with remedialtreatment against Dry rot. Sodiumpentachlorophenate dust or solution spray isdistinctly irritant and there have been severalattempts to develop more pleasant alternatives.Sodium o-phenylphenate has largely replacedsodium pentachlorophenate in remedial treatmentalthough it is little used in sapstain control; it ismore expensive but less effective than sodiumpentachlorophenate. Phenylphenol was originally aby-product of the production of phenol by theDow process which has now been replaced by thecumene process, except in Poland. Some o-phenylphenol is synthesized in the United Kingdombut Poland is now the only country in which thiscompound remains available at a realistic cost thatenables it to be used in stain control.

TC oil

While referring to phenol production it is perhapsworth mentioning that phenol itself is too solubleand too volatile to be used in wood preservation but


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the cumene process generates a phenolic residue,known in the United Kingdom as TC oil, which isvery similar to creosote in its physical properties. Itis a particularly useful preservative for exteriortimbers such as fencing, as it has excellent colourretention properties. It is also free from thecarcinogenic polycyclic hydrocarbons that limit theuse of creosote. It is therefore surprising that thesephenolic residues have not been proposed as woodpreservatives in other countries where phenol isproduced by the cumene method.

Although sodium pentachlorophenate isextensively used for sapstain control treatment,pentachlorophenol is relatively inefficient. Thisdifference can be attributed to the high pH of thesodium pentachlorophenate solution which is alonesufficient to inhibit stain and can achieve completecontrol in the presence of limited amounts oftoxicant. The addition to sodiumpentachlorophenate of a buffer such as sodiumcarbonate, to maintain the high pH, considerablyenhances the apparent stain control activity. Asuitable phosphate can function as a buffer in thesame way but the residual phosphate will encouragethe development of surface mould. A borate bufferis therefore preferred as it also contributes to thetoxicity of the formulation; borate alone is sufficientto control the stain and the sodiumpentachlorophenate is needed only to controlsuperficial moulds which are resistant to boron.

Pentabor

One of the most effective formulations consistsof 1 part sodium pentachlorophenate with 3parts sodium tetraborate decahydrate (borax)which can be used at the same concentration asthe sodium pentachlorophenate alone. Thisformulation generally achieves improved staincontrol at lower cost and considerably reducesany dangers associated with the sodiumpentachlorophenate, since this is reduced to onlya quarter of the total content. Pentabor S is aformulation of this type but with half the waterof crystallization removed, to concentrate the

product and reduce transport costs. This ratio ofthe toxic components performs well fortreatments in temperate areas but the proportionof sodium pentachlorophenate must be increasedfor the treatment of tropical hardwoods.

Chlorophenol toxicity

There are many proprietory wood preservativescontaining chlorophenols and it is unrealistic to listthem. Pentachlorophenol is the most importantorganic compound used in wood preservation. Inthe United States the wood preservation industrystill uses about 20 000 tonnes ofpentachlorophenol annually, despite concernregarding possible health hazards; whilst it iscertainly true that chlorophenols are toxic, this is aproblem that applies to all wood preservatives andthey should therefore be handled with care. Somefatalities have occurred but these have all beenassociated with abnormal absorptions ofpentachlorophenol, through a failure to takenormal precautions. There are also fears that thedioxin impurities in chlorophenols may beparticularly hazardous but, whilst there may bedistinct dangers associated with 2,4,5-T, the wellknown herbicide which is a trichlorophenylacetate, there is no evidence of similar dangersassociated with pentachlorophenol or even thetrichlorophenol with which it is sometimescontaminated—apparently because the latter is2,4,6-trichlorophenol and generates a differentrange of dioxin impurities which are actually lesstoxic than the chlorophenols from which they arederived. Environmental theoreticians havesuggested that pentachlorophenol should bereplaced in stain control by less persistenttrichlorophenol but this compound is less effectiveand must be used at higher concentrations, and thedioxin impurities tend are more toxic than thoseassociated with pentachlorophenol. The continueduse of pentachlorophenol would appear to be bestin terms of both handling safety and environmentalprotection. Tetrachlorophenol is also sometimesused and represents an intermediate risk.

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Pentachlorophenol (PCP)—pentachlorophenyl esters—Mystox LPL—Fungamin

Pentachlorophenol is essentially a fungicidalwood preservative possessing only limitedactivity against borers, principally those that aredependent on previous fungal decay or thepresence of an intestinal flora. It is thereforefrequently formulated with insecticides,particularly the contact insecticides that aredescribed later in this chapter. It has noappreciable fixation to wood and is essentially asimple toxic deposit which normally possessesreasonable life through its relatively lowvolatility coupled with fairly deep penetration.Pentachlorophenyl laurate (Mystox LPL) androsin amine-D pentachlorophenate (Fungamin)are compounds in which the pentachlorophenolhas been reacted with other organic groups toincrease the molecular weight and reducevolatility but solubilization is also assisted andthe need for special solvents to prevent surfaceblooming is avoided. Copper and zincpentachlorophenate are also useful preservatives;these are described in detail later in this chapter.

Cumullit—Parol

p-chloro-m-cresol was first proposed as a woodpreservative in 1913 and was the main toxicant inCumullit, first marketed in Germany in about1917. This product performed well when appliedby normal pressure impregnation methods butlow retentions from brush treatment wereinadequate; it is not known whether this was dueto inadequate solution concentration. Parol wasthe potassium salt and was used for the treatmentof transmission poles in Germany in World War I;in world War II it was again used either as thesodium salt or dissolved in ethanol before dilutionto a concentration of about 1%. p-chloro-m-cresol is a fungicide and not significantlyinsecticidal. The compound does not appear to becurrently used but it is mentioned here, as are

several other compounds, as a potential woodpreservative for use when established compoundsbecome scarce or too expensive.

Chloronaphthalenes

The chloronaphthalenes were proposed as woodpreservatives following the successfulchlorination of creosote and the development ofCarbolineum Avenarius. Individualchloronaphthalenes are rarely used commerciallyand mixtures resulting from the chlorination ofnaphthalene are usually described in terms oftheir melting point which increases with thedegree of chlorination. Thus the mono- anddichloronaphthalenes are liquids whereas tri-andtetrachloronaphthalenes are solid waxes.

The mono- and di- compounds are distinctlyfungicidal but this activity appears to beassociated in part with their volatility and thefungicidal properties are reduced in the tri- andtetra-compounds, despite the higher chlorinationwhich would normally suggest greater fungicidalactivity. The explanation probably lies in theirmanner of use and evaluation—these compoundsare used in superficial brush and spraytreatments where the deeper distribution of thelower viscosity compounds accounts for theirapparent fungicidal activity, whereas in animpregnation treatment the solid compoundscombine greater protection against fungi withmuch greater permanence through theirresistance to volatilization.

The insecticidal properties increase steadily withthe degree of chlorination, whether the compoundsare applied by impregnation or superficialapplication, and this fact is reflected in, for example,the South African standard which requiresprincipally tetrachloronaphthalene to be used andthus defines a minimum chlorine content of 47%and a minimum softening point of 90°C (194°F).

The volatility of the mono- anddichloronaphthalenes results in a characteristicodour which is a distinct disadvantage. As thechloronaphthalenes are largely used in


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preservatives for superficial application theirperformance depends largely on theirpenetration. There have been several suggestionsthat this can be improved by the addition ofvarious resins and waxes, and particularly byadding stearic or palmitic acids or esters; thelatter act as surfactants, condensing onto thehydrophilic wood components and permittingthe hydrophobic solution to penetrate.Surfactant systems of this type are extensivelyused as additives to road tar and bitumen toassist wetting of damp crushed stone.

Halowax—Wykamol (Anabol)

Halowax, consisting mainly of trichloronaph-thalene gave excellent preservation againsttermite attack when evaluated in Panama in1913, but the advantages of this material do notappear to have been immediately appreciatedcommercially. Chloronaphthalene wax was theprincipal toxicant in Anabol, a remedialtreatmentwood preservative introduced in England in 1934and subsequently renamed Wykamol. Theexcellent termite resistance of chloronaphthalenewax was also established in South Africa in about1950; a mixture of 3.5% tetrachloronaphthaleneas an insecticide with 2.0% pentachlorophenol asa fungicide in organic solvent was approved foruse by pressure impregnation to meet regulationsthat required all wood in certain areas to bepreserved against termites and Longhorn beetles.

Xylamon—Olimith C20—Ridsol

Mono- and dichloronaphthalene were firstproposed as wood preservatives in about 1920and were the principal toxicants in Xylamonwhich was introduced in 1923. Similar productshave been introduced more recently such asOlimith C20 and Ridsol in the Netherlands.Whilst the smell of these compounds is adisadvantage, the toxic vapour diffuses deeplyafter superficial treatment and these products aretherefore particularly suitable for the eradication

of House Longhorn beetle and other borers inremedial treatments.

Chlorobenzenes -Rentokil

Whilst referring to remedial treatments it should beadded that, whilst chloronaphthalene wax was theprincipal toxicant in the product Anabol, laterrenamed Wykamol, this formulation also originallycontained o-dichlorobenzene to give a deeplypenetrating insecticidal vapour action. Thiscomponent was replaced in Wykamol in 1939 byRotenone and, after World War II, by Lindanecontact insecticide, although it continued in use inother similar formulations such as Rentokil. o-dichlorobenzene (ODB) is a liquid whereas p-dichlorobenzene (PDB) is a solid but more volatile.The eradicant activity of PDB against CommonFurniture beetle was first established in about 1915but the excessive volatility results in a powerfuleradicant action with only limited persistence. ODBis slightly less active but penetrates well into woodand gives excellent insecticidal preservative actioncoupled with good persistence. The fungicidalactivity of the chlorobenzenes increases with thedegree of chlorination as might be expected and thetrichlorobenzenes have been proposed as woodpreservatives. Hexachlorobenzenes have also beenconsidered but their low volatility and low watersolubility substantially reduce their effectivenesswhen applied as superficial preservatives, althoughthey are probably much more effective when usedfor impregnation.

Gamma hexachlorocyclohexane—Lindane Gammexane

Hexachlorobenzene must not be confused withbenzenehexachloride, a rather misleading namethat was used in the past for hexachlorocyclo-hexane. The gamma isomer of haxachloro-cyclohexane has been known variously as γ-BHC,Gammexane, γ-HCH and Lindane, although thetwo latter terms are now preferred. Lindane hasbeen extensively used as a contact action

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insecticide in formulated organic-solventpreservatives, particularly for superficialapplication in remedial treatments and usually at aconcentration of 0.3–0.5%. Activity decreasesprogressively through volatile losses that areinitially very rapid but if reasonable penetration isachieved the treatment will give protection againstegg-laying or emergence of adults followingpupation for many years. This activity is affectedby the presence of other components, especiallysome waxes, resins and non-volatile oils, whichtrap the Lindane, usually giving a lower initialtoxicity but greater persistence; the value ofdifferent proprietory products may vary greatly,despite a similar Liridane content, through thepresence of these apparently non-functionalcomponents.

Rotenone—Dicophane (DOT)

It is often suggested in literature on woodpreservatives that Lindane and other contactinsecticides were first considered as insecticidalcomponents in about 1947–1953 but, whilst thiswas apparently the situation in Germany andperhaps the United States. Lindane was already inuse in the United Kingdom in 1945 where it hadreplaced Rotenone, a natural contact insecticidederived from the Derris root, in Wykamol. Priorto the introduction of Lindane, Dicophane orDDT (dichlorodiphenyltrichloroethane) and theless effective contact insecticide 666 had also beenconsidered but were less effective. DDT must beused at 2%, or even 5% where there is a severeinsect hazard such as a risk of termite attack, toachieve the same protection as Lindane at about0.5%, the insecticidal activity depending on thepp-DDT content.

Cyclodiene insecticides—Heptachlor(Chlordane)—Dieldrin (HEOD)—Aldrin(HHDN)—Endrin

The cyclodiene insecticides Chlordane(Heptachlor), Aldrin (HHDN) and Dieldrin

(HEOD) have all been widely employed as soilpoisons in termite control treatments but Dieldrinwas preferred as a wood preservative because ofits lower vapour pressure and better persistence.This persistence has been a considerabledisadvantage when Dieldrin has been used inagriculture and horticulture as the insecticide hasaccumulated in natural food chains, causingconsiderable infertility where these chains end in,for example, birds of prey and causing concernthat mankind may be affected in a similar way.Endrin, a similar insecticide, is less persistent thanDieldrin and has therefore been more widely usedin agriculture and horticulture but it is not soactive as an insecticidal wood preservative. WhilstDieldrin is not popular in wood preservationbecause of the bad publicity associated with theseagricultural problems, none of the chlorinatedhydrocarbon contact insecticides representsignificant environmental risks when employed inwood preservation as these products controlinsects only within the confined environment ofthe wood so that they are unable to enter thenatural food chains.

Whilst it may be an advantage to develop non-persistent insecticides for use in agriculture andhorticulture it is clearly essential that extremelypersistent insecticides should be developed for use inwood preservation. In most countries these contactinsecticides continue in use as wood preservatives,perhaps subject to special licensing if their use isforbidden for other purposes, although generallyLindane is now preferred to Dieldrin. One difficultyis the limited availability of these insecticides nowthat they are no longer manufactured foragriculture, since it does not appear to have beengenerally appreciated that the wood-preservationmarket for these compounds is larger in manycountries than for agriculture. Chlordane(Heptachlor) has been recently considered an asalternative to Dieldrin as it is less volatile and thusmore persistent that Lindane, but there is somecurrent concern regarding possible carcinogenicdangers associated with this compound.

Among the chlorinated hydrocarbon contact


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insecticides Dieldrin, Aldrin and Chlordane aremost persistent. Lindane is less persistent andDDT is not lost too rapidly but it is much lessactive. Marine exposure tests have shown thatthese contact insecticides possess excellentactivity against the crustacean borers such asgribble, but little activity against the molluscanshipworms which are more susceptible tofungicidal preservatives. It has been found that achlorinated hydrocarbon contact insecticide suchas Dieldrin or Lindane in creosote providesexcellent preservation in marine situations.

Organophosphorus and carbamateinsecticides

The problems with chlorinated hydrocarboninsecticides have prompted the development ofmany alternatives, mainly for agricultural andhorticultural uses, where only limited persistence isconsidered to be advantageous, so that most havebeen unsuitable for wood preservation wherepersistence is essential. Phenthoate, Fenitrothion,Malathion, Dichlorvos, Carbaryl, Diazinon,Fenthion, Arprocarb, Bromophos and Fenchlorphoshave been considered, as well as many otherorganophosphorus and carbamate insecticides. Themost promising were organo-phosphoruscompounds; some of them such as Malathion wereunsuitable due to their persistent and unpleasantodour but Fenitrothion and particularly Phenthoatewere active and acceptable alternatives to Dieldrinand Lindane, although all organophosphorusinsecticides are very toxic to humans.

Pyrethroids

The pyrethroids are much safer. They wereoriginally derived from the pyrethrum daisy butthe natural extracts are not persistent on woodalthough they have a very powerful knockdowninsecticidal action which is valuable in flying-insect control. Synthetic pyrethroids have beendeveloped with a range of properties and severalof these compounds are now used in forest and

mill treatments against pinhole and lyctid attack,in remedial treatments, and even in preservationtreatments against termite and House Longhornbeetle attack. Decamethrin has rather highmammalian toxicity but Deltamethrin,Cypermethrin and particularly Permethrin havelow toxicity and are now extensively used; theycan give both eradication and long-termprotection against wood-boring beetles, termitesand crustacean marine borers such as gribble.

Many organic fungicides have been developedsince World War II and some have been used inwood preservation. Most of these compoundsoriginate from agricultural and horticulturalresearch programmes aimed at developingfungicides with low persistence and it is nottherefore surprising that very few of these newcompounds have proved effective as woodpreservatives, although some of them are used instain control. Modern performance and safetyapproval schemes involve substantial cost so thatdevelopments for wood preservation alone areunrealistic as expensive new products cannotcompete with established products, and only a fewcompounds developed since World War II justifymention here. Suspensions of Benomyl (Benlate) andCaptafol (Difolatan) were introduced as alternativesto chlorophenols but they are not generally veryeffective; they leave only a superficial deposit which,whilst it may control surface stain and mould, hasno influence over deep stain within the wood, andthese fungicides are only reliable when formulatedwith borates which will control deep sapstain.Carbamates such as IPBC (Polyphase),chlorothalonil (Tuffgard, Tuffbrite), variousisothiazolone (ITA) compounds, thiazoles such asMBT and TCMTB, and triazoles such ascebuconazole, propiconazole and azaconazole(Madurox) have performed well in stain controltrials alone or in various formulations, and are alsoclaimed to be suitable for wood preservation use butthey are never as persistent and reliable as inorganic,organometal or even quaternery ammoniumsystems. The trihalomethylthio- compounds must bementioned as they have been used particularly to

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control stain in service beneath paint and varnishcoatings. Captan is probably the least efficient ofthis group but this may be only a solubility factor asFolpet (Fungitrol 11) with the sametrichloromethylthio- radical is very active. Thedichlorofluoro- compounds Fluorfolpet (PreventolA3) and Dichlofluanid (Preventol A4) are also veryeffective and are preferred by many manufacturersas they are more readily soluble in most organicsolvents whereas Captan and Folpet are virtuallyinsoluble and can usually be applied only inrelatively high viscosity systems, such as pigmentedcoatings or formulations with a relatively high resincontent. Various metal soaps and organometalcompounds are used in organic solvent systems butthey will be considered separately in the next sectionof this Chapter, together with organo-nitrogencompounds such as quaternery ammoniumcompounds.

4.5 Organometal compounds

Various organometal compounds have been usedin wood preservation, particularly mercury andtin compounds. Metal soaps, particularly copperand zinc naphthenates, have also been used and,whilst they are metal esters rather than organo-metal compounds, they are considered to be moreappropriate to this section. Similarly, theorganonitrogen compounds are not organo-metalbut they are included in this section as they aresimilar in many respects to organotin compounds.

Copper and zinc soaps

The preservative activity of metal soaps can beattributed principally to the metal content andthis is now recognized in most specifications.Following treatment, the soaps hydrolyse andthe acid is slowly lost by volatilization, leavingonly the metal. Whereas the acid has animportant eradicant function when metal soapsare used for remedial treatment, care must betaken to condition test blocks to ensure that

most of the acid has dispersed when assessingpreservative activity, to avoid enhancedperformance which will be absent in actualservice. Copper and zinc are effective fungicidesbut if they are applied at inadequate retentionsthey tend to be detoxified by tolerant fungi suchas Poria species, and this can be clearly seenwhen inadequate loadings of green coppernaphthenate are used, as fungal infections maydecolourize the treated wood. Other metalnaphthenates have been employed, such ascalcium and barium, but the activity of thesecompounds must be attributed to the acid alonewhich is ultimately lost by volatilization.

Cuprinol

Metal naphthenates were first proposed as woodpreservatives by von Wolniewicz in Russia in1889 but they were first marketed commerciallyas Cuprinol in Denmark in 1911 and wereintroduced from there into Sweden in about1920 and into England in 1933. Coppernaphthenate is produced either by the fusionmethod in which copper oxide or carbonate isdissolved in heated naphthenic acid or by theprecipitation method following doubledecomposition when sodium naphthenate ismixed with copper sulphate in aqueous solution.The very distinctive green copper naphthenate isan important organic-solvent wood preservativeand clear zinc naphthenate is also extensivelyused. Both these soaps possess low insecticidalactivity, except when free acid is still present, butadequate retentions give good fungicidalprotection. Failures of treated wood in serviceare usually associated with the development ofresistant fungi on wood treated by superficialbrush or spray application.

Oborex Cu and Zn

Copper and zinc naphthenates are marketedthroughout the world under a variety of names,such as Oborex Cu and Zn in the Netherlands,


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but it is unrealistic to list them—there were at onetime 23 copper naphthenate and a further eightzinc naphthenate formulations registered inSweden alone! In the United Kingdom theproduction of copper naphthenate increasedconsiderably during World War II, largely as aresult of the demand for preserved ammunitionboxes and other military packaging which wouldnot deteriorate in service, but also for treatmentof canvas. Copper naphthenate has not been usedin heavy petroleum oil to the same extent aspentachlorophenol, although such formulationsinhibit the volatilization of the naphthenic acidand prolong the acid activity.

Acypetacs copper and zinc

Naphthenic acid has not been readily availablein recent years and various synthetic acids havebeen considered as alternatives. Many organicacids are suitable but octanoic and versatic acidshave been preferred in some countries, probablybecause they have been used as alternatives tonaphthenic acid in soaps used as paint driers.Cuprinol in England has been particularly activein the search for advantageous acids, eventuallyadopting a mixture of linear and branched chainsaturated aliphatic carboxylic acids derived frompetroleum, the resulting soaps being described asacypetacs copper or zinc.

Copper and zinc pentachlorophenates

The formation of copper pentachlorophenate byprecipitation from mixtures of copper salts andpentachlorophenates will be described shortly butcopper and zinc soaps are also mixed withpentachlorophenol in organic-solvent solutions inthe hope that the metal pentachlorophenates willbe formed following the volatilization ofnaphthenic acid after treatment. Theunpredictability of the reactions is clearly apparentwith the copper formulations as they often retainthe green colour of the copper naphthenatewhereas copper pentachlorophenate is dark red.

However, even in acid conditions, where theformation of pentachlorophenate is prevented inthis way, these mixed preservatives haveconsiderable advantages over the use of theirindividual components alone as they possess abroader spectrum of activity and greaterpersistence, and numerous clear organic-solventpreservatives are therefore based on mixtures ofzinc naphthenate and pentachlorophenol. Thesepreservatives are often prepared by adding zincnaphthenate to a normal pentachlorophenolsolution containing the usual co-solvents and anti-blooming agents, yet if the toxicants are present atan appropriate ratio the zinc naphthenate contentalone is sufficient to carry the pentachlorophenolwithout the need for these additives, anobservation that is put to good effect in the SouthAfrican Standard for such mixtures, which requires0.31% zinc and 2.5% pentachlorophenol innormal preservative solutions applied by pressureimpregnation. Naphthenates are also similarlyused in conjunction with other anionic fungicidessuch as o-phenylphenol but many preservativesalso contain trihalomethylthio type fungicides toimprove the resistance to staining fungi.

KP-Cuprinol—Penta-Tetra-Copper

KP salt, which is known in Sweden as KP-Cuprinol, is another invention of Häger who wasresponsible for K33 and the earlier Bolidenproducts BIS, S and S25. KP salt was firstintroduced in Sweden in 1955 and was supplied astwo components—the K salt contained copper asthe ammoniacal carbonate whilst the P saltcontained sodium chlorophenate, usuallytetrachlorophenate. The two salts were dissolvedseparately and then mixed to prepare a solutionwith a pH of 8.0–8.5 for full-cell impregnationcontaining 0.3% copper and 0.15% chlorophenol.After treatment the ammonia evaporated, reducingthe pH and causing the precipitation of copperpentachlorophenate. As the fixation processoccurred only after the treatment cycle wascompleted, KP salt could be applied by empty-cell


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processes, although the Lowry process wasnormally preferred. KP salt was applied in this wayat the commencement of the Royal process for thetreatment of joinery (millwork) in whichimpregnation is followed by heating under oil toremove the water and ammonia so that, when thewood is removed from the treatment vessel, it isalso deeply impregnated with the oil. Penta-Tetra-Copper is a similar product, forming a mixture ofcopper penta- and tetrachlorophenate.

Cuprinol Tryck (Cuprinol KPN)

Cuprinol Tryck, originally Cuprinol KPN, wasdeveloped by Häger as an alternative to bothCCA products and his earlier KP Cuprinolsystem. It is an ammoniacal formulationcomprising copper carbonate dissolved in caprylicacid and ammonia added to form cuprammoniumcaprylate, the eventual volatilization of ammoniaprecipating cupric caprylate.

Copper 8-hydroxyquinolinolate (oxinecopper)—Cunilate—Nytek GD—Cupristat

Copper 8-hydroxyquinolinolate, also known asoxine copper, has been introduced in recent yearsas an alternative to copper naphthenate. Thiscompound does not present the distinctivenaphthenic acid odour of copper naphthenateand it is also resistant to hydrolysis so that it canbe solubilized in water; Cunilate 2174 is 10% ofthis compound solubilized in water. Woodpreservatives of this type such as Nytek GD andCupristat are also used for stain control. Theyhave very low mammalian toxicity and arevirtually free from tainting problems so that theycan be used for the treatment of food packagingsuch as fruit boxes, sometimes with the additionof water-repellent components.

Organomercury compounds

The use of mercury compounds in woodpreservation has already been described in anearlier section of this chapter and it will thereforebe appreciated that mercuric chloride (corrosivesublimate) was an important wood preservative in

the late 19th century. In 1910 mercurychlorophenate was developed to avoid thecorrosive properties of mercuric chloride but it wasfound to be comparatively uneconomic and wasnever used commercially. Various organomercurycompounds were patented in Germany in 1926 butwere not used significantly in Europe, althoughethyl mercury and phenyl mercury compoundswere introduced in North America in about 1930and were soon extensively used in stain-controltreatments. Ethyl mercury chloride, sulphate,phosphate and acetate have all been used, as wellas phenyl mercury acetate and oleate, usually atconcentrations of about 0.1% to achieve staincontrol on freshly felled green wood. Thesecompounds are still used for this purpose in someareas but they are very toxic and were largelyreplaced by sodium pentachlorophenate afterabout 1940. The more volatile compounds such asethyl mercury acetate possess limited effective lifeand, where organomercury compounds continue tobe used, phenyl mercury acetate or preferablyoleate are normally employed. Pyridyl mercurychloride and stearate have also been proposed, anddiphenyl mercury and phenyl mercury chloridehave been reported as possessing good preservativeactivity against termites.

Organotin and organolead compounds

Silicon, germanium, tin and lead form Group IV/IVb in the periodic classification. Generally, thestability of organic compounds of these metalsdecreases with increasing atomic weight from siliconthrough germanium and tin to lead but in otherrespects the metals behave in a fairly analogousfashion to give four types of tetravalent compounds;RMX3, R2MX2, R3MX and R4M, where M is themetal, R is an alkyl or aryl group and X is the‘anionic radical’ which differs from the R groups asit is not attached to the metal by a C-M bond.

Biological activity increases steadily fromgermanium to lead, being moderately developed inR2MX2 and highly developed R3MX, the siliconcompounds and the other two structures exhibiting


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virtually no activity. The greatest activity isassociated with the tin and lead compounds but, aslead is far less expensive than tin, it would appearto be the more economic. In fact, the leadcompounds tend to be more reactive, introducingmanufacturing difficulties and instability in use sothat the tin compounds are generally preferred. Inaddition, residues from the degradation oforganolead compounds are toxic inorganiccompounds, although formed in only insignificantamounts from biocidal treatments, whereas theultimate degradation products from tincompounds are generally innocuous.

Triphenyltin compounds have found use inagriculture and triphenyllead compounds havebeen used in anti-fouling compositions but, inother biocidal applications, the trialkyltincompounds have been most widely used. Theirmicrobiological activity depends on the totalnumber of carbon atoms in the alkyl chains forboth symmetrical and asymmetrical compounds—the greatest fungicidal activity develops with 9–12carbon atoms, an observation made in about 1952which prompted proposals for their use as woodpreservatives. The mammalian toxicity decreasessharply with increase in total carbon atoms,perhaps being related to the water solubility of thestable hydrolysis products—trimethyl and triethylcompounds hydrolyse to the water-soluble andhighly toxic hydroxides, tripropyl compoundsform stable oxides or hydroxides, whilst tributyland longer chain compounds form oil-solubleoxides of lower toxicity, and trioctyl compoundsare completely non-toxic. The insecticidalproperties decline steadily as the number of carbonatoms increases and reduce sharply when theseexceed about 15, while general wood preservativeactivity decreases conversely to the increasingequivalent weight, and then decreases moresharply in excess of 15 or 18 carbon atoms.

Tributyltin compounds

Tributyltin compounds are therefore preferred asthey offer the greatest separation between

mammalian toxicity and useful biocidal orpreservative action, and tri-n-butyltin compoundsare the only organometallic compounds of GroupIV metals that are extensively used in woodpreservation. Although they were proposed aswood preservatives following observations oftheir exceptional fungicidal and insecticidalactivity, they are actually non-toxic when appliedto wood at low retentions, yet the wood does notnecessarily decay. It appears that the tributyltingroup may have a chemical affinity for woodcellulose, causing modification which inhibitsdecay, although fungal hyphae may penetrate intothe treated wood. There is thus a distinct dangerthat internal decay can occur if the preserved zoneis relatively shallow, a danger that is normallyavoided by using a much higher retention oftributyltin oxide or by the addition of other non-fixing fungicides. For example, a 0.1% organic-sol-vent solution of tributyltin oxide can beshown in laboratory experiments to giveprotection to a completely impregnated pineblock but in commercial treatments 1% is morenormal, or alternatively 0.5% when otherfungicides are present.

Experience since 1959 when tri-n-butyltin oxidewas first introduced commercially in the UnitedKingdom suggests that the addition of otherfungicides is advantageous, particularlypentachlorophenol, o-phenylphenol and borates asthese appear to give considerably improvedresistance to White rots, whereas the organotincompounds give particular protection against theBrown rots which attack only the cellulose inwood. These mixed formulations are particularlypreferred for remedial treatment preservativeswhilst the higher concentration of 1.0% tri-n-butyltin oxide is now used extensively for thetreatment of external joinery (millwork),particularly by the double vacuum process.Formulations sometimes also contain contactinsecticides, although tri-n-butyltin oxide isdistinctly insecticidal and can give excellentprotection on its own when applied at adequateretentions. The House Longhorn beetle is readily


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controlled in this way at retentions of about 1 kg/m3 whereas twice this concentration or more isrequired to control the Common Furniture beetle.Numerous proprietory organic-solvent woodpreservatives now contain tri-n-butyltincompounds as their principal toxicants, and suchformulations are extensively applied by pressureimpregnation, double vacuum, immersion or spray.

The volatility of tri-n-butyltin oxide can betroublesome during hot weather and less volatilecompounds have been used such as naphthenatesand phosphates, although they all eventuallyhydrolyse to the oxide. Problems at lowtemperatures usually indicate contaminationwith more volatile compounds such as halidesthrough poor manufacture or reaction withadditives; some stabilizers that are added toTBTO can cause increased volatility. There isalso some evidence of degrade on treated woodto di- and monobutyltin but, whilst these formshave lower microbiological activity, there is noevidence that the long-term preservative actionhas been affected, probably because it dependson blocking hydroxyl groups on cellulose andlosses by volatilization and degrade affect onlythe excess unreacted compound.

The fungicidal preservative action of tri-n-butyltin oxide is enhanced when it is applied inthe presence of a swelling solvent, particularlywater. Permapruf T, known in the Nordic area asBP Hylosan PT, was the first proprietarypretreatment product to take advantage of thisobservation and consisted of tri-n-butyltin oxidesolubilized in water using quaternary ammoniumcompounds. While other surfactants can beemployed, they are generally less reliable andcannot contribute to the preservative activity inthe same way. A sapwood retention in pine of1.2 kg/m3 TBTO in Permapruf T is equivalent tothe normal required retention of 9 kg/m3 activeoxides in conventional CCA formulations, whichis achieved with retentions of 15 kg/m3 TanalithC or Celcure A; the overall retentions areapproximately half these figures. Theseretentions, based on stake trials, suggest that

Permapruf T, which contains 10% TBTO, isabout 25% more effective than a BritishStandard 4072 CCA salt product, such asTanalith C and Celcure A, which contain about60% active oxides.

The systematic studies by the author in 1960–70 on the organic compounds of the Group IV/IVb elements silicon, germanium, tin, and leadprompted interest in similar studies on othergroups in the periodic classification. In Group IIIboron has been extensively used, as boratesderived from boric acid but not as organiccompounds, but aluminium has proved useful inwater-repellent and film-forming compounds asdescribed later in this chapter. Phosphorus,sulphur and chlorine from Groups V, VI and VIIare used as component elements in many of themodern complex organic fungicides andinsecticides, but only Group V gives rise to arange of compounds which have similarity to theorganometal compounds of Group IV. Group V/Vb comprises nitrogen, phosphorus, arsenic,antimony and bismuth, but only the great varietyof organic compounds of nitrogen have attractedspecial attention.

Organonitrogen compounds—quaternaryammonium compounds—alkylammonium compounds (AAC)

The organic compounds of nitrogen are generallyknown as amines and do not usually have anyspecial biocidal activity, but the quaterneryammonium compounds, sometimes known as alkylammonium compounds (AAC), are distinctlydifferent. The greatest biological activity isassociated with compounds with a single anionicgroup such as chloride or bromide and a cationcomprising nitrogen with four organic groups. Thesimplest cation is ammonium, NH4. With largerorganic groups water solubility is associatedusually with a benzyl group and these compoundsare favoured for wood preservation as they avoidthe need for a co-solvent such as an alcohol. Theprecise structure of the other three organic groups


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attached to the nitrogen is relatively unimportantin the sense that greatest biocidal activity developswhen the carbons in these groups total about 16.An alkyl-benzyl-dimethyl ammonium compoundwith an alkyl group with a chain length of about14 therefore represents optimum fungicidal andbacte-ricidal activity; compounds of this structureare known as benzalkonium compounds and areused as antiseptics. They are very effective as woodpreservatives, achieving an eradicant action butalso a persistent action through condensing ontohydroxyl groups on wood cellulose. A compoundwith a slightly different structure, benzalkyl-trimethyl ammonium, in which the benzalkylstructure forms a single group on the nitrogen, hasbeen extensively used in wood preservation,particularly in Europe where it was known asGloquat C but it has now been withdrawn as thereare health problems associated with thepreparation of the raw materials from which it wasmanufactured.

Preservative activity has been reported for awide range of amines and quaternery ammoniumcompounds but this activity is associated inmany cases with a surfactant effect or degrade toammonia with no true preservative action. Thefailure of some of these compounds has led todoubts regarding the reliability of the quaterneryammonium compounds but, if compounds ofsuitable structure are selected, they have a broadspectrum of activity, although some of thecompounds can be degraded by resistant fungi. Itis therefore advisable for quaternery ammoniumcompounds to be used only in association withother fungicides such as borates.

4.6 Carrier systems

The carrier system is as important as thetoxicant system in a wood preservativeformulation. The advantages and disadvantagesof various systems have been discussedtheoretically in Chapter 3 and there are alsoappropriate notes in Section 4.2 of this chapter.

However, formulated preservatives need acarrier solvent, the most important choice beingbetween polar solvents such as water andalcohols, which have reactive hydroxyl groupswhich will cause swelling in wood, and non-polar solvents such as petroleum distillateswhich will avoid swelling.

Organic solvents—proprietary advantages

Whilst an organic solvent may not contributedirectly to the preservative properties it is certainlyof considerable importance. Non-polar lightpetroleum distillates have low viscosities and areable to penetrate rapidly into dry wood so thatthey are particularly suitable for use in preservativeformulations that are designed for superficialapplication by brush, spray or immersion.However, it is not sufficient to achieve deeppenetration by the preservative formulation as thesubsequent volatilization of the solvent may causethe toxicants to return to the surface where theywill be particularly susceptible to losses byvolatilization and leaching. Co-solvents arefrequently used, sometimes to assist in the solutionof the toxicants during manufacture but often toprovide high viscosity residues or tails which willretain the toxicants whilst the carrier solventvolatilizes, ensuring proper distribution. In somecases the use of co-solvents distinctly reduces theapparent toxicity of an active component, perhapsdue to deeper distribution or a protective actionwhich also ensures greater persistence and a longereffective life. For example, pentachlorophenol isoften used in heavy oil as an alternative forcreosote, the heavy oil persisting and protecting thepentachlorophenol from leaching andvolatilization. While many products may be basedon the same concentration of a particular toxicant,their performance may vary widely and it must notbe assumed that apparently similar proprietaryproducts will achieve similar performance.

The penetration of polar organic solventsystems can be improved by the addition ofvarious resins and waxes, particularly by adding

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stearic or palmitic acids or esters; the latter actas surfactants, condensing onto the hydrophilicwood components and permitting anhydrophobic solution to penetrate. Surfactantsystems of this type are extensively used asadditives to road tar and bitumen to assistwetting of damp crushed stone.

There have been many attempts to use watermore extensively to avoid the high cost oforganic solvent systems and to reduce fire andhealth risks. In suspension systems, liquid orsolid toxicants are dispersed directly in the waterusing only a surfactant; the toxicants do notpenetrate and are simply deposited on the woodsurface. In emulsion systems the toxicants aregenerally dispersed as a concentrated solution insolvent emulsified in water. When applied to thesurface of wood the emulsion breaks, allowingthe solvent phase to penetrate whilst the waterevaporates. The fire risk associated with the useof sprayed organic-solvent preservatives isconsiderably reduced but in spray applicationthe penetration depends directly on the solventconcentration that is present and can never be asgood as with organic solvent systems.

Bodied mayonnaise-type emulsions(BMT)—Woodtreat

One problem in remedial treatment preservationis the deep penetration that is required wherelarge wood sections are involved which may besuffering from deep borer infestations or fungalinfections, and in the treatment of externaljoinery which may also be suffering frominternal fungal decay, concealed by a paintcoating. The bodied mayonnaise-type (BMT)emulsion products are pastes which can beapplied to the surface of wood with a trowel orgun, the emulsion breaking in contact with thesurface so that the organic-solvent phasecontaining the toxicants is able to penetrateslowly. Organic solvents of low volatility areused as very deep penetration can be achieved inthis way, although many proprietary products

contain inadequate concentrations of toxicant—the concentration must depend on the requiredretention related to the degree of penetrationthat may be achieved. The best known productof this type is Woodtreat.

Wykamol injectors—borate diffusiontreatments—Boracol—Borester 7—Timber rods

Wykamol remedial products were usually appliedin the past by drilling deep holes which were theninjected under pressure using a conical nozzle.This system has been replaced by a mouldedplastic injector which is driven into each hole,leaving a projecting nipple to which a pressuregun is fixed. Organic-solvent products areappreciably compressible and the injector is fittedwith a non-return valve, thus trapping within thehole a reservoir of preservative which is then ableto disperse deeply within the wood. Unlike theBMT systems this technique can be used for thetreatment of window frames and other externaljoinery; after treatment is complete each nipple isremoved with a chisel and the hole stopped andpainted to conceal the remains of the injectorwithin the wood. Concentrated borate solutionssuch as Boracol 20 and 40 and Borester 7 can beused in downward holes, slowly diffusing; Timborrods can also be inserted into holes, diffusing ifthe wood becomes wet. The borate products aredescribed earlier in this chapter.

Smoke treatments

Insecticidal smokes must also be mentioned asthey are claimed to have advantages over the useof formulated organic-solvent preservatives. Asmoke can achieve only a finely dispersed soliddeposit concentrated largely on upper horizontalsurfaces. With contact insecticides such asLindane or Dieldrin any insect settling on thetreated surface will be killed, but the finelydispersed and superficial nature of the depositensures only transient protection.


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Gas treatments—methyl bromide—Dichlorvos—methyl borate

Fumigant gases are attractive because of theirvery low viscosities compared with liquids and thepenetration that they can achieve. Methylbromide is very effective in eradicating insectinfestations such as termites but it is extremelypoisonous. A stack of wood or an entire structuresuch as a house must be sealed for several days topermit the methyl bromide to entirely eradicateany deep-seated borer infestation as there is noresidual action, and the gas must then be entirelyremoved by ventilation. An alternative system fora building is to install porous strips impregnatedwith a volatile insecticide such as Dichlorvos,although the strips must be replaced annually,shortly before the flight season, as with a smoketreatment, and a more realistic use for thisparticular insecticide is at low concentrations inmulticomponent products as an initial eradicantinsecticide. Methyl borate is a much more realisticgaseous preservative. Although it will penetratedeeply into wood it will subsequently hydrolyse todeposit boric acid, but realistic retentions of boricacid can only be achieved if this preservative isapplied by impregnation processes to achievedeep penetration, the gaseous phase only assistingwith subsequent diffusion.

4.7 Water repellents, stabilizersand decorative systems

Changes in moisture content up to fibre saturationpoint invariably involve movement, shrinkage withdrying and swelling with wetting. Although it isnormal to dry wood to a moisture contentequivalent to the average atmospheric relativehumidity anticipated in use, it is common toencounter movement problems. Faults such as gapsappearing between floor blocks or boards are dueto the wood drying after installation, eitherthrough inadequate kilning or perhaps re-wettingbetween kilning and installation. A door or drawerjammed in humid weather may be exceedingly

slack under drier conditions. Frames whichintroduce an end-grain surface in contact withside-grain will inevitably result in cracking of anysurface-coating system. In other situations thecross-sectional movement may become apparent aswarping through twisted grain effects. The obvioussolution to all these problems is to use only woodwith low movement but this is not always realistic.The alternative is to impregnate the wood withchemicals which induce stabilization.Unfortunately these processes are also frequentlyunrealistic because of the difficulty of achievingcomplete impregnation, a problem that has alreadybeen discussed in connection with normal woodpreservation.

Paint and varnish

One obvious solution is to enclose the woodwithin a protective film to stabilize the moisturecontent. Paint and varnish coatings will act in thisway, provided they completely cover the woodand remain completely undamaged.Unfortunately, whilst these coatings give goodprotection against rainfall, they are unable toprevent moisture content changes resulting fromslow seasonal fluctuations in atmospheric relativehumidity. As a result the painted wood will shrinkor swell with changes in relative humidity, causingthe surface coating to fracture wherever a jointinvolves stable side-grain in contact with unstableend-grain. Rain is absorbed by capillarity into thecrack, yet the remaining paint coating restrictsevaporation, so that the moisture content steadilyincreases until fungal decay is sure to occur if thewood is non-durable. It is frequently suggestedthat preservation provides a simple solution tothis problem, but this ignores the fact that wateralso damages the paint coating. It is explained inChapter 2 that wood is an hygroscopic material,covered with hydroxyl groups which have astrong affinity with water so that penetratingwater will tend to coat the wood elements,displacing paint and varnish coatings. This failureis known as preferential wetting and is

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responsible for blistering and peeling inpaintwork and the loss of transparency invarnishes.

Water repellent preservatives

The best solution to both the decay andpreferential wetting is treatment of externaljoinery (millwork) and cladding before painting,with a formulation that is both a preservativeand a water repellant. A water-repellenttreatment coats the pores of a structuralmaterial, reversing the angle of contact so thatcapillary absorption of water is prevented; waterrepellency is associated with water globulationon the surface but absence of globulation on aweathered surface does not necessarily meanthat a treatment is no longer effective as thepores may still be water repellent.

Waxes and resins

Various waxes, particularly paraffin waxes, are themost commonly used water-repellent componentsin wood preservative formulations, although atreatment based on a wax is generally assusceptible to preferential wetting failure as thesurface coating that it is designed to protect.Treatments of this type are reliable only if theypenetrate deeply and are applied at sufficientretentions to ensure that the wood elements areentirely inaccessible to even changes in the relativehumidity of the atmosphere. High-wax retentionscannot be used if wood is to be finished with apaint or varnish coating as the adhesion is seriouslyaffected—the coating cannot adhere to the waxdeposit and is unable to penetrate if the waxretention is too high, but in addition, the wax maymigrate into the coating solvents, affecting bothsolvent loss and the ability to absorb the oxygenrequired for drying so that the coating may remaintacky. The wax will continue to migrate throughsubsequent coatings, affecting inter-coat adhesion,perhaps even causing cissing, the situation when acoat is unable to wet a surface and tends to

concentrate in globules, leaving other areasuncoated. Yet another problem with migratingwax is the tendency to prevent the development ofgloss in the final top coat. For these variousreasons waxes are generally used at low retentionsand the desired pore-sealing action is achieved bythe addition of resins.

Resin selection is critical in terms of waterand water vapour resistance as well aspaintability. The aliphatic and aromatichydrocarbon resins are inexpensive and efficientbut they do not dry; they solidify only by loss ofsolvent and may be re-dissolved by coatingsolvents, perhaps interfering with the drying anddurability of the coating system. Natural dryingoils such as boiled linseed oil can also be usedbut paintability problems may arise throughslow drying. The use of suitable modern alkydresins can avoid this difficulty but they are alsoexpensive. The most realistic systems thereforetend to be based on mixtures of waxes,hydrocarbon resins and alkyd resins to avoidthese problems, and there are therefore distinctdifferences between proprietary products.

It is particularly important to appreciate thatunsaturated or drying resins are likely tosignificantly reduce the activity of some cationicpreservatives such as zinc, and particularlytributyltin oxide. In addition, alkyd resins willsolidify only in the presence of driers or catalystssuch as metal naphthenates, and these catalystsmay be inactivated by tributyltin oxide which is abase and will thus absorb their acids. Suchproblems can be avoided by using other toxicants,yet tributyltin oxide is particularly suitable as ittends to improve the resistance of the formulationto preferential wetting, and it is therefore betterto use tributyltin compounds other than the oxidesuch as the naphthenate or o-phenylphenatewhich do not suffer from these disadvantages.

Silanes (silicones)

There have been several attempts to developmore suitable water repellent components in


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view of the difficulties associated with the use ofwaxes. Tributyltin oxide orientates onto thewood fibres, giving a water-repellent surfacethrough the presence of the hydrophobic butylgroups. However, this compound is expensive,and toxic at high retentions, so that use as awater repellant is unrealistic, but other Group IVorganometal compounds can be used. Theorganosilicon compounds, the silanes orsilicones, are the best known water repellents inthis group but the very stable silicone oils tend topossess many of the disadvantages associatedwith heavy organic oils and waxes. The onlysilicones suitable are those which have a highdegree of functionality so that they are able toattach themselves to the wood components inthe same way as tributyltin oxide, thus givinggood resistance to preferential wetting failure.They have not been extensively employed,probably through disappointing resultsfollowing the use of unsuitable silicone oils andresins.

Organoaluminium compounds—Manalox

Organic compounds of aluminium, titanium andzirconium can also be used but the water-repellentgroups in typical available commercial productsare usually long-chain fatty acids such as stearatewhich give a waxy treatment and are moresusceptible to oxidation when applied at lowretentions than the short-chain alkyl groups ontypical silicone resins. However, aluminiumcompounds can incorporate unsaturated chainsand, when used for preservative, water-repellent orpriming treatments, they can provide excellentadhesive bonding between the wood elements andalkyd systems, giving resistance to preferentialwetting. Even toxic groups such aspentachlorophenate can be incorporated, thusavoiding the need for special co-solvent or anti-blooming systems. These principles are most highlydeveloped in various Manalox products which canbe described as polyoxoaluminium compounds.These advantages of organoaluminium compounds

are not apparent in the normal aluminium stearate,which performs only in the same way as a wax.

Stabilizers

If tributyltin oxide is applied at retentions inexcess of the toxic limits required to protectwood against fungal decay, the treatmenteventually saturates all the free hydroxyl groupson the cellulose chains which are responsible forhygroscopic movement and the wood becomescompletely stabilized. Such treatments areuneconomic but there are other possible systemsfor chemically reacting these troublesomehydroxyl groups. Formaldehyde treatment in thepresence of an acid catalyst will cross-linkhydroxyl groups on adjacent chains, reducingthe dimensions of the wood in the process butalso reducing the movement to less that 10% ofnormal. Acetylation involves the treatment ofwood with acetic anhydride in the presence of astrong acid catalyst, a process that considerablyreduces the hygroscopicity of wood and alsoincreases its resistance to fungal attack.However, all these chemical modificationtreatments suffer from the severe disadvantagethat they are effective only if the wood iscompletely impregnated and they can thereforebe used realistically only on permeable species;acetylation is being used in this way to anincreasing extent on radiata pine.

Bulking—Impreg—PEG—Carbowax—MoDo

In bulking, the wood is impregnated with a veryhigh retention of material which will physicallyrestrain movement. Several resin systems havebeen employed in this way such as the phenolicresin in Impreg and a styrene/polyester co-polymer system used for the impregnation of floorblocks, in Finland. These systems rely on physicalrestraint and are reliable if deep penetration isachieved, although complete penetration is notessential. The polyethylene glycol waxes are also

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bulking treatments but they are applied in waterand retain the wood in the expanded wet state.Treatments of this type such as PEG, Carbowaxand MoDo are generally applied by prolongeddiffusion. The compounds with low molecularweight of 200–600 are readily soluble in waterand diffuse reasonably quickly but 1000 is lesssoluble and gives slower diffusion, although it isless hygroscopic so that after drying the wood isnot so tacky as with the lower molecular weighttreatments. These systems are used particularlyfor the stabilization of archaeological specimens,the largest to be treated so far being the warshipsWasa in Stockholm and Mary Rose in Portsmouthwhich were spray-treated with a mixture ofpolyethylene glycol and borate whilst theatmospheric relative humidity was maintained ata high level to prevent drying. Although thissystem has been used successfully for thestabilization of gun stocks, the relationshipbetween molecular weight, treatment time andhygroscopicity is a distinct disadvantage. In onesystem for the treatment of floor blocks the lowmolecular weight compounds are employed,followed by complete drying, and theintroduction of isocyanate vapour which reactswith the glycol to form a polyurethane resin,avoiding all the disadvantages and giving atreatment which is stable and resistant to heavyfloor wear. There are many other polymer systemsthat have been or could be used but they aregenerally unrealistic, combining the need for highretentions with expensive chemical compounds.

Decorative preservatives—Madisonformula

While many water-repellent preservatives aredesigned specifically for use as pretreatmentsprior to painting or varnishing, perhaps in placeof conventional priming treatments, other systemsare designed as complete maintenance treatments,frequently serving a decorative as well as aprotective function; these decorative preservativesare particularly popular in the Nordic countries.

The two types of water-repellent preservative arenot necessarily similar; the first type must becompatible with subsequent paint or varnishcoatings whilst the second type must clearly havegood resistance to weathering. The Madisonformula, developed in the United States as amaintenance treatment for western red cedarcladding, is perhaps the best known. It consists ofparaffin wax, pigments and boiled linseed oilbinder with pentachlorophenol as the preservativeand zinc stearate to give water repellency, colourretention and freedom from stain. It has now beenlargely replaced by various improved proprietaryproducts, those containing trihalomethylthio-compounds being much more efficient incontrolling stain, as explained in Section 4.9.

Royal process

Weather resistance is poor with systems that aresimple deposits of hydrophobic components suchas waxes which are susceptible to preferentialwetting, but can be improved by using a binder asin the Madison formula or by fixation to thewood as with silicone resins, although deeppenetration will improve the performance of mostsystems. In the Royal process developed by Hägerfor the treatment of external joinery (millwork) awater-borne preservative treatment is followed bydeep treatment with a drying oil. This is a veryeffective process but involves a complex multi-stage treatment and the need for a multiple oil-storage system to provide finishes in differentcolours. Whilst the Royal process gives anexceptionally durable decorative finish it is alsovery expensive. The main problem is thattreatment is carried out in two stages, the firstintroducing large quantities of water which mustbe removed before the second oil impregnationstage can be satisfactorily achieved. There is noreason why similar reliability could not beachieved by single impregnation with an organicsystem, designed to achieve both the preservativeand the decorative functions, but commercialcompanies are reluctant to invest in systems that,


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because of the need for different colours, involvemultiple storage tanks and a danger ofcontamination in the impregnation cylinder.

4.8 Fire retardants

Modern water-borne fire-retardant formulationsoriginated in 1821 when Gay-Lussac reportedthat ammonium phosphate, ammoniumphosphate with ammonium chloride, andammonium chloride with borax (sodiumtetraborate) were excellent fire retardants whenapplied at adequate retentions to cellulosic fibres.These substances were not so effective when usedon wood but this was due to difficulty inachieving adequate retentions from superficialtreatments. They were far more effective whenapplied by pressure impregnation to give higherretentions but, as with the salt preservatives,corrosion problems were encountered and the useof ammonium sulphate was discontinued for thisreason shortly after it was introduced in 1880.

Oxylene—Minolith—Celcure F—Pyrolith—Fyre Prufe—Minalith—Pyresote

The Oxylene process was introduced in 1905,followed by Minolith in about 1915 which consistedof Triolith wood preservative with the addition of alarge concentration of rock salt, to give a combinedpreservative and fire retardant for use in mines. Inabout 1930 Celcure F was developed, in which theacetic acid in normal Celcure was replaced by boricacid which, with added phosphates and zincchloride, performed well as a flame retardant.Various competitive systems followed such asPyrolith, Fyre Prufe, Minalith and Pyresote, allbased on similar mixtures of soluble salts, usuallyadded to established preservative formulations. Fire-retardant salt components are leachable but alsohygroscopic so that normal coating systems cannotbe used on treated wood to give protection againstleaching. The compositions of the products havevaried with the availability of individual chemical

compounds. The most popular components areammonium phosphates, ammonium sulphate, zincchloride, boric acid and borates. A fire retardantmust suppress both flaming and after-glow but onlya few compounds can achieve this when used alone.

Non-Com

Generally, formulations containing zinc chloridesuch as Pyresote, which also contains ammoniumsulphate, boric acid and sodium dichromate, andmight be described as a fire-retardant version ofchromated zinc chloride, are declining in use,whereas the less sophisticated mixtures ofammonium phosphates, ammonium sulphatesand borates are becoming more popular. Moreexpensive systems such as Non-Com, whichpolymerizes within the wood, have beenintroduced to avoid the problems associated withsoluble and hygroscopic salts. This improvementhas certainly increased the scope of fire-retardanttreatments but it is clear that there is far moreinterest in North America than in Europe.

Halogenated compounds

An alternative method for achieving leachresistance is to use only water-insoluble organiccompounds. Generally, fire retardants can beprepared by using high loadings of halogenatedcompounds such as chloronaphthalenes andchlorinated paraffins, although their effectivenessas fire retardants is greatly increased if theyincorporate catalysts, particularly antimony orzinc compounds. Brominated compounds are alsoused. These are the only formulations that may besuitable for remedial use in building structures,but it is difficult to find solvents that do notthemselves introduce a fire hazard during theapplication process. The very high retentions thatare necessary unfortunately make thesetreatments very expensive and they have not beenused to any significant extent. In recent yearsmore sophisticated organic polymer treatmentshave been developed, primarily for the use on

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textiles—whilst these are efficient on wood, theyagain suffer from relatively high cost and the needfor high retentions. It is therefore usually morerealistic to use intumescent systems for in-situtreatments but, as these are essentially coatingsrather than preservatives, their composition is notconsidered in detail here; their mode of action hasalready been discussed in Chapter 3.

4.9 Stain control

Organomercury compounds—sodiumpentachlorophenate

The normal wood preservative systems givecomparatively poor control over the sapstainfungi and superficial moulds that are principallyresponsible for stain in freshly felled green wood,and under coating systems in service. It waseatablished in about 1935 that organomercurycompounds were extremely effective incontrolling these mixed fungal infections andchlorophenols were introduced about five yearslater. The organomercury compounds are verytoxic and, despite various attempts to reducemammalian toxicity, by replacing ethyl mercuryby phenyl mercury compounds and acetates byoleates, their use is now forbidden or officiallydiscouraged in most countries. Despite the irritantnature and moderately high toxicity of sodiumpentachlorophenate, it remains in use in mostcountries as efficient and economic alternativesare not readily available. However, thiscompound is not permitted in some countries suchas Sweden, where there are fears of health risksassociated with dioxin impurities, as described inthe earlier section on organic compounds.

Fluorides and bifluorides

Fluorides and particularly the so-calledbifluorides such as ammonium hydrogendifluoride have been extensively used for sapstaincontrol but they are not very reliable. Ammonium

hydrogen difluoride gives good control of stainfungi but actually stimulates growth of somesurface moulds such as Trichoderma viride whichcan cause severe problems unless very highfluoride concentrations are used.

Pentabor

In the absence of any obvious simple alternativesco-formulations with other compounds weredeveloped, originally as a means to reduce thetoxicity of the organomercury compounds andthe irritancy associated with sodiumpentachlorophenate. Some co-formulations,particularly those based on combinations ofsodium pentachlorophenate and borax, havebeen widely used throughout the world and offertreatments of lower toxicity than with sodiumpentachlorophenate alone. Co-formulationssuffer from the obvious disadvantage thatseveral separate components must be measured,perhaps in small quantities, when topping uptreatment tanks. The most popular co-formulation consists of 1 part sodiumpentachlorophenate with 3 parts borax (sodiumtetraborate decahydrate). The cost ofmanufacture has tended to discourage the use ofready mixed compositions but Pentabor, a co-formulation of this type developed in England,has half the water of crystallization removed, toreduce transport costs. Bromophenols have alsobeen used, particularly tribromophenol, bothalone and in combination with borates, but theyhave no significant advantages over the lessexpensive and more effective chlorophenols,except that their toxic dangers are less wellknown and their use is therefore less restricted.

Borates

Chloro- and bromophenols are generally used assodium or potassium phenates, the alkalinity orhigh pH greatly enhancing their stain controlactivity. The addition of inactive sodium carbonateprolongs control, apparently by maintaining this


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high pH. Borax was originally added to sodiumpentachlorophenate to broaden the spectrum ofactivity but the ability of borax to maintain thehigh pH is probably equally important. Boratecompounds cannot be used alone in stain controltreatments because they are not effective againstsome surface moulds, but they are very effectiveagainst sapstain itself and are now used as themain toxicant in many formulations, additionalfungicidal components effectively controlling onlythe surface-mould problem.

Benomyl (Benlate)—Captofol(Difolatan)—quaternary ammoniumcompounds—zinc borate—Polyphase(IPBC)—Chlorothalonil (Tuffbrite)—isothiazolones (ITA)—thiazoles (MBT,TCMTB)—triazoles (Madurox)

Suspensions of Benomyl (Benlate) and Captafol(Difolatan) were introduced as alternatives tochlorophenols but they are not generally veryeffective; they leave only a superficial depositwhich, whilst it may control surface stain andmould, has no influence over deep stain within thewood. These fungicides are therefore most reliablewhen formulated with borates which will controldeep sapstain. The most effective stain controltreatments are therefore mixtures of borates whichwill control deep sapstain with other fungicidessuch as Benomyl, Captafol or even quaternaryammonium compounds which will control surfacemould growth. Completely inorganic systems suchas zinc borate formulations are not currently usedbut are probably the most promising sapstaincontrol treatments for the future. Thisdevelopment has been largely ignored, probablybecause it is well known that cations such ascopper and zinc are not effective alone againstsapstain, and chemical manufacturers havetherefore concentrated on research on increasinglycomplex organic compounds, which are generallyexpensive and lack permenance when widelydispersed on a wood surface exposed to strongsunlight, as a stain control treatment. Some of

these organic compounds have proved effectiveand marketable for stain control. They are alldescribed in more detail in the earlier section onorganic compounds. Carbamates such as IPBC(Polyphase), chlorothalonil (Tuffbrite), variousisothiazolone (ITA) compounds, thiazoles such asMBT and TCMTB, and triazoles such ascebuconazole, propiconazole and azaconazole(Madurox) have performed well in trials, alone orin various formulations.

Stain in service (under coatings)—trihalomethylthio- compounds—Captan—Folpet (Fungitrol 11)—Fluorfolpet—Dichlofluanid (Preventol A3, A4)

It will be appreciated from the comments inChapter 3 that the control of stain under paintand varnish on exterior joinery (millwork) isessential to achieve a reasonable life for adecorative coating system. One of the best waysto apply a stain control treatment is as a normalwood preservative, perhaps in a water-repellent orpriming formulation, as a pre-treatment prior topaint or varnish. Normal organic-solvent woodpreservatives possess poor activity against stainfungi and mould, even if they containpentachlorophenol at 5% or tributyltin oxide at2%, although both these toxicants are also usedfor mould control in emulsion paints. Copper 8-hydroxyquinolinolate and Thiram are moreefficient, whilst diphenyhl mercury dodecenylsuccinate (Nuodex 321 Extra) gives excellentresults initially, but lacks persistence. Only thetrihalomethylthio- compounds have provedconsistently reliable. Captan is probably the leastefficient of this group of compounds but this maybe only a solubility factor as Folpet (Fungitrol 11)with the same trichloromethylthio- radical is veryactive. The dichlorofluorocompounds Fluorfolpet(Preventol A3) and Dichlofluanid (Preventol A4)are also very effective and are preferred by manymanufacturers as they are more readily soluble inmost organic solvents, whereas Captan andFolpet are virtually insoluble and can usually be

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applied only in relatively high-viscosity systems,such as pigmented coatings, or formulations witha relatively high resin content. However, none ofthese treatments give permanent protection asthey are lost by volatilization and oxidativedegradation, in common with almost all organiccompounds; only inorganic systems such as zincborate formulations are able to providepermanent protection.

Stain in service is perhaps most apparent whensimple clear or pigmented preservative systems areapplied to external cladding and joiner (millwork).The pigmented and water-repellent Madisonformula was introduced in the United States someyears ago as a maintenance treatment for westernred cedar cladding, relying on pentachlorophenoland zinc toxicants. Performance has beenunpredictable—the formula contains boiled linseedoil which is readily attacked by some of the stainfungi on wood, particularly Aureobasidiumpullulans, and there have been many attempts todevelop improved proprietory products. In somecases the risk of stain and mould development hasbeen reduced by the use of alternative binders, suchas the Manalox compounds described as waterrepellants earlier in this chapter, but a great varietyof toxicants has also been used. Some of these havebeen introduced without proper evaluation whilstothers, such as the organomercury compounds,have shown good initial activity but poor life.Some proprietory treatments have actuallyincreased the stain risk! The most successfulproducts generally contain one of thetrihalomethylthio- compounds.

4.10 Remedial treatments

Remedial treatment preservative formulationshave been mentioned up to now only in passingas they actually represent a distinct and ratherspecialized development route. They will bedescribed only briefly; a more detailed account isavailable in the book Remedial Treatment ofBuildings by the present author.

Xylamon—Rentokil—Wykamol (Anabol)

Few products were produced specifically for thispurpose before about 1920. Xylamon wasintroduced in Germany in about 1923 as aneradicant for House Longhorn beetle using mono-and dichloronaphthalenes with their characteristicpungent odour. This development was followed inthe United Kingdom by the introduction ofRentokil based on o-dichlorobenzene, and in 1934by Anabol, later called Wykamol, which consistedprincipally of chloronaphtalene wax with a smallamount of o-dichlorobenzene. From that pointRichardson & Starling Limited, the manufacturersof Wykamol, became leaders in the development ofnew remedial treatment preservatives. In 1939 theo-dichlorobenzene in Wykamol was replaced byRotenone, a natural insecticide extracted fromDerris root. DDT was considered as a replacementwhen it was introduced during World War II but itwas not very effective. In 1945 the Rotenone wasreplaced by Lindane which was found to be farmore effective, particularly in combination withchloronaphthalene wax which tended to protect itfrom volatilization, increasing its effective lifewithout significantly reducing its activity.

Cuprinol—Reskol

The Cuprinol copper and zinc naphthenateproducts had been widely used previously for theeradication of fungal infections in building timber,although failures occasionally occurred as it isdifficult to achieve adequate retentions bysuperficial brush or spray application. In 1936Richardson & Starling introduced Reskol whichwas designed specifically as an eradicant fungicidalwood preservative. It consisted originally ofbarium naphthenate and p-dichlorobenzene inlight creosote. In about 1955 the formulation waschanged to 5% pentachlorophenyl laurate in alight petroleum distillate but this proved ratherunsatisfactory, although it is still used by othermanufacturers. It was therefore replaced in 1957by 5% o-phenylphenol; many other manufacturers


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have used 5% pentachlorophenol but it is veryunpleasant to apply by spray during remedialtreatment.

Insect infestations are often encouraged by ordependent on fungal decay and a combinedinsecticide and fungicide treatment was thereforeintroduced by adding pentachlorophenyl laurateand later o-phenylphenol to Wykamol, resultingin Wykamol PCP and Wykamol Plus. Thiscombined product eventually replaced Wykamoland Reskol. In 1961 tri-n-butyltin oxide wasadded, originally as a persistent fungicide,replacing a proportion of the o-phenylphenol.Wykamol Plus has continued to contain twocomplementary fungicides and has thus largelyavoided the problems encountered by productswhich rely upon tri-n-butyltin oxide alone, o-phenylphenol was derived from the manufactureof phenol by the Dow process and became scarceand expensive when this process was replaced bythe cumene method. This stimulated yet anothernew development with the replacement of the o-phenylphenol by a borate ester.

In 1967 Wykemulsion was introduced,incorporating the same toxicants as WykamolPlus but with a reduced amount of solventemulsified in water. When sprayed on the surfaceof wood the emulsion breaks, allowing thesolvent phase to penetrate whilst the waterevaporates. The fire risk associated with the useof sprayed organic-solvent preservatives isconsiderably reduced. This type of emulsionmust not be confused with products containingonly a small amount of solvent as these aresuitable only for pressure impregnation; in sprayapplication the penetration depends directly onthe solvent concentration that is present.

Injection and diffusion treatments—bodied mayonnaise-type emulsions(BMT)—Woodtreat

One problem in remedial treatmentpreservation is the deep penetration that isrequired where large wood sections are

involved which may be suffering from deepborer infestations or fungal infections, and inthe treatment of external joinery which mayalso be suffering from internal fungal decayconcealed by a paint coating. The bodiedmayonnaise-type (BMT) emulsion products arepastes which can be applied to the surface ofwood with a trowel or gun, the emulsionbreaking in contact with the surface so that theorganic-solvent phase containing the toxicantsis able to penetrate slowly. Organic solvents oflow volatility are used as very deep penetrationcan be achieved in this way, although manyproprietory products contain inadequateconcentrations of toxicant; the concentrationmust depend on the required retention relatedto the degree of penetration that may beachieved. The best known product of this typeis Woodtreat.

Wykamol injectors—borate diffusiontreatments—Boracol—Borester 7—Timber rods

Wykamol remedial products were usuallyapplied in the past by drilling deep holes whichwere then injected under pressure using a conicalnozzle. This system has been replaced by amoulded plastic injector which is driven intoeach hole, leaving a projecting nipple to which apressure gun is fixed. Organic-solvent productsare appreciably compressible and the injector isfitted with a non-return valve, thus trappingwithin the hole a reservoir of preservative whichis then able to disperse deeply within the wood.Unlike the BMT systems this technique can beused for the treatment of window frames andother external joinery; after treatment iscomplete each nipple is removed with a chiseland the hole stopped and painted to conceal theremains of the injector within the wood.Concentrated borate solutions such as Boracol20 and 40 and Borester 7 can be used indownward holes, slowly diffusing; Timbor rodscan also be inserted into holes, diffusing if the

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wood becomes wet. The borate products aredescribed earlier in this chapter.

Smoke and gas treatments

Insecticidal smokes and gas treatments are usedin remedial treatment in some circumstances butsmokes give superficial deposits with onlytransitory protection and gases area generallyeradicants giving no protecting. They are

described in more detail in the earlier section oncarrier systems.

This account of remedial treatments has beenlimited to building treatments. Other remedialtreatments, of poles at the ground line usingbandages and injection treatments into poles andrailway sleepers (ties), are described elsewhere inthis chapter and in Chapter 3, together withdescriptions of termite remedial treatments usingsoil poisoning.

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5.1 General principles

Deterioration risk

Before the most suitable preservation systemcan be selected the deterioration risk must beclearly defined. With normal structural woodit is possible to define situations wheredeterioration will certainly occur, and in thesesevere hazard conditions the use of naturallydurable or adequately preserved wood isessential. Perhaps the most important severehazard situation is ground contact andtransmission poles, fence posts, railwaysleepers (ties) and construction piles must beproperly protected. In most marine situationsprotection is required against marine borers,and building structures require protectionagainst the House Longhorn beetle and againstDry Wood termites in areas where there is adanger of attack by these insects. In all thesesituations the use of naturally durable wood isunrealistic as it is both scarce and expensive,and in practice preservative treatment isessential and often required under localregulations.

A moderate hazard exists if deterioration ispossible rather than probable. In normalbuilding construction in temperate climatesthere is the danger that sapwood may becomeinfested by wood-borers and fungal infectionmay occur if wood becomes wet, perhapsthrough poor maintenance or condensation. In

such situations treatment is desirable ratherthan essential, although there is one situation inwhich there may be a stronger case fortreatment. External joinery (millwork) such asdoor and window frames is usually protected bypaint or varnish coatings but cracks candevelop through movement at joints, permittingthe penetration of water and introducing adecay risk. While it is theoretically possible toavoid this danger, by careful maintenance or bythe use of wood of low movement, it is clearlydesirable to reduce the decay danger by the useof naturally durable or adequately preservedwood, and this is now mandatory in severalcountries.

Where the deterioration danger is only slightthere can be no justification for treatment. Themost obvious example is the structuralwoodwork in a normal dry building. Generally,leaks will become readily apparent before fungaldecay can develop and, if the sapwood contentof the woodwork is limited, Common Furniturebeetle infestation, the normal borer hazard intemperate climates, will be structurallyinsignificant. In these circumstances treatmentcannot be justified; if decay occurs it will be aclear indication of defective design, constructionor maintenance. Another slight hazard involvesfurniture constructed from wood species that aresusceptible to attack by Common Furniturebeetle or Powder Post beetle; the latterrepresents a risk only within two or three yearsafter felling.

5

Practicalpreservation

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Stain control

Freshly felled green wood is also subject to aslight hazard, both in the forest and afterconversion at the mill. Whilst the moisturecontent remains high there is a danger that stainfungi can develop and if logs are likely to remainin the forest or in storage for a significant periodbefore conversion the cut ends should be sprayedimmediately after felling. Freshly sawn woodmust also be treated, although it is frequentlyclaimed that stain can be avoided by rapiddrying. In fact, even in kiln-drying there is adanger that stain will develop before themoisture content is significantly reduced and, ifkiln-dried wood is wrapped in an attempt toretain the low moisture content, there is a dangerthat condensation will occur beneath thewrapping and a treatment is still necessary iffreedom from stain is to be assured. In practice itis normally more realistic to treat all sawn woodto prevent stain, permitting normal sawn woodfor use as carcassing (framing) and other generalpurposes to dry naturally during storage andtransport. Wrapped kiln-dried wood, preferredfor joinery (millwork) manufacture, must betreated before and after kilning if completefreedom from staining is required.

Organomercury compounds were used in thepast for stain control but the most widely usedtreatment today in temperate areas is 2% sodiumpentachlorophenate solution. In some countriessuch as Finland only 1 % is often used as it isargued that the lower cost and improved safetyjustifies increased risk of stain development. InMediterranean-type climates the concentrationmust be increased to about 3% on softwoods, andin tropical areas about 5% is required onhardwoods. Sodium pentachlorophenate can bereplaced by various mixtures incorporating thiscompound, as described in the previous chapter,but the most efficient consists of 1 part sodiumpentachlorophenate with 3 parts sodiumtetraborate decahydrate (borax), a mixture thathas been in use since World War II and is more

effective, more economic and safer than sodiumpentachlorophenate alone. Pentabor S is a mixtureof this type, which is concentrated by a reductionin the water of crystallization; 1.3% Pentabor S isapproximately equivalent in performance to 2%sodium pentachlorophenate. If these mixtures areused on hardwoods in the tropics it is moreeffective to change the proportions rather thansimply increase the use concentrations. ThusPentabor SA is based on a mixture of 1 partsodium pentachlorophenate with 2 parts borax.When sodium pentachlorophenate is applied towood the sodium ions are quickly neutralized bythe natural acidity, resulting in a relativelysuperficial precipitate and poor penetration. Themixtures with borax delay this precipitation andthus achieve better penetration and improvedcontrol over internal staining which can developunder a superficially treated zone.

Limited penetration is also the maincriticism of Benomyl (Benlate) and Captafol(Difolatan) suspensions which have been usedin recent years as replacements for sodiumpentachlorophenate, although they areeffective when mixed with borates whichcontrol deep stain. Bifluoride mixtures havealso been used such as Improsol and BPMykocid BS. These solutions and the hydrogenfluoride that they liberate can penetrate verydeeply and they give excellent control overinternal stain, although they are much lesspersistent than sodium pentachlorophenateand must also be used at comparatively highretentions to control mould growth so thatthey tend to be rather expensive.

Pinhole borer control

Pinhole borers will rapidly attack logs while thebark is still adhering. They introduce stain fungito their galleries, causing both staining andboring damage. Whilst this damage can beprevented by the rapid removal of the bark thisencourages stain development so that a staincontrol treatment is then necessary (Fig. 5.1). It

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is often considered more realistic to spray thefreshly felled logs, particularly in tropical forests,with a stain control treatment incorporating aninsecticide, perhaps as an emulsion or suspension.Lindane has been used for this purpose for manyyears but it is now being replaced by syntheticpyrethroids such as Permethrin.

Powder Post beetle control

The sapwood of large-pored hardwoods issusceptible to attack by Lyctus Powder Post

beetle and it is advisable to treat wood with aninsecticide (Fig. 5.2) where this is a local danger,even if wood is to be kiln-dried; indeed thisinsect depends on starch within the wood whichtends to be lost during air seasoning, butretained during kiln-drying. Whilst stain controltreatment may not be considered necessary whenhardwoods are being rapidly kiln-dried aninsecticidal treatment is often essential. Boratesare particularly suitable for this purpose and arewidely used in Australia and New Zealand, andone advantage of the pentachlorophenate and

FIGURE 5.1 Pinhole borer prevention on tropical hardwood logs by spraying with Protostan insecticide afterremoval of bark. (Stanhope Chemicals Limited)


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borate mixtures such as Pentabor is the effectivecontrol of these Powder Post beetles that can beachieved at the same time as stain control withair-seasoned wood; it is not necessary to add anadditional insecticide. Susceptible hardwoodsinclude the temperate oak, elm and walnut aswell as the tropical light-coloured hardwoodssuch as obeche and ramin.

Preservation requirements

True preservation treatments generally involveimpregnation using vacuum and pressure systems.

Throughout the world the most important risksituation involves the use of wood in groundcontact as there is then a severe danger of fungaldecay, principally by Basidiomycetes. These fungiare generally unable to develop when wood isimmersed in water but there is then a danger ofSoft rot and borer attack in marine situations.Above ground level there is still a danger of decaywhere rainwater may remain trapped in joints orsplits, and this danger can be considerablyenhanced where a split occurs through a relativelyimpermeable coating such as paint or varnishwhich will tend to restrict dispersion of the water

FIGURE 5.2 Spraying susceptible tropical hardwood with Lyxastan insecticide to prevent Powder Post beetleattack. Immersion treatment is also used. (Stanhope Chemicals Limited)

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by evaporation. In most temperate areas thesapwood of many hardwoods and softwoods issusceptible to Common Furniture beetleinfestation but a much greater structural riskoccurs in temperate and tropical areas wherethere is a danger of infestation by the HouseLonghorn beetle and Dry Wood termites. Manytermites are, however, unable to fly and are morereadily controlled by soil poisoning around thestructure or the installation of termite shieldsrather than by treatment of the susceptible wood.

Need for permanence

In all preservation processes there is a need forpermanence. Thus all preservatives must haveadequate resistance to volatilization andoxidation, depending on the particularconditions of use, and also resistance to leachingif they are likely to be exposed to a highmoisture content in exposed or ground contactconditions or in marine environments. While itmay appear adequate to enclose wood in asuperficial envelope of protective treatment itmust be appreciated that deeper penetration willsignificantly improve the life of treatments whichare slightly volatile or water soluble, and naturalsplits or accidental damage may penetratethrough a superficial treatment, perhapspermitting internal decay to develop.

Preservative selection—wood selection

Table A.1 in Appendix A is a schedule of some ofthe most widely used preservative systems withtypical recommended retentions on Europeanredwood or Scots pine. These retentions alsoapply on most other softwoods, although theactual preservation process may vary dependingon the properties of an individual wood species,as shown in Table A.2. For example, Corsicanpine Pinus nigra, South African pine Pinuspatula, Shortleaf or Southern pine Pinusechinata all possess non-durable heartwood andsapwood but both are also permeable and these

species can therefore be readily preserved,although it will be appreciated that the sapwoodretentions shown in Table A.1 must be achievedthroughout in such woods. European redwoodor Scots pine, Pinus sylvestris, possesses rathersimilar properties when fast grown, although inslow-grown wood the heartwood is much lesspermeable and almost completely resistant topenetration, but also possesses moderate naturaldurability. It is therefore generally consideredthat adequate protection can be achieved bytreating the permeable sapwood alone. InDouglas fir, Pseudotsuga menziesii, theheartwood has good natural durability but thenon-durable sapwood is also resistant toimpregnation. Adequate penetration can beachieved if the sapwood is incised to give accessto routes for tangential penetration.

Unfortunately the same system is less effectiveon European spruce Picea abies; although incisingwill enable the sapwood to be treated theheartwood is non-durable and there is thus adanger that the outer sapwood will split throughshrinkage, following periodic wetting and drying,exposing unprotected and non-durable heartwood.The development of splits is not very likely withspruce as it possesses low movement as shown inTable A.2 and this wood is therefore more suitablefor superficial treatment than woods with mediumor large movements. If deep penetration intospruce can be achieved with a non-fixedpreservative leaching is unlikely following initialdrying or seasoning in view of the impermeabilityof the wood and its natural resistance to re-wetting. This advantage of impermeable woods isoften ignored, particularly when considering thereliability of unfixed treatments such as borates,which can be applied readily to green spruce bydiffusion to give complete impregnation.

Round poles typically involve relativelyimpermeable heartwood, perhaps with somenatural durability, surrounded by relativelypermeable sapwood which enables a preservedzone to be achieved. In sawn wood the surfacepossesses a random distribution of sapwood and


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heartwood and, whilst the heartwood may bemoderately durable as in slow-grown Europeanredwood, it cannot be readily treated and willdeteriorate when exposed to a continuous decayrisk such as in ground contact. There is a shortageof redwood of suitable size for transmission polesand a proposal that poles should be constructed bylaminating smaller pieces of wood is unrealistic asit exposes moderately durable heartwood whichcannot be treated, and which gives inadequateperformance when it lacks the protection ofsurrounding treated sapwood. It is perhaps strangeto consider that a more realistic technique might bethe use of spruce, incised after lamination, as thelow movement would avoid the danger of crackspenetrating through the treated zone.

Full- and empty-cell impregnation

Full-cell pressure impregnation is normallyemployed when it is required to achieve thehighest possible loading of preservative withinwood. This system is therefore used to achievevery high retentions of creosote, such as for use inmarine conditions, and also for the application ofrapid fixing multicomponent water-bornepreservatives such as the copper-chromium-arsenic (CCA) types. A full-cell water-bornetreatment results in a very high moisture contentwhich considerably increases the weight of woodas well as introducing handling and workingproblems, but it is normally considered thatempty-cell processes cannot be used as fixationwill occur within the wood and cause depletion ofthe active components in the recoveredpreservative solution. The concentration can becorrected if it is consistent but difficulties alsoarise through the nature of the fixation process;the recovered solution contains reducing sugarsfrom the cell contents which may causeprecipitation within the storage tanks. Theseproblems do not occur with preservatives whichfix by loss of a volatile component rather than byreaction with the wood elements; the ammoniacalpreservatives and the zinc and copper borate

systems which fix by acetic acid loss can bereliably applied by normal empty-cell techniques.

Empty-cell processes are used mainly forcreosote treatments for normal ground contactsituations, particularly for transmission poles; theyachieve preservative retentions that are adequatewithout wasting excessive preservative. They canalso achieve relatively clean treatments, althoughconsiderable difficulties are often encounteredwhen using the Rüping cycle which involves aninitial air compression stage and, if the pressure ofthis compressed air is not completely relievedduring the final recovery stages, there is a dangerthat bleeding will continue for a protracted periodfollowing treatment. For this reason a Lowryempty-cell cycle is preferred, although it shouldalso be noted that it achieves better distribution ofthe preservative as well as giving freedom frombleeding. The advantages and disadvantages ofthese various processes are discussed in Chapter 3.

Double vacuum impregnation—immersion treatment

Where very permeable woods are involved, suchas South African pine, or where only limitedpenetration is necessary as in the treatment ofjoinery (millwork), low impregnation pressurescan be used, even double vacuum treatmentswhich utilize only a vacuum and atmosphericpressure to achieve penetration. It has alreadybeen explained in Chapter 3 that, whenrelatively impermeable woods are involved,pressure increase is not very effective inachieving increased penetration and retention,and to extend the treatment time is far moreeffective. This is clearly demonstrated in non-pressure immersion techniques where completeimpregnation can be achieved, providedsufficient time is allowed.

Diffusion treatment

Prolonged used of immersion plant is economicallyunrealistic but diffusion techniques can still

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be used. A concentrated water-borne preservativesolution can be applied by immersion or spray togreen wood possessing a high moisture content inexcess of perhaps 50%. The wood is then closestacked and wrapped or placed in special sealedbuildings to inhibit evaporation so that thepreservative can penetrate deeply by slowdiffusion ( Figs 5.3 and 5.4). Timbor boratetreatment is applied in this way and is perhaps themost realistic method for the treatment ofspruce—complete penetration can be achievedand the relative impermeability of the spruce,following drying, means that losses throughleaching are unlikely, provided that the source ofmoisture is not prolonged and continuous. Sprucetreated in this way is not sufficiently reliable forground contact conditions but is ideal for othersituations in buildings as it provides protectionagainst both insect attack and fungal decaycaused by accidental leaks or condensation.Bifluorides are often applied similarly but muchof their deep penetration can be attributed tohydrogen fluoride which is readily lost byvolatilization and leaching. Potassium fluoridemixtures such as Osmose can also be applied inthis way but, if fixed treatments are required,preservative fixing by ammonia or acetic acid lossare most suitable provided that fixation is delayed

by wrapping for a sufficient period to permitcomplete diffusion.

Spray treatment

Spray treatments are generally unsuitable forsevere hazard preservation as they are onlysuperficial and the protection can be readilydamaged by the development of shakes or splitsand by woodworking. However, spray treatmentscannot be seriously faulted if they are appliedsufficiently generously to a completed structureand they are thus perfectly adequate for remedialin-situ treatments; these are described in moredetail in the book Remedial Treatments ofBuildings by the present author. Brush treatmentsshould never be considered for the application ofpreservatives as they cannot achieve loadingsufficient to give even superficial protection.

Ideal preservation

The ideal situation would be to use only naturallydurable wood but adequate supplies are notavailable and it is economically unrealistic to rejectall sapwood and use only durable heartwood. Analternative ideal situation would be for all wood tobe treated throughout its thickness at the mill, so

FIGURES 5.3 and 5.4 Diffusion treatment at a modern sawmill in Papua New Guinea. Sawn wood isimmersion-treated at the end of the production line and then close stacked in sealed sheds to permit diffusionof the preservative.


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that it would remain reliably preserved even ifsubjected to extensive wood-working. This can beachieved in permeable species such as Corsican,South African and Southern pine using normalpressure impregnation techniques, even usingstandard copper-chromium-arsenic (CCA)preservatives. Alternatively many species,including some normally impermeable species suchas spruce, can be treated by diffusion with boratesto give wood that is completely reliable inrelatively protected above-ground situations,including joinery (millwork) where the necessaryprotection is provided by a coating system.

5.2 Uses of preserved wood

Ground contact

Preservation was originally introduced as a means toavoid the deterioration that occurs when untreatedwood is used in various service conditions, but theintroduction of reliably preserved wood effectivelyintroduced an entirely new structural materialwhich can be used in new situations for whichuntreated wood was never previously considered—such as for durable wood house foundations. Themajor use of preserved wood is certainly in groundcontact conditions. In many respects poles, postsand piles present similar technical problems as theyall involve ground contact conditions and they varyonly in their dimensions and in the fact that poleshave most of their length above the ground incontrast to piles which usually have most of theirlength below ground.

Poles, piles and posts

Round wood transmission poles (Fig. 5.5)are used throughout the world, the principaladvantages of wood being excellent strength-to-weight properties and elasticity under load.Naturally durable wood is rarely used andmost poles are vacuum/pressure treated withcreosote or water-borne salt preservatives.In relatively permeable woods such as Southern

FIGURE 5.5 Eucalyptus transmission poles inAustralia impregnated with Tanalith C (CCA)preservative. (Hickson’s Timber Products Limited)

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pine complete impregnation is achieved andalmost any fixed preservative is suitable; theseare the Class A conditions shown in Table A.2 inAppendix A.

Where the sapwood alone is permeable butthe heartwood is moderately durable, as inEuropean redwood, resistance to movement isdesirable to reduce the tendency for checks orsplits to develop and expose the untreatedheartwood. Water-borne preservatives are lesseffective than creosote, which is particularlyefficient in reducing movement, although water-borne systems are efficient on species possessinglow movement where there is little likelihood ofthe development of checks. Checking is oftenignored, yet in tropical areas a wood with a largemovement can fail structurally, simply due to thephysical damage that results. Whilst Europeanredwood is normally considered to possesspermeable and non-durable sapwood,impermeable and moderately durable heartwoodand only medium movement, these propertiesapply only to wood that has grown relativelyslowly. With fast-grown wood the heartwoodmay be non-durable but it is also morepermeable so that reasonable penetration can beachieved with normal vacuum/pressureprocesses, although it should be appreciated thatwhere this fast-grown wood is included in atreatment charge the absorption of preservativemust be increased to ensure reliable protection.

Douglas fir also possesses non-durablesapwood and durable heartwood but even thesapwood is resistant to impregnation and polescan be reliably preserved only if they are incised.Spruce possesses sapwood that is resistant toimpregnation but incising has only limitedadvantages as the heartwood is also non-durable. However, as the wood is relativelyimpermeable a deposit of unfixed preservativewithin the heartwood possesses excellentresistance to leaching—rapidly fixed copper-chromium-arsenic (CCA) preservatives will treatthe sapwood only to the depth of incising butwith copper-chromium-boron (CCB)

preservatives the boron component will continueto diffuse, significantly improving the durabilityof the heartwood. The slow fixationpreservatives which fix by ammonia or aceticacid loss are more efficient as they can combinethis protracted diffusion with ultimate fixation.However, it must be appreciated that higherconcentrations are required if such diffusionoccurs. The wrapping of the treated wood toprevent drying and to allow for diffusion isprobably best avoided for poles so that higherloadings can be achieved through relatively rapidfixation in the external sapwood zone, wherethere is the greatest decay risk, but slow airdrying ensures that the inner moisture contentchanges only slowly and significant diffusion isstill able to occur.

The increasing scarcity of suitable sizes andspecies of wood for transmission poles meansthat there is increasing interest in the use of morereadily available species, such as relativelyimpermeable spruce, and in the manufacture ofpoles by laminating smaller sections. Inlaminated poles complete impregnation isessential and it is important to appreciate that aspecies such as European redwood with animpermeable but moderately durable heartwoodis entirely unsuitable—the lamination ensuresthat the heartwood is exposed to ground contactwhich represents the greatest decay risk, whereasin round poles the heartwood is protected byreliably preserved sapwood. It may seem strangebut spruce is likely to be more reliable thanEuropean redwood laminated poles becauselaminated spruce poles can be incised aftermanufacture to give a treated zone to a fixeddepth and the low movement of spruce avoidsthe danger of the development of shakes thatmay penetrate through this relatively superficialtreatment.

In tropical areas problems are frequentlyencountered in the treatment of hardwoods,particularly Eucalypts treated with water-bornepreservatives. Soft rot frequently develops at theground line, apparently because this fungus is able


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to invade the cell walls which have not beenpenetrated by the toxic components in thepreservative—this is a problem of micro-distribution of the preservative within the woodand is currently the subject of extensiveinvestigation. Class AS preservatives in Denmarkare considered to be those that are particularlysuitable for use in situations where there is adanger of soft rot; this generally means continuousimmersion in water in the Danish context.

In the Poulain process, described in detail inChapter 3, a relatively light overall treatment of apole is followed by a second and more thoroughtreatment of the butt. This originally involved aRüping empty-cell treatment of the entire pole witha light creosote followed by a further treatment ofthe butt with a heavier oil. In more recent yearscreosote has been used following a water-borneand sometimes unfixed treatment to give thedesired additional protection at the ground line. Inthe Dessemond process in France, poles were firsttreated with copper sulphate but mercuric chlorideKyanising or zinc chloride Burnettising treatmentswere also used. The Card, Tetraset and othersimilar processes are described in detail in Chapter4. In recent years it has often been argued thatcreosote butt treatment should be applied to allpoles treated with water-borne preservatives butthis achieves little advantage—a fixed water-bornetreatment such as CCA will give reliable protectionat the ground line in normal conditions and it is theexposed part of the pole that suffers from thedevelopment of checks. Butt treatments withcreosote or non-toxic bitumen have an advantageonly when the pole is treated with a poorly fixedpreservative which is unable to withstand theground contact conditions.

Creosote treatment (Figs 5.6 to 5.8), has thedistinct disadvantage that it is dirty and likely tobleed, perhaps causing serious damage toclothing where poles are erected in areas withheavy pedestrian traffic; the problems ofbleeding with empty-cell and particularly Rüpingtreatments has been discussed in detail inChapter 3. Preventing bleeding only reduces the prob-

FIGURE 5.6 Peeling transmission poles prior tocreosote treatment. (Industri- og ByggnadsaktiebolagetSuecia, Sweden)

FIGURE 5.7 Poles on bogies being loaded into thetreatment cylinder. Creosote is heated electrically toreduce viscocity and improve penetration. (Industri-og Byggnadsaktiebolaget Suecia, Sweden)

FIGURE 5.8 The stock yard with modern handlingequipment. (Industri- og ByggnadsaktiebolagetSuecia, Sweden)


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lem as the poles still remain dirty. Water-bornetreatments are clean and attractive but shakes orsplits may develop and many of these systemscontain arsenic which is a danger to livestock,even if it is proved to be reliably fixed. Fenceposts can be a source of arsenic poisoning just asmuch as transmission poles; the easiest way toavoid arsenic toxicity is to use only arsenic-freepreservatives such as the copper-chromium (ACC)types are completely reliable, except in situationswhere there is a serious hazard from termites orcopper resistant fungi such as Poria species.

Fences receive little attention yet theyrepresent a very large volume of treated wood.Usually, local species are used and there is thus atendency for their value to be largely ignored asreplacements can be readily obtained. However,any replacement or repair of a fence representssubstantial labour costs and there is alwaysjustification for the selection of naturally durablewood or the use of a preservation process.Round posts are always best and as they haveonly a small section they usually consist almostentirely of sapwood. In view of the short lengthof the posts even relatively impermeable speciescan be treated by penetration through the morepermeable end-grain. In many countriesstandard fence posts, pressure treated withcreosote or water-borne preservatives, representa normal commodity which can be readilypurchased by the agricultural community, but inother countries there is a tendency to prepareposts on each individual farm. In this casepreservative treatment, if it is used at all, isnormally applied by the butt hot and coldmethod described in Chapter 3.

Although preserved construction piles arecompletely reliable and widely used in America,they are not popular in Europe where tubularsteel and concrete piles are preferred. However,wood piles are widely used in marine situationsthroughout the world. Some naturally durablewoods are used such as greenheart but preservedpiles, particularly incised creosoted Douglas fir,are most popular. In situations where there is a

risk of marine borers it is necessary to use eithervery high retentions of creosote or additives suchas arsenic or contact insecticides which willprevent damage by gribble. Whilst marinedefence works such as groynes must be similarlyprotected, the superficial and decking timbers inwharfs, jetties and marinas represent onlynormal decay hazards and Class A preservativesare completely satisfactory for such structures.

Railway sleepers (ties)

The first use of pressure creosoted wood was forrailway sleepers (ties). Wood sleepers (Fig. 5.9)are still extensively used but the decliningavailability of large-section wood hasprogressively increased their cost and metal andparticularly concrete sleepers have been adoptedin several countries for economic reasons,although without taking proper account of thelife of the sleeper which is almost indefinite withcreosote treatment to a suitable wood. In recentyears the system of mounting the rails in chairsscrewed to the sleeper has been abandoned inmany countries in favour of the use of flat-bottomed rails secured with spikes.Unfortunately, the spikes do not hold so well increosoted wood and there are severaldisadvantages with water-borne treated woodsuch as movement splits, exposing onlymoderately durable heartwood in Europeanredwood sleepers, and electrical insulationproblems with salt systems, where signallingsystems operate through the rails. With newhigh-speed tracks, where spiked flat-bed rails areunsuitable, and where there are doubts about thelife of concrete sleepers, there is now a tendencyto return to creosoted wood sleepers.

With European redwood transmission polesand sleepers there is a danger, particularly withwater-borne treatments, that untreatedmoderately durable heartwood will be exposedby cracking and will slowly decay. When suchcracking is observed in adequate time it ispossible to carry out remedial treatments using


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the Cobra and similar injection processesdescribed in Chapters 3 and 4. Diufix, aspreadable mixture of creosote, tar, pitch andfiller, was developed for coating railway sleepersto fill existing cracks and reduce the tendency forfurther shakes to develop.

With some preservatives a further problem isslow and progressive Soft rot attack at theground line. With softwood poles this damagegenerally occurs only with the old fluorine-chromium-arsenic-dinitrophenol (FCAP)treatments; with copper-chromium (ACC) andcopper-chromium-arsenic (CCA) preservatives itoccurs only on hardwoods. In 1928 AllgemeineHolzimpragnierung GmbH introduced AHIG,the first pole bandage for wrapping round theexposed ground line of a pole to control Soft rotdamage (see Fig. 3.5). AHIG, which consists of awater-proof bandage lined with Wolman salts,has since been followed by many similarproducts such as the Osmose bandage and Pile

Card. In the Mayerl process a trough is fixedround the pole at the ground line and filled withcreosote which is then slowly absorbed into thepole. These processes are specific remedies forprogressive surface Soft rot and injection isessential if an attempt is to be made to controlheart rot developing in untreated heartwood.

Road works

Wood blocks treated with creosote and tar wereextensively used in the past as road pavingblocks. They were laid with the end-grainupwards and gave an exceptionally durable andresilient road. Whilst they are no longergenerally used for public roads they are still usedin parts of Europe as flooring in heavy industrialworks (Fig. 5.10). In modern road building,preserved wood is most extensively used forfencing and for crash-barrier posts—large-section wood posts can be installed directly into

FIGURE 5.9 Using preservative cartridges of Wolmanit TSK to protect heartwood at risk in sleepers (ties)through checks extending through the outer preserved zone. (Dr Wolman GmbH)


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the soil whereas steel posts must be mounted inconcrete if they are to provide adequateresistance to impact damage. Both fencing andcrash-barrier posts are usually treated withwater-borne preservatives, particularly copper-chromium-arsenic (CCA) types at the Class Aretentions shown in Table A.2 in Appendix A.

Bridges

Bridges are also sometimes constructed fromwood but it is important to be aware of thedangers of exposing heartwood which is onlymoderately durable when, for example, sawnEuropean redwood is employed. Incised Europeanwhitewood or spruce is more reliable, eventhough it is classified as non-durable andimpermeable, than European redwood or Scotspine with easily penetrated sapwood andmoderately durable heartwood; the lowmovement of spruce means that incising results intreatment to a controlled depth which is unlikelyto be penetrated by the development of shakes.

Buildings

Many exposed structures represent problemssimilar to those for bridges but in buildings (Figs5.11 to 5.14) there is usually less risk as theyshould be designed to ensure that wood remainsdry. For this reason a special Class B is shownin Table A.1 in Appendix A for buildings,including cladding and structural elementsexposed to the weather, as above-groundconditions are generally less severe than groundcontact. Wood preservation is required in flatroofs, swimming pool roof linings and industrialbuildings where a decay risk may arise throughcondensation, but in other circumstances themain reason for a preservative treatment maysimply be the desire to guard against futuredamage by accidental leaks or by HouseLonghorn beetle in temperate areas and DryWood termites in the tropics. Where insecticidalprotection is required, preservatives meeting theClass I requirements in Table A.1 are required.Generally, water-borne preservatives containing

FIGURE 5.10 End-grain wood blocks impregnated with creosote and used as a heavy industrial floor. (OrbenBois SA)


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FIGURE 5.11 Pine, pressure impregnated with anorganic solvent preservative BP Hylosan, used ascladding for a prize-winning housing project inSweden. (Svenska BP Aktiebolag)

FIGURE 5.12 Preservative-treated framing andDouglas fir plywood used in Canada for constructionof durable house foundations, a system widely used inNorth America. (Council for Forest Industries ofBritish Columbia)

FIGURE 5.13 Pine, impregnated with BP Hylosan,used as ceiling and wall lining to a swimming pool inSweden, (Svenska BP Aktiebolag)

FIGURE 5.14 Pine, impregnated with Boliden K33preservative used as a durable cladding for a factorybuilding in Sweden. (Anticimexbolagen)


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arsenic or boron are most economic but these arestomach poisons and tasting damage may besignificant, particularly in the case of exposureto termites. The use of contact insecticides suchas Lindane, Dieldrin or Permethrin is desirableto avoid the damage but their effective life isconsiderably less. With non-flying termites,damage is normally avoided by poisoning thesoil around the building or by using shieldswhich will prevent the termites from gainingaccess; these are illustrated in Fig. 5.15 and arerather reminiscent of the staddle stones used inold English barns to prevent rats from gainingaccess to the stored grain.

Fencing

Whilst wood used above ground level inbuildings generally represents a considerablyreduced risk compared with that in groundcontact conditions, there are situations wherewood is exposed to the weather and may decaywhere rainwater can penetrate into joints orcracks. It is perhaps worth mentioning that oneexample of this risk occurs in fencing where themortices used in the construction of gates and

for fixing rails to posts represent water traps andare therefore invariably the areas where decayprogresses most rapidly, even if naturally durablewood such as oak is employed.

Joinery (millwork)

In buildings, the external cladding does notusually present any serious problems and themain danger is associated with the joinery(millwork) such as the window and door frames.Decay develops under the paint or varnishcoating when water is trapped followingabsorption through splits at the joints. Thenatural reaction is to introduce a preservativeto prevent decay but splits then continue tooccur at the joints and water is absorbed which,even if it is unable to cause decay, results inpreferential wetting failure of the paint coating.The use of water-repellent organic-solventpreservatives tends to prevent the water frombeing absorbed, although cracks at the jointsstill occur so that water repellents tend to delayrather than completely prevent failure. The bestway to reduce cracks is to use only wood withlow movement and natural durability; suitable

FIGURE 5.15 Protecting buildings from non-flying termites. Shields (or caps) on supporting walls and all pipesand cables passing from the oversite to the floor will give some protection but regular inspection of the crawlspace is necessary and any mounds or tunnels found, which tend to by-pass the shields, must be destroyed. Soilpoisoning is more reliable and now more commonly used. Trenches are dug, then the oversite is levelled and thesite sprayed. Treatment is also applied to all fill returned to the trenches. In this way the building is isolated bya poisonous zone.


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tropical hardwoods are available. Alternatively,preservative treatment can be used but it is stillessential to use wood with low movement toavoid the splitting of the coating system at thejoints so that a longer decorative life can beachieved.

Remedial treatments

Remedial treatments in buildings (Fig. 5.16) aredescribed briefly in Chapters 3 and 4, but theseare extremely complex processes which are alsorelated to dampness problems and masonrydeterioration and they are best consideredentirely separately, as in the book RemedialTreatments of Buildings by the present author.The economics of preservation versus remedialtreatments in buildings must also beconsidered—is it more sensible to takeprecautions during construction or simply toremedy defects if they develop? In ground

contact conditions it is clear that the use ofnaturally durable or adequately preserved woodis essential. European redwood or Scots pinejoinery (millwork) is almost certain to decay,despite regular painting, and preservation isjustified. Where there is a risk of HouseLonghorn beetle preservation is again justifiedfor the carcassing or framing components of abuilding but it cannot be justified where there isa risk of attack only by Common Furniturebeetle which can do less damage to the structure.Sometimes preservation is applied as there maybe a slight danger of an accidental leak but it isunlikely that it can be justified, although if thereis known to be a condesation problem, as in flatroof construction, precautions preservation isessential.

The conclusion that must be reached is thatthere are many situations in buildings wherepreservation is essential or clearly desirable,but it is equally apparent that in many countries

FIGURE 5.16 A water-in-oil emulsion Wykemulsion being used for remedial treatment of a roof. Organicsolvent preservatives are often preferred but this emulsion reduces the fire risk. (Cementone-Beaver Limited)


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the need for preservation is largely ignored. Theproposal that treatments should be introduced inunder-developed countries is particularlyinteresting; at present a large proportion of thenational effort is devoted to replacing decayedbuildings and even a preservation treatment oflimited efficacy may result in a substantialimprovement in national prosperity.

Boats

In boats (Fig. 5.17) it is usual to use onlynaturally durable wood to avoid fungal decay.The main danger arises through penetration ofrain through decking and upper works togetherwith poor ventilation within the hull. There isno reason why properly preserved wood shouldnot be used as an alternative to naturallydurable wood, a fact which is generallyaccepted in the case of plywood, but the use ofwood in boat building has progressivelydeclined in recent years, largely as a result ofthe unreliable performance of wooden boats.

This can be attributed partly to the seriousdecay that occurred regularly some years ago innon-durable core veneers in plywood. Whilstnon-durable veneers are no longer permitted,plywood still possesses a poor reputation. Theother problem with wooden boats is the need topaint them regularly; this could be avoided bythe use of treatments which would preventpreferential wetting failure. These problems aredescribed in more detail in Chapters 3 and 4.There are opportunities for preservationtreatments to be used more extensively but it isfirst necessary to re-establish the goodreputation of wood in boat building. Oneserious problem concerns the attitude of paintmanufacturers; they are reluc-tant to adoptsystems that will reduce paint failure as they arelargely dependent on maintenance painting fortheir turnover, although this short-sightedattitude has resulted simply in the abandonmentof painted boats and is the main reason for thesubstantial reduction in the use of wood in boatbuilding.

FIGURE 5.17 Pine, pressure treated with Boliden K33 salt preservative, used for piles, beams and decking of ayacht jetty. (Anticimexbolagen)


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Agriculture

Agricultural buildings (Figs 5.18 and 5.19) areoften constructed from wood in all parts of theworld. In recent years there has been a tendencyto abandon the use of frame buildings and toadopt pole construction instead, an examplebeing the pole barn where pressure-treatedroundwood is used for the basic structuralmembers. The use of preserved clapboard

cladding enables this form of construction to beadopted for other purposes such as cow-housesand implement sheds. There are a multi-tude ofsmall uses for preserved wood in agriculture andhorticulture, a few being the construction ofgreenhouses, mushroom boxes, stakes for fruittrees and vines, and fences and gates.

Mines

In mines the dampness and constant temperatureconditions favour decay. Two basic types of supportare required, the working-face supports or propsand the linings to the main roadways. Wood is stillextensively used for props but it is usuallyconsidered that there is little justification forpreservation as only a relatively short life isrequired. In the case of the more permanentroadways there is a tendency for steel to be adoptedbut wooden boards are often placed between steelarches and these must be treated. Usually, water-borne preservatives are used, particularly thecopper-chromium-arsenic (CCA) types, and there isalso a demand for fireproofing treatments for woodused in principal roadways, although it is not clearthat wood necessarily contributes to fires in mines asit is normally too damp.

Australian quarantine requirements

Two special preservation requirements must bementioned. The first concerns the AustralianQuarantine Regulations which require that allimported wood forming part of disposable or re-usable packaging (cargo containers) must betreated to ensure that it does not introduce awood-borer risk. The reasons for theseregulations have been discussed in detail inChapter 2 and suitable Class 1 type preservativeretentions are shown in Table A.1 in Appendix A.

Cooling towers

Another special use for preserved wood is incooling towers where the fill slats are exposed to

FIGURE 5.18 A simple pole barn constructed fromwood impregnated with Tanalith C (CCA)preservative. Hickson’s Timber Products Ltd)

FIGURE 5.19 Silos in Sweden contructed from pineimpregnated with Boliden K33. (Anticimexbolagen)

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hot water and are thus particularly susceptible toSoft rot degradation. Whilst there have beenexperiments with a number of alternativematerials such as glass, asbestos and concrete, thelow density of wood makes it particularlyattractive for this purpose and it is entirelyreliable if treated with a suitable preservative suchas a copper-chromium-arsenic (CCA) system.

5.3 Health and the environment

In recent years the health and environmentaldangers associated with wood preservation haveattracted particular attention. Restrictions onthe use of existing preservatives and therequirements for approval of new preservativeshave become increasingly stringent and are nowcausing serious difficulty to the industry. Thesechanges have not necessarily resulted in reducedrisks to health and the environment, as thedevelopment of safer preservative systems is nowdiscouraged by the costs involved in submittingnew preservatives for approval and it has beennecessary, for economic necessity, to extend thelife of established preservative systems whichwould not be acceptable if they were submittedfor safety approval today.

All wood preservatives contain toxiccomponents but there is no justification for theirprohibition. Regulations should specify theprecautions that are necessary to ensure their safeuse in terms of the hazards to operatives duringformulation and use, to the users of treated wood,and to the environment. In some cases theseprecautions may mean that it is uneconomic touse a particular product and realistic control istherefore achieved. For example, arseniccompounds are very toxic but they can be safelyused in wood preservation with appropriate strictcontrols on the handling of treating solutions, butmodern preservative formulations ensure that thearsenic is ultimately fixed in the wood so that itcannot easily affect users or the environment byleaching or volatilization. Control is much easier

at treatment plants than during the handling andworking of treated wood or service in a structure,remedial wood treatment in buildings presentingthe greatest risks so that it is discussed later indetail, although solution spills and wind-blowndust from treatement plants can cause problems;dust dangers are not significant with mostpreservatives as powder formulations have beenlargely replaced by pastes and solutions.

Arsenic

The arsenic content in CCA, FCAP, CAA, ZAA,ACA, ACZA and similar formulations is a seriousproblem, it is unfortunately true that cattle havebeen poisoned as a result of licking treatedtransmission poles and fence posts but thisnormally occurs only with preservatives in whichthe arsenic is not fully fixed, such as K-33 whichhas now been largely withdrawn, and only inareas where there is a natural salt deficiency—thedanger can be completely avoided by providingproper salt licks and using only highly fixedpreservatives. Arsenic preservatives are banned inbuildings in some countries such as Finland, yet inothers they are used even for the treatment ofplayground equipment. In Switzerland, arsenicpreservatives are banned for the treatment oftransmission poles as they may introduceenvironmental pollution. In most countries thesehazards are considered to be insignificant withCCA preservatives in view of their excellentfixation, although fixation is only reliable withCCA-C and BS 4072 formulations, and the mainfears are related to the possible volatilization ofarsenic when treated wood is destroyed byburning.

Yet another fear is the danger of arsinepoisoning. In about 1890 many fatalities occurredin homes and these were eventually attributed toarsine poisoning. Wallpaper decorated witharsenical dyes had been attacked by the fungusScopulariopsis brevicaulis. at that time known asPenicillium brevicaule, it is now known that otherfungi can generate arsine in this way. There is no


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danger of arsine poisoning from preserved woodprovided that the fungicidal components are ableto control the fungi that generate arsine. Ifpreservation is required against insect attackalone, as for the treatment of wood in accordancewith the Australian Quarantine Regulations orfor Furniture Beetle control in New Zealand,there is clearly a temptation to use just a simplearsenical preservative and there is then a dangerof arsine poisoning if fungi are able to develop.Many insects are dependent on or encouraged bythe presence of fungal attack and fungicidalcomponents in multi-component formulationstherefore assist in their control. This factor iscompletely ignored in Australia and New Zealandwhere the minimum retentions of approvedpreservatives are based solely on an arsenicretention of about 0.97 kg/m3 As2O5.

There have been fears, particularly in theUnited States, concerning the carcinogenicdangers associated with the arsenic contents inwood preservatives. There have been extensiveenquiries and the current evidence suggests thatthe carcinogenic properties are largely associatedwith arsenic trioxide, As2O3, and arsenites ratherthan with the arsenic pentoxide, As2O5, and thearsenates that are used in modern preservativeformulations. The carcinogenic dangersassociated with arsenical wood preservatives areslight but arsenic pentoxide is prepared from thetrioxide and increasing controls on the lattercompound are likely to introduce manufacturingdifficulties, perhaps leading to scarcity andincreased cost. The dangers associated with fixedarsenic wood preservatives are often exaggerated,sometimes by manufacturers of competitiveproducts; the enquiries in the United States wereprompted by the cement and concrete industrywhich feared the competition of pressure-treatedwood foundations in domestic construction!

Chromium

Arsenic dangers always attract most attention butchromium may represent the greatest hazard in

wood preservation. If properly formulatedpreservatives are used in a competent andresponsible manner the dangers are very slightand this is confirmed by the very low level ofillness or injury in this long-established industry,but it must be recognized that dangers exist.Tropical conditions discourage the use ofprotective clothing and operatives then frequentlysuffer from chrome ulcers which are painful anddifficult to heal but it is interesting to note thatthere are no arsenic problems, and the injuriessustained are an indication of poor plant hygieneand control rather than serious criticism of thepreservative formulations involved.

Creosote

In recent years health risks associated with the useof creosote have been more closely scrutinized,the main risks being identified as the reportedcarcinogenic properties of polycyclic aromatichydrocarbons, including the benzopyrenes increosote. The benzopyrene content can be limitedby using only creosote distilling at highertemperatures, a restriction that does not affectimpregnation grades but which is now restrictingthe availability of lighter creosote for surfaceapplication, for maintenance of fencing. In otherrespects the use of creosote does not present anyunusual health hazards, provided that plants areoperated with proper care and persons handlingcreosote and treated wood observe normalpersonal hygiene precautions.

Chlorophenols

In the United States the wood preservationindustry still uses about 20 000 tonnes ofpentachlorophenol annually despite concernregarding possible health hazards. Whilst it iscertainly true that chlorophenols are toxic, this is aproblem that applies to all wood preservatives andthey should therefore be handled with care. Somefatalities have occurred but these have all beenassociated with abnormal absorptions of


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pentachlorophenol through a failure to takesensible precautions. There were several fatalitiesin France when operatives who were stripped tothe waist in very hot weather became heavilycontaminated with sapstain treatment, and severalfatalities occurred in Germany in a similar waythrough people bathing in water contaminatedwith pentachlorophenol remedial treatment. Thereare also fears that the dioxin impurities inchlorophenols may be particularly hazardous but,whilst there may be distinct dangers associatedwith the herbicide 2,4,5-T, which is atrichlorophenyl acetate, there is no evidence ofsimilar dangers associated with pentachlorophenolor even the trichlorophenol with which it issometimes contaminated. Apparently, this isbecause the latter is 2,4, 6-trichlorophenol andgenerates a different range of dioxin impuritieswhich are actually less toxic than thechlorophenols from which they are derived.Environmental theoreticians have suggested thatpentachlorophenol should be replaced in staincontrol by less persistent trichlorophenol but thiscompound is less effective and must be used athigher concentrations, and the dioxin impuritiestend to be more toxic than those associated withpentachlorophenol, so that the continued use ofpentachlorophenol would appear to be best interms of both handling safety and environmentalprotection. Tetrachlorophenol is also sometimesused and represents an intermediate risk.

Chlorinated hydrocarbon insecticides

The chlorinated hydrocarbon insecticides DDT,Dieldrin and Lindane have attracted particularattention at times because of their interference inenvironmental food chains when used inagriculture or horticulture. Wood preservationtreatments do not normally interfere in theenvironment in this way because woodpreservation insecticides are designed to achievecomplete control of borers within the treatedwood. In contrast, in agriculture and horticulture,killed insects and treated survivors may be eaten

and enter food chains. Lindane is still permittedfor many wood preservation purposes but muchsafer pyrethroid insecticides have been introducedand are now preferred, particularly Permethrin;various natural and synthetic pyrethroids havebeen approved for many years for use in the foodindustries for insect control.

Organotins

The organotin compound tri-n-butyltin oxide hasbeen in use in wood preservation since about 1960without any serious health problems being reported,other than dermal and respiratory irritation whichcan be attributed to poor operative technique andinadequate personal hygiene, although otherorganotin compounds are exceedingly toxic. This isa further example of the situation that has beenencountered in relation to chlorophenol dioxins—although there may be some exceedingly toxiccompounds within a group, that does not mean thatall compounds in that group are similarly toxic andsome may be virtually non-toxic.

Copper and zinc soaps

Copper and zinc naphthenates have been used formany years in the wood preservation industrywithout reports of unusual health problems,although the naphthenic acid liberated from thesetreatments has a distinct musty pungent odourwhich is unpleasant and irritating. A leadingmanufacturer of preservatives of this type hasreplaced the napthenates in recent years withacypetacs compounds which avoid this mustyodour, although they produce instead a slightsickly odour to which some individuals seem to beparticularly sensitive, and which has causeddifficulty in houses where excessive preservativehas been applied.

Remedial treatments

Clearly, remedial treatment contractors have aduty to ensure the good health and safety of their


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own operatives as well as the occupiers oftreated buildings. Proper protective clothingmust be provided, although it is difficult toensure that it is worn at all times. For example,during the spring roof spaces may become ratherwarm in contrast to the winter period andoperatives may be tempted to remove clothing,perhaps during spraying. The operative thenleaves the roof and perhaps sits in the sunwithout a shirt or vest and there is then a dangerof mild sunburn, accompanied by considerableirritation if the skin is affected by organicsolvents. Nose bleeding can also occur whensome preservative vapours are encountered inhigh temperature conditions, but it must beappreciated that some operatives are moresensitive than others, and particularly sensitivepersons should never be employed for this typeof work. Masks and barrier creams are oftenrecommended but neither is really advisable.Simple gauze masks tend to absorb treatmentfluids, perhaps exposing the user to abnormalconcentrations of toxicant vapours; without amask, the operative would probably take morecare to avoid unnecessary breathing ofpreservative spray or vapour. Barrier creams canalso give operatives unjustified confidence and itis far better to train operatives to take thenecessary care. Obviously all operative gangsmust be aware of the health and fire dangers,and must be aware of the action that should betaken in an emergency; a first aid kit and fireextinguishers should always be available toremedial treatment operatives.

There are several important points thatshould be borne in mind when carrying outremedial treatment in buildings using organicsolvent preservatives. Low-pressure spraysshould be used with coarse jets to ensure that themaximum volume can be applied to the timbersurface without the excessive volatilization ofsolvents that occurs if high pressures and finejets or air-entrained paint sprays are used. Whilstit is essential to achieve the maximum loading ofpreservative on the wood to ensure maximum

penetration, dripping of excess fluid must beavoided and care must be taken to ensure thatelectrical cables are not treated unnecessarilyand preservative does not enter junction boxesor other electrical fittings. Treated areas must befreely ventilated to disperse solvent vapourwhich is a fire hazard and which may affectelectrical cables and cause staining aroundceiling roses and wall switches. Electricalinstallations in the treated areas should bedisconnected during treatment and even forseveral days afterwards as there is a danger thatsparks may ignite solvent vapour. Smoking,naked lights and plumbing activities must beprohibited in the area for 7 to 14 days dependingon the nature of the solvents involved, andnotices to this effect should be posted at theentrances of the property and at the entrances toroof spaces and other treated areas. Insulationmaterials must always be lifted before treatmentand replaced later after the solvent hascompletely dispersed, certainly not less thanseven days after treatment in any circumstances,and insulation must never be sprayed wihpreservative as there is then a real danger ofspontaneous combustion.

Some of the phenolic preservatives,particularly the chlorophenols, can causetreatment operatives severe respiratory anddermal irritation, particularly if excessive spraypressures are used which result in preservativeatomization and spray drift. Dermal irritationproblems are often due to the solvents alone andenquiries usually disclose that the individuals aresensitive to similar solvents such as gasoline,kerosene, white spirit and turpentine; suchproblems are usually associated with fair skinand are aggravated by exposure to sunlight. Thisdermal irritation is also aggravated by somepreservative biocides, particularly chlorophenolssuch as pentachlorophenol (PCP) and organotincompounds such as tri-n-butyltin oxide (TBTO),although sensitivity to these biocides variesenormously and, if normal precautions areobserved in use, problems are only encountered


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by particularly sensitive individuals. In extremecases, respiratory irritation can occur and causecoughing and bleeding from the nose but suchreactions are usually related to extreme exposuresuch as spraying preservatives in roof spacesduring very hot weather; these problems can bereduced by using coarse low pressure sprayapplication to flood the surface of the wood withpreservative which can then be absorbed bycapillarity, avoiding high pressure sprays whichcause atomization and rapid volatilization, butimproved ventilation may also be necessary.

If sufficient ventilation is provided followingtreatment the solvents will rapidly disperse,leaving only preservative deposits of lowvolatility which do not normally causepersistant odours. There have been varioussuggestions in recent years that these treatmentsare dangerous to health but treatments must beapproved by the health and safety authorities inmost countries. The health risks associated withcurrent preservatives have therefore beencarefully assessed and there is no reason tosuppose that they present significant risks,either to treatment operatives or to personsresident in treated buildings. Obviously,treatment operatives are severely exposed andwould be expected to suffer most seriously fromany health hazards but, although there areperhaps 5000 to 10 000 operatives employed inthe remedial timber treatment in the BritishIsles, reports of health problems are very fewindeed, despite the fact that most operativeswork within the industry for many years; on thecontrary it seems that operatives suffer lessfrom some common illnesses such as colds andinfluenza!

There were several proprietary remedialtreatment preservatives some years ago which werebased on o-dichlorobenzene or on monoordichloronaphthalene. These biocides are oils whichcan be readily absorbed through the skin and theycertainly presented a danger of liver damage totreatment operatives. The dangers were much lesswith the solid polychloronaphthalene waxes which

could not be absorbed in this way and no illnesseswere reported despite their extensive use at veryhigh concentrations over many years.Pentachlorophenol attracted attention in the pastbecause of its pungent and irritating odour whenapplied, but in recent years attention hasconcentrated on the dioxin impurities that may bepresent in chlorophenols, as described earlier inthis section.

Health problems are not confined, of course,to remedial treatment wood preservation butobviously spraying in confined spaces representsthe most intensive exposure that is likely to beencountered. The other common problems thatarise are dermal and respiratory problems due tohandling timber treated with organic solventpreservatives, particularly in hot weather, andcareless operation of treatment plants involvingpreservative spillage, vacuum pumps discharginginto working spaces, and timber dried aftertreatment in working spaces, all problems thatare associated with careless handling rather thanany defect in the preservative system. However,there are periods when a series of complaintsarise, apparently associated with a particularpreservative biocide. Obviously, reports ofproblems can prompt further unjustifiablecomplaints, but a series of incidents usually havesome common cause which is often very difficultto identify. Some of the complaints in recentyears seem to be associated with Lindanetreatments and others with TBTO treatments. Inboth cases unusual volatility seems to beinvolved which sometimes affects treatmentoperatives but which is also readily apparent tothe occupiers of treated buildings. In such casesit must be suspected that the Lindane and TBTOwere poor quality products containing impuritieswhich have caused these problems as suchproblems are not associated with the purecompounds. Lindane is defined as 99% pure γ-isomer of hexachlorocyclohexane, previouslyknown as gamma-benzenehexachloride, and itseems that some material contains much higherconcentrations of other isomers. TBTO often


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contains a stabilizer, and some stabilizercompounds interfere with the fixation of theTBTO to the treated wood so that the TBTOremains volatile. Such problems may beindications of inadequate quality control but

they are very rare; when complaints areinvestigated it is generally discovered thatnormal precautions have not been observed,particularly in relation to excessive treatmentlevels and ventilation following treatment.

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Wood preservation is continuously developing inresponse to new research findings and economic andsafety pressures. Any list for suggested further readingwould be largely obsolete and it is therefore moreappropriate to give guidance on sources of furtherinformation.

Guidance on current approval requirements can beobtained from national organizations responsible forstandards and regulations.

Any reader of this book can obtain currentinformation on wood preservation from paperspresented during the proceedings of the followingorganisations:

The International Research Group on WoodPreservation,

Box 5607,S-114 86 Stockholm,Sweden.

American Wood Preservers’ Association,P.O. Box 849,Stevensville,MD 21666,U.S.A.

British Wood Preserving and Damp-proofingAssociation,

Building 6,The Office Village,4 Romford Road,Stratford,London,E15 4EA,England.

The reference lists at the ends of the papers willsuggest further reading.

Further reading

179

Decay hazard

Throughout the world the most important risksituation involves the use of wood in groundcontact, where there is a severe danger of fungaldecay, particularly by Basidiomycetes. Thesefungi are generally unable to develop when woodis immersed in water, although there is then adanger of Soft rot and borer attack in marinesituations. Above ground level there is still adanger of decay where rainwater may remaintrapped in joints or splits in wood exposed to theweather, and this danger can be considerablyheightened where a split occurs through arelatively impermeable coating such as paint orvarnish, which will tend to restrict the dispersionof the water by evaporation. In structures, theframing or carcassing components are generallyprotected from the weather and there is no risk ofdecay provided care is taken to avoid leaks andthe structure is designed to avoid condensation.

Borer hazard

In most temperate areas the sapwood of manyhardwoods and softwoods is susceptible toCommon Furniture beetle infestation but a muchgreater structural risk occurs in temperate andtropical areas where there is a danger ofinfestation by the House Longhorn beetle andDry Wood termites. Many termites are, however,unable to fly and they are more readily

controlled by soil poisoning around a structureor the installation of termite shields rather thanby treatment of the susceptible wood.

Permanence—penetration

In all preservation processes there is a need forpermanence. Thus all preservatives must haveadequate resistance to volatilization andoxidation, depending on their particularconditions of use, and also resistance to leachingif they are likely to be exposed to a high moisturecontent in ground contact conditions or in marineenvironments. Whilst it may appear adequate toenclose wood in a superficial envelope ofprotective treatment it must be appreciated thatdeeper penetration significantly improves the lifeof treatments which are slightly volatile or watersoluble, and it must also be appreciated thatnatural splits or accidental damage may penetratethrough a superficial treatment, perhapspermitting internal decay to occur. All thetreatments considered in this appendix must beapplied by a technique that will ensure deepimpregnation, usually one which involves the useof vacuum and pressure cycles in closed cylinders.

Wood properties

Table A.1 gives a schedule of some of the mostwidely used preservative systems and typical

Appendix A

Selection of apreservationsystem

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TABLE A.1 Typical preservative retentions for Baltic redwood*

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recommended retentions on European redwoodor Scots pine. These retentions also apply onmost other softwoods, although actualpreservation processes may vary depending uponthe properties of an individual wood species, asshown in Table A.2. For example, Corsican pine,Pinus nigra. South African pine, Pinus patula,and Shortleaf of Southern pine, Pinus echinata,all possess non-durable heartwood andsapwood. However, both heartwood andsapwood are also permeable and these speciescan therefore be readily preserved, but it will beappreciated that the sapwood retentions shownin Table A.1 must be achieved throughout insuch woods. European redwood or Scots pine,Pinus sylvestris, possesses rather similarproperties when fast grown, but in slow-grownwood the heartwood is much less permeable andalmost completely resistant to penetration. Thisslow-grown heartwood also possesses moderatenatural durability and it is therefore generallyconsidered that adequate protection can beachieved by treating the permeable sapwoodalone. In Douglas fir, Pseudotsuga menziesii, theheartwood again has good natural durability butthe non-durable sapwood is also resistant to

impregnation. However, adequate penetrationcan be achieved if the sapwood is incised to giveaccess to routes for tangential penetration.Unfortunately, the same principle is less effectiveon European spruce, Picea abies—althoughincising enables the sapwood to be treated theheartwood is non-durable, and there is thus adanger that the outer sapwood could splitthrough shrinkage following periodic wettingand drying, exposing unprotected and non-durable heartwood.

Spruce

In fact, however, the development of splits is notvery common with spruce as it possesses lowmovement, as shown in Table A.2, and it istherefore more suitable for superficial treatmentthan woods with medium or large movement. Ifdeep penetration into spruce can be achieved witha non-fixed preservative, such as the boroncomponent in a copper-chromium-boron (CCB)preservative or a simple borate preservative appliedby diffusion, leaching is unlikely once initial dryinghas occurred, in view of the impermeability of thiswood and its natural resistance to re-wetting.

TABLE A.1 (continued)

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TABLE A.2 Properties of principal construction woods used in northern and southern hemisphere temperatezones

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Round or sawn

Round poles typically involve relativelyimpermeable heartwood, perhaps with somenatural durability, surrounded by relativelypermeable sapwood, which enables a deeppreserved zone to be achieved. In sawn woodthe surface possesses a random distribution ofsapwood and heartwood and, whilst theheartwood may be moderately durable as in


slow-grown European redwood, it cannot bereadily treated and deterioration will occurwhen it is exposed to a continuous decay risk,as in ground contact, Thus piles, transmissionpoles and posts in ground contact shouldalways be made of round wood with agenerous zone of permeable sapwood, unless itis possible to use one of the previously-mentioned completely permeable species, suchas Southern pine.

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A person involved in the wood preservationindustry is normally well acquainted with thewood-borers of local significance but may knowlittle of rare species, particularly if they arefound in wood imported from another country.If preservatives or preserved wood are to beexported the habits of particular borers in thedestination country may also have considerableimportance. The purpose of this appendix istherefore to provide a reasonably thoroughdescription of the borers that are of significanceto the European wood preservation industry. Itdoes not, however, include a description of themany other insects which are found, particularlyin domestic premises, and confused with wood-borers; a description of these insects appears inRemedial Treatments in Buildings by the presentauthor, a book which also includes a moredetailed account of the methods for identifyingborer damage.

Classification

Before describing the individual borers it isnecessary to explain the method of classificationthat is used by zoologists. The animal kingdom isdivided into three sub-kingdoms, the protozoa,the parazoa and the metazoa, but wood-borersare confined to only two of the twenty phylathat comprise the metazoa. The most importantphylum is the Arthropoda which includes themost important group, the terrestial insectborers, as well as the marine crustacean borers.The phylum Mollusca includes only a fewmarine borers. In fact, with the exception of themarine crustacean and molluscan borers, allother wood-borers are terrestial insects.

Insects—beetles

The class insecta includes five orders containingwood-borers. All insects possess six legs and oneor two pairs of wings. The life cycle commenceswith an egg which hatches to produce a larvawhich feeds and grows, eventually pupating andmetamorphosing into an adult insect. The mostimportant order is the Coleoptera, the beetles. Inthis order the fore-wings are modified to formhard elytra or cases over the folded hind-wings.The beetles include the most important borersand these can be classified into ten families. ThePlatypodidae represent one of the two majorgroups of pinhole borers, the other being theScolytidae. The Bostrychidae include two veryimportant sub-families, the Lyctidae or PowderPost beetles and the Anobiidae or Furniturebeetles. The Cerambycidae or Longhorn beetlesare also very important in two ways; manyspecies cause damage to freshly felled greenwood and sickly trees in the forest but a singlespecies, Hylotrupes bajulus, represents one of themost serious wood-borer pests, in that it infestsdry softwood in the warmer temperate zones.The other six families are far less important; theyare the Curculionidae or weevils, the Buprestidaeor flat-headed borers, the Oedemeridae, whichinclude the wharf borer, and the minorLymexylonidae, Dermestidae and Tenebrionidaefamilies.

Termites

The termites, order Isoptera, are the most seriouswood-borer pests in all tropical and sub-tropicalzones. These are all social species living in largecommunities which include both the winged and the

Appendix B

Wood-borers

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non-winged forms. Although they are known aswhite ants they are not directly related to the trueants, which are in the order Hymenoptera. The mostimportant Isoptera families are the Kalotermitidaeor Powder Post termites, the Rhinotermitidae orMoist Wood termites and the Termitidae, whichinclude both subterranean and mound species. TheHodotermitidae are a semi-desert species known asharvester or forager ants, whilst the Termopsidaeare of minor importance and the Mastotermitidaeconsist of only one species in tropical Australia.

Wasps, bees, ants—moths—flies

The order Hymenoptera, the wasps, bees and ants,includes the Wood wasps, Carpenter bees andCarpenter ants, which bore into wood, as well asvarious predators which are found in borergalleries. The order Lepidoptera consists of thebutterflies and moths, whose larvae, or caterpillars,sometimes bore into wood, usually when it issoftened by decay, in order to obtain protectionduring pupation. The Carpenter moths, however,are true wood-borers as the larvae hatch from eggslaid on the bark, producing galleries in the wood,in which they feed and develop until they pupateand emerge as adult moths. The order Diptera, thetrue flies possessing only a single pair of fore-wings, the hind-wings being represented byhalteres or lumps on the thorax, contains only afew minute species which can be classified aswood-borers, these attacking the cambium of sometrees when the bark is damaged.

Marine borers

The class Crustacea includes a number of wood-borers in the order Peracarida. They are allmarine borers, similar in appearance to woodlice or fleas. The sub-order Isopoda are thegribbles, which are true wood-borers whereasthe sub-order Amphipoda do not damage woodbut are often found in gribble burrows.

Within the phylum Mollusca the wood-borersare confined to the class Lamellibranchiata,

molluscs with a symmetrical body which isnormally enclosed by a shell that develops in twoparts or valves. In the family Teredinidae, whichincludes the shipworms and pileworms, thesevalves or shells are used as cutters to form aburrow, and the body grows progressively to fillthe burrow as it is formed. The familyPholadidae, the boring mussels, tend to formrather more shallow burrows than those of theTeredinidae, usually attacking sedimentary rocksrather than wood.

Although there are a great number of specieswhich can be classified as wood-borers there areonly a few of serious economic importance, yet itis necessary to consider the minor species as theyare sure to create interest and perhaps problemswhen encountered, and their identification isnaturally more difficult than that of the well-known species. Latin names are deliberatelyquoted as these will enable readers to search forfurther information if they wish; common namesfrequently vary in different countries. It must,however, be appreciated that this does notattempt to be a comprehensive account of allwood-borers. Only the borers of significance inwood preservation are described in detail andothers are mentioned only in passing so that thereader is aware of their existence.

Ambrosia beetles

According to the systematic approach brieflyoutlined earlier in this appendix the first wood-borers that must be considered are the Ambrosiabeetles, members of the Scolytidae andPlatypodidae, which are also known as pinholeor shothole borers after the damage that theycause. The adult beetles are generally 3–6 mm(1/8 in-1/4 in) long and bore round holes throughthe bark of fresh, green logs, particularly inhardwoods rather than softwoods. These holessometimes penetrate deep into the wood, formingextensive branched galleries (Fig. B.1), theshape often being characteristic of the species.Eggs are laid in the galleries and the ‘ambrosia’

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fungus introduced through spores carried by theadult. The development of this fungus is, ofcourse, dependent upon the presence of sugar,starch and a high moisture content in freshlyfelled wood and this is, in fact, a condition forinfestation by these beetles as the larvae hatchingfrom the eggs browse on the fungus. Some larvaeextend the galleries to form niches in which theypupate.

Pinhole, Shothole

The damage consists of galleries which aresometimes just beneath the bark and thuscommercially insignificant but in some speciesthe galleries penetrate into the sapwood,particularly in tropical hardwoods and, forexample, in European oak attacked by Platypuscylindrus. The bore holes vary in size, thesmaller ones being described as pinholes and thelarger ones in tropical species being sometimesknown as shotholes. The galleries are free frombore dust and stained brown or black internally

by the Ambrosia fungus, and this infection canalso extend along that grain adjacent to thegalleries to form a characteristic candle-shapedstain. Pinhole or shothole damage is neverstructurally significant but frequently occurs indecorative woods and results in lower grading.

Infestation ceases as the wood dries anddamage can be avoided only by treatment of thelogs immediately after felling. Some species ofAmbrosia beetles are dependent upon bark andcan be readily controlled by its removal. Damageby other species can be prevented to some extentby rapidly drying logs or immersing them inwater, or, in the case of temperate climates, byextraction during the winter months. Theseprecautions are not always possible andtreatment of freshly felled logs with suitableinsecticides is frequently employed, perhapscoupled with a fungicidal treatment to preventthe development of Ambrosia and other stainingfungi. A few Platypodids have been found boringin living trees. This rarely happens but somedamage to tropical hardwoods may be caused inthis way and cannot therefore be controlled bylog treatments. Only a few holes may be presentin the bark of an apparently sound tree so thatthe damage cannot be detected until after fellingand conversion of the wood.

Oak and chestnut from North America isoften damaged and graded as ‘sound wormy’,whereas damaged mahogany from Africa isgraded as ‘pin wormy’, both gradings indicatingthat the wood is structurally sound butunsuitable for use in solid furniture or veneer.Damage is normally caused by Scolytids, butsome Platypodids are also important, anddamage is most severe in sub-tropical andtropical climates. The most important Scolytidgenera are Trypodendron, Pterocyclon, Webbia,Anisandrus and, particularly Xyleborus (Fig.B.2). The most important Platypodid generaare Platypus, Crossotarsus and Diapus. SeventyScolytid species have now been identified inthe British Isles alone, but the most seriousdamage in Europe can be attributed to a single

FIGURE B.1 Ambrosia beetle galleries beneath bark.(Wykamol Ltd)

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Platypodid, Platypus cylindrus (Fig. B.3), which isfound particularly in more southerly, oakwoodlands. This species, which can often befound in wind-blown trees, stumps and logs,principally infests oak but also occasionallybeech, ash and elm. Graphium spp of fungi arealways found in the galleries and oftenCephalosporium and Ceratostomella spp inaddition, the latter being well known as a cause of

sapstain, even in the absence of Ambrosia beetleattack. Although several other non-indigenousPlatypodids are now found in Europe, almost allother damage can be attributed to Scolytids.Scolytus (Hylesinus) fraxini frequently causesdamage in ash logs, largely in the cambiumimmediately under the bark, whilst Scolytusdestructor causes similar damage in elm.

A beetle that has attracted considerableattention in recent years is Scolytusmultistriatus, as heavy infestations are alwaysfound in trees infected by the Dutch Elm diseasefungus, Ceratocystis ulmi. As spores of thisfungus are always found on these beetles theyhave been frequently described as the cause ofthe disease, yet Scolytids do not normally attackhealthy trees and it is far more likely that thebeetles are attracted to a tree which is alreadyinfected and which will thus provide a good sitefor boring and egg laying. This is perhapsconfirmed by the observation that this species isfound throughout Europe and North America, aswell as in Australia, where the disease isunknown. Generally, the species forms a verticalgallery under the bark of a standing tree, withlarval galleries branching off to form acharacteristic fan shape.

Other Scolytids causing damage tohardwoods in Europe are Xyloterus signatus anddomesticus, Xyleborus saxeseni andmonographus, and Anisandrus dispar.

Scolytids also cause damage to softwoods, forexample Ips typographus and Pityogenesbidentatus in spruce and Myelophilus spp inpines in the British Isles. Occasionally, severelocal infestations have occurred, such as that ofTrypodendron lineatum in Argyllshire, but inEurope generally Xyloterus lineatus probablyrepresents the most serious Ambrosia beetleproblem in softwood. In North America themost serious damage is caused by Dendroctonusspp, D. frontalis being known as the Southernpine beetle, D. breviconis as the Western pinebeetle and D. ponderosae as the Mountain pinebeetle respectively; damage by all of these species

FIGURE B.2 A Scolytid, Xyleborus saxesini.

FIGURE B.3 A Platypodid, Platypus cylindrus.

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occasionally being observed in wood importedinto Europe. Generally, a wide variety of speciesis involved in damage caused in importedtropical hardwood, but occasionally anindividual species may attract particular interest,such as Diapus furtivus, which was at one time aparticular problem in hardwood imported fromMalaysia.

Melandryid bark-borers

The family Melandryidae are bark-borers, andare often confused with the Ambrosia beetles orthe Anobid bark-borer, Ernobius mollis, whichwill be described later. Thirteen genera of veryvariable form and habits have been reported inthe British Isles but they are all rare and the onlyspecies likely to be observed as a bark-borer isSerropalpus barbatus.

Bostrychid family

The family Bostrychidae includes a number ofimportant species which can be most convenientlydivided into two groups, the Powder Post beetlesand the Furniture beetles. The Powder Postbeetles can be divided in turn into two sub-families, the Bostrychids and the Lyctids.

Bostrychid Powder Post beetles

The Bostrychids, the Auger beetles, are alsoknown as Shothole borers in some areas such asSouth Africa and this can lead to confusion withthe Ambrosia beetles. However, in the areasconcerned the Ambrosia beetles, the Scolytidsand Platypodids, are invariably small and knownas Pinhole borers so that the possible confusionis confined only to the larger borers. Most Augerbeetles are small, 3–6 mm (1/8 in-1/4 in) long,although one species, Bostrychopsis jesuita, isabout 20 mm (3/4 in) long. The beetles arecylindrical in section with spines on the frontedge of the thorax, which is hooded so that isconceals the head from above, the bostrychoid

form, from which this sub-family and familyderive their name. These features and the three-jointed antennae enable the Bostrychids to bereadily identified. The European Bostrychids aredark brown or black in colour with the singleexception of a species attacking oak, Apatecaputina, which has brown or red elytra.

Adult Bostrychids tunnel in bark in order to layeggs, producing tunnels which are free of dust.The hatching larvae then bore in the sapwood insearch of starch, producing tunnels which arepacked with fine bore dust, as is the case with theLyctid Powder Post beetles. This pattern oftunnelling and the four-jointed legs of the curvedlarvae enable this damage to be distinguishedfrom that of the Lyctids. In fact, Bostrychiddamage is not so common as Lyctid damage,probably because infestation commences with atunnel bored by the adult in contrast with aLyctus infestation which is initially completelyinvisible. Damage is principally confined to thesapwood of green hardwoods, althoughsoftwoods are occasionally found to be attacked,particularly if they have bark adhering. The adultbeetles are able to bore into wood treated withsome preservatives, such as creosote and copper-chromium-arsenic salts, but the egg larvae die andare unable to cause further damage.

The most common Bostrychid damage intropical hardwoods is caused by Heterobostrychusbrunneus, Xylopertha crinitarsis andBostrychoplites cornutus in West African woodsand Heterobostrychus aequalis in woods fromIndia, Malaysia and the Philippines. NorthAmerican hardwoods are infested by Schistoceroshamatus and occasionally Xylobiops basilare inash, hickory and persimmon. The only Bostrychidcommon in Europe is Apate capucina (Fig. B.4),which attacks oak. Dinoderus spp, particularlyD. minutus, are sometimes found in bamboo andbasketwork. In Australia Mesoxylion collaris,a red-brown Bostrychid about 6mm (1/4 in) longis sometimes found in the south-east, attackingthe sapwood of sawn wood in buildings, andMesoxylion cylindricus, a larger and deeper

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brown Bostrychid about 12 mm (1/2 in) long issometimes found in poles and mining timber.The very large Bostrychopsis jesuita, a darkbrown or black beetle 20 mm (3/4 in) long andproducing 6 mm (1/4 in) diameter holes is oftenfound in Australia. It is not common in sawnwood but is occasionally found in sub-floorstructures, where the relatively high moisturecontent permits its continuing development.

Lyctid Powder Post beetles

The Lyctid beetles are all small with a length ofonly about 4 mm (1/6 in). These beetles areelongate but flattened in appearance and, fromabove, the head is clearly apparent, protrudingin front of the thorax. The colour for the variousspecies varies from mid-brown to black and theantennae possess a distinct two-jointed club. Thebeetles are active fliers, particularly on warmnights. After mating the female lays 30–50 eggsin large pores on the end-grain of suitablehardwoods containing adequate starch. The eggsare elongate, oval or cylindrical with a strand atone end and they hatch after 8–14 days. Thelarvae feed on the yolk at first until they growsufficiently to gain a purchase on the side of thepore. At this stage the larva is straight-bodiedand moves along the grain but, after an initial

moult, it becomes curved and commences toburrow across the grain, and eventually grows toa length of about 6 mm (1/4 in). It can bedistinguished from other similar larvae by theprominent spiracle on either side of the eighthsegment, immediately before the last segment,and the three pairs of minute three-jointed legs.

Pupation occurs in a chamber immediatelybeneath the surface and an adult beetle emergesafter 1 or 2 years in the spring, summer orautumn, usually between late May and earlySeptember in Europe, leaving a flight hole 0.8–1.5mm (1/32 in-1/16 in) in diameter. The lifecycle is shorter in warmer conditions. Thegalleries are packed with soft, fine bore-dust butthe tunnels are not distinctly separate as arethose of the Furniture beetles and all thesapwood may be completely destroyed exceptfor a surface veneer, accounting for the name ofPowder Post beetle. As the initial attack consistsonly of an egg laid in an open pore it will beappreciated that the first sign of damage iscollapse or alternatively the appearance of aflight hole, which is an indication of extensivedamage within the wood.

It is unlikely that Lyctid beetles are indigenousin Europe and new species are being continuallyintroduced in infested wood. The two mostimportant species are Lyctus brunneus (Fig. B.5)and L. linearis, the latter being readilydistinguishable by the long rows of hairs on theelytra. Lyctus planicollis and L. parallelopipeduswere originally confined to North America buthave been identified extensively in Europe sinceWorld War I, whilst L. cavicollis from the UnitedStates and L. sinensis from Japan have also nowbeen introduced in imported oak. Minthea spp,which are largely confined to tropicalhardwoods, are difficult to distinguish fromLyctus spp and cause exactly the same damage;M. rugicollis is most common. However,Minthea adult beetles burrow, as does Lyctusafricanus, whereas only the larvae of otherLyctids do so.

The Bostrychid Powder Post beetles will at-

FIGURE B.4 A Bostrychid, Apate capucina.

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tack most freshly felled hardwoods, preferablythose with bark adhering, provided the moisturecontent remains reasonably high and stable. Incontrast, the Lyctid Powder Post beetles confinetheir infestations to dry, felled wood still retainingstarch but only of those species which have largepores, such as oak, ash, walnut, elm and hickoryin temperate climates. Numerous tropical woodsare susceptible, particularly light-colouredspecies. Susceptible African species are obeche(wawa), agba, afara (limba), African mahogany,antiaris, iroko, afzelia, albizia, panga panga,gaboon (okoume), Rhodesian teak and, in SouthAfrica, black wattle. In the Far East susceptiblewoods include ramin and the many Shorea spp,such as seraya, meranti and lauan. In Australiamany of the Eucalypts are susceptible.

Starch is essential to the development of allPowder Post beetles, whether Lyctids orBostrychids. In temperate forests the felling ofhardwoods in early winter when the starch contentis higher will obviously encourage infestation.Starch is confined to the sapwood so that sapwoodremoval represents one method of control, butusually insecticide treatment is applied if wood is

to be air-seasoned. The starch degeneratesprogressively and wood becomes immune to attackafter a period of several years. Air-seasoned woodis, in fact, less susceptible as the cells remain aliveduring the early stages of seasoning, reducing thestarch reserves. Kiln-seasoning is sometimesadvocated as a means of controlling Powder Postbeetles but, whilst it achieves complete control ofBostrychids, the wood cells are kilned attemperatures in excess of 40°C (104°F) allowingthe starch to remain so that kiln-dried wood isparticularly susceptible to a new infestation.

Heavy Powder Post beetle infestations aregenerally accompanied by parasites andpredators, including the mite, Pyemotes(Pediculoides) ventricosus, the Clerid beetles,Tarsostenus univitartus and Paratillus carus, aswell as ant-like Hymenoptera, the winglessSclerodermus domesticus and S. macrogaster,and the winged Eubadizon pallipes, predatorsare mentioned later in this appendix.

Anobid Furniture beetles

The Furniture beetles, comprising the sub-familyAnobiidae, are probably the best known wood-borers in temperate areas, probably becausedamage occurs in furnishings and is thus readilyapparent to the householder. This sub-familycan be further divided into the Anobiinae, whichproduce elongated ovoid or rod-shaped pellets,and the Ernobiinae, consisting of Ernobius,Xestobium and Ochina spp, which produce bun-shaped pellets, a useful diagnostic feature whenonly damaged wood is available. All the Furniturebeetles possess the hooded bostrychoid thoraxconcealing the head from above. The last threejoints of the antennae are always larger thanthe other joints, except in Ptilinus pectinicornisand Lasioderma serricornia, which are readilyidentified by their comb-shaped antennae. Allthe larvae are curved and the life cycles tendto be long, with a very slow build-up ofinfestation over many years. All Furniture beetleslay eggs in cracks or open pores and consid-

FIGURE B.5 A Lyctid, Lyctus brunneus.

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arable damage may occur before the infestationbecomes apparent through the development offlight holes and bore dust discharges.

Anobium punctatum (domesticum, striatum),the Common Furniture beetle (Fig. B.6),probably originates in the northern temperatezones but it is now widely distributedthroughout the world, infesting sapwood ofhardwoods and softwoods, as well asheartwoods of some light-coloured hardwoodsand heartwood of most species in dampconditions. The beetle is 2.5–5.0 mm (1/10 in-1/5 in) long—the female tending to be larger—reddish to blackish brown in colour and with afine cover of short, yellow hairs over the thoraxand elytra, particularly on freshly emergedspecimens. When viewed from the side thehooded thorax has a distinct hump and there arealso rows of pits on the elytra which account forthe names punctatum and striatum.

The beetles emerge from the wood in northerntemperate climates between late May and earlyAugust and can often be seen crawling on wallsand windows. The beetles are strong fliers onwarm days and live for 3–4 weeks during which

they mate and the female lays up to 80 lemon-shaped, white eggs about 0.3 mm (1/80 in) longin cracks, crevices, open joints and old flightholes, usually in small groups. The eggs hatchafter 4–5 weeks, the larva breaking through thebase of the egg and then tunnelling within thewood in the direction of the grain. The galleryincreases in diameter as the larva grows,occasionally running across the grain. Thegalleries are filled with loosely-packed, grittybore dust consisting of granular debris plus ovalor cylindrical pellets, compared with the finepowder in the case of Powder Post beetle attack.When fully grown the curved larva is about 6mm (1/4 in) long with five-jointed legs.Eventually the larva forms a pupal chamber nearthe surface about 6–8 weeks before emergencethrough a flight hole about 1.5 mm (1/16 in) indiameter, larger than a Lyctid flight hole butsmaller than that of the Death Watch beetle.Under optimum conditions the life cycle can beas short as one year but it is usually longer andup to four years.

As infestation is largely confined to sapwoodthe damage is not usually structurally important,except where individual components in furnitureare composed entirely of sapwood or where oldtypes of blood or casein adhesives have beenused, as these considerably encourageinfestation. Infestation is also encouraged bydampness, slight fungal or bacterial activityenabling the infestation to extend into normallyresistant heartwood. All these situations tendingto favour Common Furniture beetle attackappear to be related to nitrogen availability andresult in shorter life cycles; such an exaggerationof activity is particularly noticeable in stablesand byres, which are often extensively damaged.As in the case of other Anobids, activity isindicated only when a bore dust dischargesuggests recent emergence. It is therefore oftendifficult to decide whether a remedial treatmentis necessary or, if it has been completed, whetherit has been effective.

Common Furniture beetles (Fig. B.7(a)) suf-

FIGURE B.6 Common Furniture beetle, Anobiumpunctatum.

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fer a very high natural mortality, perhaps only60% reaching the larval stage, where they maybe further reduced in numbers by the action ofpredators. The most common predators are twoHymenoptera, the flying, ant-like Theocolaxformiciformis and Spathius exarator, which canoften be seen exploring flight holes. Predatorybeetles are also sometimes found, such as Opiliomollis (domesticus) and Korynetes coeruleus, butthe latter is more often associated with the DeathWatch beetle; predators are described later inthis appendix.

In New Zealand Anobium punctatum isknown as the Common Houseborer, anunfortunate name as it leads to confusion withthe House Longhorn beetle, which is known inNorthern Europe and many other countries, suchas South Africa, as the Houseborer. TheCommon Furniture beetle is a particularlyserious problem in New Zealand as it can causeextensive structural damage to the light-framed,softwood buildings that are so often used. Theclosely related Anobium pertinax is sometimesfound, particularly in buildings in Scandinavia,but only in association with fungal attack so thatsevere infestations are normally confined topoorly ventilated cellars or sub-floor spaces, orin timber subject to periodic rainwater orplumbing leaks.

Xestobium rufovillosum (tesselatum) , the

Death Watch beetle, (Figs B.7(b) and B.8), is thelargest Anobid, with a length of 6–8 mm (1/4 in-1/3 in). The Death Watch beetle attacks onlywood that is subject to dampness and some decay;indeed, a common characteristic feature of DeathWatch beetle attack is a brown colouration in theinfested wood, arising from the fungal decay.Infestations in the British Isles occur mostcommonly in oak, probably because this woodused to be extensively employed in construction,but infestations can also occur in elm, walnut,chestnut, alder and beech. Sapwood andheartwood can be infested if previously infectedby decay, and the infestation can spread intoadjacent softwoods, though infestation is alwaysconfined to damp or decayed areas. It frequentlyappears that this insect favours churches but thisis really a combination of circumstances whichresults in church timbers being particularlysuitable; the roofing frequently consists of sheetsof lead or other metals and the periodic heatingresults in condensation, which causes the incipientdecay that encourages infestation.

The Death Watch beetle is chocolate brown incolour and has patches of short, yellow hairs,which give a mottled appearance. The thorax

FIGURE B.7 Side views of (a) Common Furniture and(b) Death Watch beetles, showing distinctive shapes ofthe hooded thorax.

FIGURE B.8 Death Watch beetle, Xestobiumrufovillosum.

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conceals the head but is very broad, appearingfrom above to form a hood over the front endsof the elytra, whereas other Anobids have adistinct waist between the thorax and the elytra.The larvae are very similar to those of theCommon Furniture beetle but attain a far largersize, eventually growing to about 8 mm (1/3 in).

The length of the life cycle depends upon thequantity of nitrogen available in the form offungal attack and can be a single year underoptimum conditions, but it is usually far longerand perhaps as much as ten years. In northerntemperate climates the adult emerges betweenthe end of March and the beginning of June and,after mating, the female lays about 40–60 white,lemon-shaped eggs 0.6 mm (1/40 in) long incracks, crevices and old exit holes. The larvaehatch after 2–8 weeks and explore the surface ofthe wood before commencing to bore. Wherelonger life cycles are involved the fully-grownlarva pupates in July or August, meta-morphosing into an adult after only 2–3 weeks,but it remains in the wood and graduallydarkens in colour until it emerges the followingspring, leaving a flight hole 3mm (1/8 in) indiameter.

The adult is not a free flier and tends to matewith other beetles emerging from the same pieceof wood, attracting their attention by movingthe legs so that the head is struck on the woodsurface, producing a series of 8–11 taps in aperiod of about 2 seconds. Tapping with the tipof a pencil can generate a similar noise and canstimulate a response. This tapping noiseprobably accounts for the name of Death Watchbeetle, perhaps because the sound is apparent ina house which is quiet through a recent death.The tapping should not be confused with thesound produced by Psocids such as the book lice,Trogium pulsatorium, which is more like awatch tick than a tap. Although the insects canfly reasonably well when the weather is verywarm, it is probable that this pest is spreadlargely by re-use of old infested wood.

The Death Watch beetle has a much more

limited distribution than the Common Furniturebeetle and in the Britih Isles it is confined toEngland, Wales and part of southern Ireland. Insome areas, such as Germany, the Death Watchbeetle is frequently confused with the CommonFurniture beetle although the flight holes aremuch larger and the galleries are packed withmuch coarser bore dust with distinct bun-shapedpellets, as opposed to the oval or cylindricalpellets of the Common Furniture beetle. TheDeath Watch beetle is always associated withdampness and decay and can therefore be readilydistinguished from Ernobius mollis, whichproduces a similar sized flight hole, as the latteris confined to softwoods with adhering bark.Wood infested by the Death Watch beetle mayalso be attacked by other insects, such as theCommon Furniture beetle and the Wood weevils,and perhaps Helops coeruleus in the damperzones of the wood. Predators may also bepresent, particularly Korynetes coeruleus, adistinctive blue beetle which is an active flier andoften the first sign that a Death Watch beetleinfestation is present in concealed damp timbers.

Ernobius mollis, sometimes known simply asthe Barkborer (Fig. B.9), is an Anobid beetle 3–6mm (1/8 in-1/4 in) long, midway in size betweenthe Common Furniture and the Death Watchbeetles. It is reddish brown in colour with yellowhairs. Compared with that of the CommonFurniture beetle the thorax forms a distinct

FIGURE B.9 The Barkborer, Ernobius mollis.

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triangle and is not so hooded so that the headprotrudes slightly when seen from above. Theantennae are also proportionately longer. Thelarvae are similar in appearance and eventuallygrow to about 6mm (1/4 in) long.

Eggs are laid in cracks in the bark of freshlyfelled softwood, the larvae tunnelling throughthe bark and up to about 1 cm (1/2 in) into thesapwoods. This is typical of Europeanobservations but this beetle also occurs inSouth Africa, Australia and New Zealand,where it is reported to bore rather moredeeply, perhaps because the softwood speciesin these countries have a deeper sapwoodzone. The galleries are filled with bore dustwhich contains bun-shaped pellets which areeither brown or white in colour, dependingupon whether the larva has been feeding onbark or xylem. The life cycle is typically oneyear, the adult beetle emerging in northerntemperate climates between May and Augustfrom a flight hole 2.5 mm (1/10 in) in diameterusually close to an area of retained bark. Thisbeetle is widely distributed throughout Europebut attacks only softwood with bark adheringto it, often causing the bark to strip away fromrustic poles and posts. It is dependent uponstarch and, just as the Lyctid Powder Postbeetles, can attack wood only for a few yearsafter it has been felled. In remedial treatmentsdamage by this beetle is often confused withthat by the Common Furniture beetle.

The Anobid beetle Ptilinus pectinicornis (Fig.B.10) is readily identified as it has a distinctlyglobular thorax and the antennae are comb-likein the male and serrated or saw-like in thefemale. The adult beetles are distinctlycylindrical, about 3–5 mm (1/8 in-1/5 in) long,with a dark brown prothorax and reddish elytra.The larvae are similar to those of the CommonFurniture beetle, but identification can be achievedif it is essential. In many respects the behaviour ofthis insect is similar to that of the Lyctid andBostrychid Powder Post beetles. Adult beetlesemerge from the wood in Europe between May

and July through exit holes about 1.5mm (1/16 in)in diameter, the same as Common Furniturebeetles. The adult female beetles are often foundwithin the wood, apparently extending oldgalleries or excavating new ones from which to laytheir long, thin, pointed eggs in adjacent vessels orpores. The life cycle can be only one year inoptimum conditions but it is usually 3–4 years.

This beetle is found in beech, sycamore, mapleand elm and can be a nuisance when it occurs infurniture. It is rarely observed in softwoods,clearly because of the limited size of the pores,but it appears that it may occasionally lay eggsin splits. The bore dust is finer than that of theother Anobid beetles and similar to thatproduced by the Lyctid beetles but more denselypacked. This beetle is frequently found inassociation with the Common Furniture beetlewhen infesting woods attractive to both species,such as beech.

There are a number of other Anobid beetleswhich are found in nature but rarely instructural or decorative wood, for exampleNicobium castaneum, which is found infestingwood in Mediterranean countries, Anobiumdenticolle and Hedobia imperialis, which arefound in hawthorn, and Ochina hederae

FIGURE B.10 Ptilinus pectinicornis.

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(ptinoides), which is found in ivy. Stegobiumpanaceum (Sitrodrepa panicea), the Drug Store,Biscuit or Bread beetle, is found in dried, woody,natural drugs, cork, dog biscuits and othersimilar materials, often appearing in largenumbers in buildings and producing fears of anextensive Common Furniture beetle infestation;the beetle is, in fact, less elongate, more reddishbrown and smaller, and lacks the distinct humpwhich is a feature of the thorax of the CommonFurniture beetle. Similar fears may arise inwarehouses where stored tobacco is infested bythe Cigarette beetle, Lasioderma serricornia,although this Anobid beetle is readilydistinguished by its serrated antennae. Theclosely related Ptinids or Spider beetles are alsosometimes confused with Anobids, although theypossess an abdomen which is distinctly morerounded, and much longer legs; household peststhat are confused with wood-borers aredescribed more fully in Remedial Treatments inBuildings by the present author.

Cerambycid Longhorn beetles

The Cerambycidae, the Longhorn beetles, are avery large family with widely varying habits.Their name arises from the fact that theirantennae are sometimes longer than the rest ofthe body. These beetles vary in length from 6 to75 mm (1/4 in to 3 in) and some species arebrightly coloured. The eggs are white, oval orspindle-shaped, and normally laid in crevices inbark. All the larvae are straight, with a slighttaper towards the rear. Larvae can be up to100mm (4 in) long when fully grown, legless orwith very short, useless legs. The flight holes arecharacteristically oval. Although there are manyspecies there are only a limited number ofeconomic significance as they are mainly forestscavengers infesting damp, rotted wood.

Some Longhorn beetles, such as Callidium,Phymatodes, and Trinophylum spp, bore aslarvae under the bark and then penetrate up to100 mm (4 in) into the wood in order to pupate.

Others, such as Ergates, Macrotoma, Cerambyx,Monohammus and Batocera spp, bore entirely inwood, often into heartwood in species such ashickory and ash where there is relatively littledifferentiation between heartwood and sapwood.Only a few species, such as Hylotrupes,Stromatium and Oemida are able to bore in drywood after removal of the bark. Often the formof the galleries enables the infesting Longhorn tobe identified, so that damage by Hylotrupes,Phymatodes and Monohammus can be readilydistinguished in this way. However, the thicknessof the bark and girth of the tree can influence theshape and nature of galleries and chambers. Forexample, the larvae of Leiopus nebulosus form apupal cell as an oval excavation immediatelybeneath the moderately thick bark in oak butpenetrate deep into the sapwood in chestnut,which has a thin bark.

When Longhorn damage is discovered it ismost important to decide whether the infestationis confined to the forest or whether it is able toprogress and spread in wood in service. As wooddries the life cycle of some of the forestLonghorns increases and many species are ableto survive, occasionally for exceptionally longperiods of 25 years or more, eventually emergingand perhaps establishing a new Longhorninfestation problem in adjacent suitable forests.Most of the serious Longhorn forest pests inEurope have been introduced in this way.

In Europe hardwoods are attacked by anumber of Longhorn beetles, but infestations inhealthy trees are rare and in most cases the treesare sickly or the wood even dead or decayed.Oak sapwood is sometimes attacked by the OakLonghorn beetle, Phymatodes testaceus (Fig.B.11). Eggs are laid in cracks within the barkand the larvae bore between the bark and thewood in standing, sickly trees, eventuallypupating in chambers within the wood. Thisspecies is also sometimes found in beech. Oakis also attacked by Cerambyx cerdo, Clytusarcuatus and C. arietus, the latter beingknown as the Wasp beetle because of its distinctive

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black and yellow bands. If oak is decayed it canbe attacked by Rhagium mordax and Leiopusnebulosus.

Sometimes, species associated with softwoodsare reported in hardwoods, for exampleHylotrupes bajulus and Callidium violaceum inoak sapwood and Molorchus minor under thebark of birch. Aspen, widely used in Scandinaviafor the manufacture of matches and saunafurniture, is sometimes attacked by Clytusrusticus, whilst Saperda carcharias is found inpoplar and willow. The latter species is ratherunusual as the adults feed on the leaves, tendingto kill the tree. The eggs are eventually laid atthe base of the young, sickly trees and the larvaebore within the wood, principally along thegrain; their galleries are often eventuallydisclosed by woodpeckers.

The most important Longhorn species toattack North American oak are Smodicumcucujiforme, Chion cinctus and Romaleumrufulum, these often penetrating the heartwoodas well as the sapwood. Eburia quadrigeminata,which is yellow or pale brown and 18–24 mm (3/4 in-1 in) long, is also occasionally found andcan survive for many years in service before itemerges; it has been found emerging from a

bedpost 25 years after manufacture and has beenreported as emerging from other items as muchas forty years often their manufacture. NorthAmerican hickory and ash are sometimesinfested by the ash borer, Neoclytus caprea orthe red-headed ash borer, N. erythrocephalus.

Longhorn infestations are less common insoftwoods, the most serious being that ofHylotrupes bajulus, the House Longhorn beetle,which will be considered in detail separately inview of its ability to infest dry softwood inservice. Occasionally Tetropium castaneum andT. fuscum are found in spruce and silver fir fromPoland, and Callidium violaceum andMonohammus spp in pine and spruce fromScandinavia and Russia; such infestations inimported wood have now resulted in these pestsbecoming established in the British Isles. OtherLonghorns introduced in this way are Tetropiumgabrieli, which is found in England and Walesattacking sickly larch trees and logs which arealso attacked by the Spruce Longhorn, Callidiumviolaceum. Acanthocinus aedilis is a cause ofdamage in pine in Scotland only; this wood ismore often attacked by Criocephalus rusticus,Asemum striatum and Rhagium bifasciatum, thelatter being one of the commonest Longhorns inBritain, though it has little economic significanceas it attacks only decayed softwoods.

Longhorn infestations are rare in softwoodimported from North America, but occasionallyDouglas fir or Sitka spruce may be infested byMonohammus titillator, M. scutellatus orErgates spiculatus; the latter species is able tosurvive in wood in service for many years andhas been found by the author emerging fromDouglas fir floorboards thirty years afterinstallation. Another species that is able tosurvive in service is the Two-toothed Longhornfrom New Zealand, Ambeodontus tristus, whichis described in more detail later.

Longhorns are not confined to temperateareas. Oemida gahani is often found in soft-wood, such as Cupressus, in Kenya, whereAndroeme plagiata, which is very similar in

FIGURE B.11 Oak Longhorn beetle, Phymatodestestaceus.

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appearance, is found in both hardwoods andsoftwoods. By about 1958 Longhorn damage hadbecome so extensive in Kenya in some woods suchas Isoberlinia that felling had to be abandoned asuneconomic. The Indian Dry-wood borerStromatium barbatum is remarkable as it isknown to attack over three hundred woodspecies. One Indian Longhorn, Trinophylumcribratum, was reported to have becomeestablished in England in 1947 on hardwoodssuch as beech; it was apparently introduced onimported wood and causes damage similar inappearance to that by Phymatodes testaceusexcept that the galleries penetrate 50 mm (2 in) ormore into the wood and it is therefore of greatereconomic importance. Longhorn damage is rarelyfound in tropical African hardwoods and then it isusually caused by Coptops aedificator orPlocaederus spp, the latter species beingcharacterized by calcareous cocoons which linethe pupal chambers in the bark of woods such asmahogany, a feature that is characteristic also ofanother Longhorn, Xystrocera.

It will be appreciated that many of theseforest Longhorns are scavengers which infestdecaying wood and which are of no significancefor wood in service, where the decay problem isfar more serious than any new borer infestation.Most of the other species mentioned are bark-borers which attack sickly trees or felled logsand cause little damage unless their pupalchambers penetrate deeply. A few species boremuch more deeply in wood but infested wood isusually readily detected and rejected, althoughoccasionally infestations survive in shipments.The principal significance of any such survival isthat it can enable infestations to spread to newforest areas; these borers are not generally ableto re-infest dry wood. Only a very few speciesare therefore likely to be encountered as activeinfestations in wood in service.

Occasionally, active infestations of Eburiaquadrigeminata and Ergates spiculatus may befound in North American oak and Douglas fir,respectively, many years after conversion but

these are examples of exceptional survival ratherthan re-infestation. The Oak Longhorn beetle,Phymatodes testaceus, is able to attackEuropean hardwoods, particularly oak, afterthey are dry but only provided the bark remains.The eggs are laid in the bark and the larvaetunnel between the bark and the wood, makingdeeper tunnels to provide a pupation chamber.This species constitutes a problem principallyduring air-seasoning, but emergence can alsooccasionally occur from new oak boards inservice; North American Longhorn species maysometimes emerge from imported oak. The twosmallest Longhorns, Cracilia minuta andLeptidea brevipennis, can also attack dry wood,but they are confined to wicker work and aretherefore of limited importance. In fact, the onlyLonghorn species of real economic significance isthe House Longhorn beetle (Fig. B.12).

The House Longhorn beetle, Hylotrupesbajulus (Callidium bajulum), is known in manyareas as the European Houseborer. Althoughoriginally confined to central and southernEurope this species has now been introduced onimported wood to North America, South Africaand Australia, it apparently reaching the lattercontinent through the importation of infestedprefabricated buildings in 1948. Even in Europeits distribution has been influenced by the

FIGURE B.12 House Longhorn beetle, Hylotrupesbajulus.

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establishment of trading routes and localconditions. For example, it was first reported inLondon in 1795, perhaps being imported inorange boxes from Spain, but the infestationsapparently died out, probably through pollutionin the nineteenth century. It is now confined to theless polluted areas to the south-west of London,where it is known as the Camberley borer, andspread to other parts of England has notoccurred, perhaps through climatic restrictions. InScandinavia it is found throughout Denmark, theBaltic Islands and in the south of Sweden andNorway around the major trading areas such asOslofjord, Stavanger and Bergen.

House Longhorn beetle attacks the sapwoodof dry softwoods. It is not as widespread as theCommon Furniture beetle but the damage itcauses is much more rapid and more severe so itcan be considered the most important wood-borer in temperate areas free from termites. Theadult beetle is somewhat flat and (10–20 mm)(2/5 in-4/5 in) long, the male being smaller thanthe female. The beetles are brown to black incolour except that they have thick, grey hairs onthe head and prothorax (the front section of thethorax), and the female has a central black lineand a black nodule on either side and the malewhite marks. There are also distinct, shapedwhite spots on the elytra.

In Europe the beetles emerge in July toSeptember and a single female can lay as manyas 200 eggs, which hatch within 1–3 weeks,these eggs being spindle-shaped and 2mm (1/12in) long. In a roof structure the larvae from asingle clutch of eggs can cause substantialdamage within a period of 3–11 years beforethey pupate and emerge as adults, perhapsentirely destroying the sapwood and leaving onlya thin surface veneer, this slightly distorted bythe presence of the oval galleries beneath. Thefirst sign of damage may therefore be thecollapse of a largely sapwood member, though inwarm weather the gnawing of the insects can beclearly heard. When fully grown the larva isabout 30mm (1¼ in) long, straight-bodied and

distinctly segmented, with a slight taper and verysmall legs. Pupation occurs in a chamber justbelow the surface and is complete in threeweeks, the emerging beetle leaving an oval flighthole about 1 cm (3/8 in) across.

The appearance of even a single flight holeindicates that severe damage has already occurredand that the condition of the structure should bechecked by thorough probing. Because of theseriousness of the damage this beetle causes, thebuilding regulations in England now require allstructural wood to be preserved against this pestin the areas to the south-west of London where itis known to occur. Similar regulations have beenintroduced in other European countries and inSouth Africa, whilst the Australian quarantineregulations are designed to prevent furtherinfestations being introduced.

Two-toothed Longhorn beetle

The Two-toothed Longhorn, Ambeodontustristus, causes serious damage to softwoods inservice in New Zealand. It thrives in similarconditions to the House Longhorn beetle andcauses similar damage, but the oval exit holesare distinctly smaller, being only about 5 mm (1/5 in) across. In 1974 this insect was found tohave caused severe damage to joists in a cellar inLeicestershire, England. The infestation wasintroduced in the joists, which were found to bemade from the wood of a Dacrydium pine,common in New Zealand.

Curculionid weevils

As wood becomes damp or decayed the activityof many wood-borers, such as the CommonFurniture beetle, is encouraged and the woodmay become infested by other species dependentupon decay, such as the Death Watch beetle andparticularly the wood-boring weevils,Curculionidae (Cossonidae). The weevil is ashiny, cylindrical beetle, 3–5 mm (1/8 in-1/5 in)long and brown to black in colour, its head

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protruding into a long snout with ‘elbowed’antennae about halfway along its length. Thelarvae are curved and legless.

Both the adults and the larvae bore, causingdamage which is superficially similar to thatmade by the Common Furniture beetle but theholes are smaller, contain finer bore dust, areoften laminar in pattern and are perhapsconfined to the spring wood. The attack is oftenlimited to sapwood but extends into heartwoodif fungal decay is more severe; however visiblefungal decay does not always appear to benecessary. Eggs are laid singly, either in smallholes in the surface or in niches within thegalleries, at any time of the year and they arehatched within 2–3 weeks, the larvae thenboring for 6–12 months before metamorphosinginto adults, usually during June to October.Unlike most wood-boring beetles the adults cansurvive for a very long period, perhaps 12months or more, actively tunnelling within thewood.

In Britain Pentarthrum huttoni (Fig. B.13) isa native species usually found in buildings indecayed floorboards and panelling, as well as inold casein-glued plywood. Caulotrupisaeneopiceus, another native species, is morerarely identified in buildings and is then onlyfound in association with very decayed wetwood in cellars and under-floor spaces. Veryrarely infestations may be found to be due toCossonus ferrugineus and Rhyncolus lignarius.However, in recent years the weevil that hasattracted most attention is Euophryum confine.This species was apparently introduced toBritain from New Zealand in about 1935 and ithas since spread very widely, apparentlybecause it is able to infest wood which is notsignificantly decayed and which may have amoisture content as low as 20%. This species istherefore of greater significance. Adult beetlesof Euophryum confine can be distinguishedfrom those of Pentarthrum huttoni by the sharpconstriction of the head behind the eyes, asillustrated in Fig. B.13.

Oedermerid beetles

The Wharf borer, Nacerda melanura (Fig. B.14),a member of the family Oedermeridae, issuperficially similar to a Longhorn beetle. It is afree flier and has sometimes been found in greatnumbers in streets and buildings close to dockareas in, for example, London and Copenhagen.The beetle attacks both softwoods andhardwoods which are decayed and apparentlyfavours wood which is wetted by sea water, brineor urine, and the large numbers which aresometimes found originate from piles, groins,quays and piers in sea or river areas. One of themysteries is that, at other times, this insect israrely observed and it is therefore sometimes

FIGURE B.13 Wood weevils, Euophryum confine andthe head of Pentarthrum huttoni.

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confused with the carniverous soldier beetles,Rhagonycha fulva, which are common insummer on garden flowers.

The Wharf borer is 6–12 mm (1/4 in-1/2 in)long, elongate and reddish brown with distinctblack tips to the elytra and long antennae. Thesoldier beetles are much redder with a widerprothorax, less flattened and without the lateralflanges possessed by the Wharf borer. The Wharfborer larvae are rather slender, eventuallyachieving a length of 12–30 mm (l/2in-l¼ in),dirty white in colour, and have a large yellowhead and three pairs of fairly long legs. The firstsegment behind the head has a distinct hump orknob in the centre of the back. Damaged wood isusually brown in colour through decay but alsodistinctly laminar as most of the attack takesplace along the grain.

Other beetles

Another insect associated with damp, decayedwood is Helops coeruleus, one of theTenebrionidae. Attack by this insect is rare, atleast in the British Isles, where it is confined to afew areas in southern and eastern England. Instructural woodwork it is almost alwaysassociated with the Death Watch beetle in oak orchestnut, the Death Watch beetle usuallyattacking areas which are reasonably sound but

infected with fungi such as Merulius lacrymans,Coniophora cerebella and Phellinusmegaloporous, whereas Helops is usuallyassociated with more friable wood in wetterareas, perhaps supporting Paxillus panuoides.

The larvae are long and slender, 25 mm (1 in)or more in length and cylindrical, with a toughskin, typical of this ‘click’ beetle family. The lastsegment is equipped with a pair of large,powerful spines curved towards the head,whereas the adjacent segment has two smallspines curved towards the rear so that the larvais capable of gripping with these appendages.The life cycle appears to be about 2 years, with ashort period of pupation and emergence in Mayto June. At first the adult beetles are brown butlater become deep black with a metallic blue tint.These are particularly handsome beetles 12–25mm (l/2in-1 in) long and very active fliers onwarm nights.

There are a number of other beetles that areof minor importance. In the family BuprestidaeBuprestis aurulenta is sometimes found in NorthAmerican softwoods even 25 years or more afterconversion of the wood, apparently becausedrying has retarded development, as is the casewith some Longhorn beetles, as previouslydescribed. This handsome beetle from westernNorth America, 15–22 mm (5/8 in-7/8 in) long,is a brilliant metallic green except that themargins of the prothorax and elytra are copperyor red. Other Burprestids sometimes causedamage, particularly in the United States, forexample the Turpentine borer, Buprestisapricans.

The Dermestidae are another family thatshould be mentioned as the Hide beetle,Dermestes maculata, is sometimes reported asboring into wood in order to pupate; there havebeen several recent reports in England of itscausing damage in hen houses, for this beetlefrequently infests hen litter. There are manyother Dermestids, such as the Carpet beetles,which are frequently confused with wood-borers, as are so many beetles found in domes-

FIGURE B.14 Wharf borer, Nacerda melanura.

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tic premises; these are described in more detail inRemedial Treatments in Buildings by the presentauthor.

Predators

A more important beetle is Korynetes coeruleus(Fig. B.15), a very active metallic blue Clerid6mm (1/4 in) long, which is a predator on theDeath Watch beetle and whose presence is oftenan indication of a substantial infestation. Therelated Opilio mollis is associated with theCommon Furniture beetle, whilst the smallerTarsostenus univitartus and Paratillus carus,blue black in colour except for a whitetransverse band on the elytra, are often foundassociated with Lyctid infestations. The larvae,which feed on the wood-borer larvae, are whiteand have a straight, cylindrical form with a pairof hooks at the posterior end.

Lyctid infestations also attract smallHymenoptera predators, such as the minute, ant-like, wingless Bethylidae, Sclerodermus domesticusand S. macrogaster, and the minute winged fliesof the Braconidae, Eubadizon pallipes (Fig. B.16).Other Hymenoptera are associated with theCommon Furniture beetle, for example the smallwingless, ant-like Chalcydidae, Theocolaxformiciformis (Fig. B.17) and Spathius exarata.All wood-boring beetle infestations also attractthe very minute mite Peymotes ventricosus,

the female becomes permanently attached to thehost larva by its mouth parts, swelling to form aball about 1 mm (1/24 in) in diameter, whilst themale lives on the body of the female.

Termites

Although various wood-boring beetleinfestations occur throughout the world thedamage caused by termites is generally far moreserious where these insects occur in tropical andsubtropical areas. Although termites aresometimes known as white ants they are, in fact,

FIGURE B.15 Korynetes coeruleus, a predator onDeath Watch beetle.

FIGURE B.16 Eubadizon pallipes, a predator onLyctid Powder Post beetles.

FIGURE B.17 Theocolax formiciformis, a predator onCommon Furniture beetle.

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members of the order Isoptera, whereas the trueants are Hymenoptera. However, they havemany similarities with ants as they are socialinsects forming communities which include themale and female reproductive individuals as wellas specialized sterile forms or ‘castes’, theworkers and soldiers. Generally the workers aresoft-bodied and wingless, confined to the groundor wood, where they devote their energy tofeeding, foraging and building. Soldiers serve adefensive role alone and are equipped with largeheads and jaws. In the Dry Wood termites of theKalotermitidae there are no true workers butnymphs instead. A female queen may producethousands of eggs each day, and the manner offeeding after hatching influences the ultimatedifferentiation of the forms. Reproductive formsare produced at certain times of the year anddisperse to found new colonies.

About 2000 species of termites have beenidentified, of which more than 150 are knownto damage wood in buildings and otherstructures. Termites are principally tropical butextend into Australia and New Zealand and arecommon in North America, though rare inCanada (Fig. B.18). Their introduction intocertain parts of France and Germany is clearly

related to trade; for example the termite ofSaintonge, Reticulitermes santonensis (Fig.B.19), which was established on the west coastof France between the rivers Garonne and Loire,is now found in Paris around the Austerlitzstation, which serves this region. Similarly,Reticulitermes flavipes from the United States isconcentrated around Hamburg. Neither speciescan spread widely as they are evidently sensitiveto temperature and tend to survive in centralheating ducts and other permanently warm areasin buildings.

A common feature of the six families oftermites is the lack of a cellulase digestive enzyme,although all these families include wood-

FIGURE B.19 Worker (a) and soldier (b), the mostcommon castes of the termite Reticulitermes santonensis.

FIGURE B.18 World distribution of termites (after W.V.Harris).

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destroyers that are dependent upon intestinalsymbionts or prior fungal decay in order to digestcellulose. Identification is difficult in view of thelarge number of species, yet some classification isnecessary in order to decide whether a particularspecies represents a significant risk justifyingsome form of preservation. It is also necessary tohave a knowledge of the areas in which wood-destroying termites occur, again in order to decidewhether preservation treatment is necessary;Table B.1 attempts to summarize this situation.

The damage by the Dry Wood termites of theKalotermitidae is similar to that caused by thelarger wood-boring beetle larvae, such as theHouse Longhorn beetle, Hylotrupes bajulus. Insome areas, such as South Africa, both theseinsects are found infesting wood but the termitedamage can be distinguished as the lining of thegalleries is smooth, and the galleries arerelatively large with distinct, small-diameterconnecting tunnels. The faecal pellets are smalland cylindrical with rounded or pointed ends butdistinct grooves down the sides. In fact, as theDry Wood termites spread through flying, egg-laying females, as does the House Longhorn

beetle, only the use of naturally durable oradequately preserved wood will avoid damage.

Almost all other wood-destroying termites aresubterranean, forming nest cavities in soil orvery rotten wood, or mound-building, mostconstructing covered walkways and allcontrollable by poisoning the soil of thefoundations of a building and a surroundingzone. Termite shields have been widely used insupported buildings to isolate the wooden partsof the structure from the soil but it is difficult toconstruct shields that are completely reliable andmost termites are able to find ways round byconstructing mounds or covered walkways;perhaps the main value of the shields is to makethe tubular walkways clearly apparent so thatthey can de destroyed during regular inspections.

European termites

There are only three species of termites whichcan be described as typically and exclusivelyEuropean. Kalotermes flavicollis occursthroughout the Mediterranean and Black Seaareas, usually in old stumps and dying trees. Ithas also been reported in vine stock but it appears

TABLE B.1 Wood-destroying termites

TABLE B.1 (continued)

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that it is actually occupying galleries bored bythe Longhorn beetle Chlorophorus varius;termites are seldom a primary cause of damagein living plants. This species does not appear tobe a true Dry Wood termite in the sense that it isnot usually found in dry wood but is apparentlydependent upon at least incipient decay.

Reticulitermes lucifugus is found in the samearea, approximately south of the Gironde riverin France, but there appear to be several races orstrains of this; the Iberian, French, Sicilian andBalkan insects are all distinctly different inbehaviour. However, they are all subterraneantermites which cause extensive damage tointerior and exterior woodwork, and they areperhaps the most serious wood-destroying pestsin southern Europe.

Reticulitermes santonensis was originallyclassified as a variety of R. lucifugus but it isnow considered to be a true species, distinctlymore active and more resistant to adverse dryand cold conditions. It is found attacking woodin the open and in buildings in the westerncoastal area of France between the riversGaronne and Loire, and has spread along theconnecting railway to Paris where infestationsare now notifiable. In 1956 it was confined tothree areas, with only 55 buildings beingaffected, but by 1965 86 buildings were knownto be infested. Two years later an additional areawas discovered and the infested buildingsincreased to 120, and by 1972 there were a totalof 384 known infested buildings and this specieswas clearly becoming firmly established as aserious wood-destroying pest. This species is alsofound in Yugoslavia.

Reticulitermes flavipes has been identifiedaround Hamburg and in Vienna but theinfestations are very confined in extent andapparently introduced from the eastern UnitedStates, where this species, R. virginicus and R.hageni are the three major subterranean termites.In fact, it is generally true to say that thesubterranean termites represent the major problemin the warmer temperate areas, while Dry Wood

termites, particularly Cryptotermes spp, formthe major risk in most tropical areas, and theKalotermes spp, perhaps not true Dry Woodtermites, are prominent in sub-tropical areas.

Carpenter ants

In many respects the insects of the orderHymenoptera are becoming of increasingsignificance as wood-borers. The Carpenter ants,Camponotus herculeanus (Fig. B.20) and C.ligniperda, have been causing increasing damage inbuildings in Scandinavia in recent years. In naturethese insects tunnel into old trees affected byinterior decay in order to establish nests. Modernforestry, however, leaves only very few suitabletrees and stumps, and the search for suitablenesting sites probably explains the increasingincidence of infestations in buildings. Summerhomes are much more frequently attacked thanpermanent homes, probably because they are oftensituated within or close to the forest.

It has been reported that Carpenter ants willattack only wood which has already decayed but thiscertainly appears to have been discounted in recentyears. It has also been suggested that they do notswallow or digest the wood in which they are boringand that they are therefore able to damage woodtreated with preservatives containing stomachinsecticides, such as copper-chromium-arsenicformulations. The probable explanation for theseconflicting reports is the habit of describing both

FIGURE B.20 Worker of Carpenter ant, Camponotusherculeanus; reproductive castes are winged.

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species collectively whereas they are, in fact, distinctlydifferent in behaviour; it certainly appears that C.ligniperda is able to attack and utilize dry wood.Damage is typically internal, being an irregular cavityin soft, decayed wood, but it is laminar and followsthe growth rings in sound wood.

Camponotus vagus is occasionally found insouthern Europe and C. pennsylvanicus in NorthAmerica. Other American species are Lasiusbrunneus, which causes similar damage toCamponotus, and L. fuliginosus, which attackswood to obtain material for constructing its nest.There is an increasing danger that these insectswill be introduced to other countries in woodshipments and there are already reports ofisolated infestations in the British Isles.

Carpenter beesThe order Hymenoptera also includes the familyXylocopidae, the Carpenter bees. These are thelargest known bees, black with dark and ofteniridescent wings, the fine hairs over the bodyfrequently being yellow, white or brown. Theygenerally resemble the Bumble bees, Bombus spp,but they are more flattened and less hairy. They arewidely distributed in tropical and subtropicalareas, but four species have also been reported inFrance, where the adults have been found boring inbeams, rafters and other structural timbers; thesebees penetrate very deeply and divide theirburrows into a series of cells using fragments ofwood. A single egg is deposited in each cell,together with a supply of pollen, and the purposeof the boring appears to be to provide a completelysafe egg-laying site. Hesperophanes cinereus andXylocopa violacea are frequently reported ascausing limited structural damage, principally incentral France and along the valley of the Loire butalso as far north as Paris; however, the damage isof very limited economic importance.

Saw filesOf similar limited importance is the familyCephidae, the Saw flies, slender-bodied, flyinginsects which frequently case damage to standing

crops. However, one species, Ametastegiaglabrata, is occasionally found boring in woodand has recently been reported to have causeddamage in England to creosote and copper-chromium-arsenic salt-treated posts in motor-way fencing. The attack takes the form of acircular entry hole about 3 mm (1/8 in) indiameter leading to unbranched blind tunnelswhich can be 30mm (1 1/4in) in length.

Wood wasps

The most important Hymenoptera are theSiricidae, the Wood wasps. The females use theirovipositers to bore into the bark, laying theireggs below this in the phloem. The latter thenbecomes infected with a fungus, such as Stereumsanguinolentuns, on which the larvae feed. Thelarvae of Sirex noctilio, which are often found inScots pine, eventually reach a length of 25 mm (1in) or more before pupating. In the meantimethey will have formed an extensive series ofgalleries which tend to loosen the bark—animportant stage in the destruction of sickly anddead trees in the forest. Adults may emerge fromwood in service; however, their galleries can bereadily distinguished from those of the Longhornbeetles as the flight holes are circular.

Urocerus (Sirex) gigas, the Giant Wood wasp,is found in Scots pine, larch, spruce and fir,producing tunnels up to 6–9 mm (1/4 in-3/8 in)in diameter tightly packed with bore dust. Theeggs may be laid as much as 25 mm (1 in) belowthe surface and the galleries may penetrate intothe heartwood, but only sickly trees or felledlogs are attacked. The adults, up to 50 mm (2 in)long, are striped yellow and black and oftenconfused with hornets. Trees killed by forest firescan attract great numbers of these insects; theyhave also been reported as boring in rafters, butit seems most likely that they are introduced inthe wood in the forest. Similar damage is causedby other species, for example the blue Sirexcyaneus, which infests larch, and also S. juvencusand Xeris spectrum.

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None of these Wood wasps are considered tobe of economic significance in Europe as theyattack only sickly or dead trees in the forest.However, it appears that they have beenintroduced into New Zealand and Tasmania,where they are considered to cause damage tohealthy trees. As a result the Australianauthorities introduced quarantine regulations,these originally being confined to Europeansoftwood, which was required to be treated toensure that it did not introduce furtherinfestations. The regulations have now beenextended to all wood imported to Australia, sothat all wood shipments must be inspected andall wooden components in packaging andcontainers must be treated by approvedmethods, as it has been appreciated that thereare many other far more serious wood-destroying pests that could be introduced.

The only other Hymenoptera that are ofsignificance in wood preservation are thepredators on Coleoptera, the beetles; these havealready been described as their significance liesin the fact that their presence indicates theexistence of a substantial infestation by a wood-boring beetle.

Wood-boring moths

The only other insect group of significance is theorder Lepidoptera, the butterflies and moths, astwo families have wood-boring caterpillars. TheSesiidae, the Clear wings, are not readilyidentifiable as types of moths as they moreclosely resemble wasps or bees in that they haverather slender bodies; however, they can beidentified by the narrow band of scales aroundthe edges of the wings. The Cossidae are theGoat moths; Cossus cossus (ligniperda) can befound boring into the base of oak, elm, willowand poplar trees. The larvae bore large galleries,frequently causing serious damage to standingtrees, principally because many of the species arevery large, some Australian adults achieving awing span of 180mm (7 in). In Europe and

North America the most important species is theWood Leopard moth, Zeuzera pyrina (coesculi),which bores in upper branches, usually in fruittrees. This species has a wing span of about 45mm (13/4 in) and is generally considered to be anoccasional serious pest of fruit trees, although ithas sometimes been suggested that it may be thecause of damage in structural wood.

The family Pyralidae should also bementioned for, although these social moths arenot wood-borers, their webs and pupal chambersare often found in deep, open joints in old rooftimbers, where they are sometimes confused withfungal growth by inexperienced surveyors. Thetwo most important species are the Bee moth,Aphomia sociella, and the Honeycomb moth,Galleria mellonella.

Marine borers—Gribble

The most important marine borers are Crustaceaof the sub-order Isoposa. They are all superficiallysimilar to the common wood louse, which is alsoincluded in this sub-order. Limnoria spp, thegribbles, are 3–5 mm (1/8 in-1/5 in) long andstrong swimmers; however water currentsgenerally have a greater influence over theirdistribution. They settle on wood, formingsuperficial burrows less than 12 mm (1/2 in) deepwith small entrance holes of less than 2.5 mm (1/10in) diameter. The galleries follow the early wood,giving it a laminar appearance, until the weakenedzone breaks away, exposing a fresh surface toattack. Gribble actually attacks wood at all depthsbelow the mid-tide level but damage is mostapparent in the tidal zone where the wave actionsteadily removes damaged wood, permittingprogressive erosion to occur. Eggs are held by thefemale in a brood pouch beneath the body untilthey eventually hatch and the young gribble arereleased into the parent’s burrow. These eventuallybore on their own and also swim freely in warmweather in search of suitable settlement sites.

Various species of Limnoria occur throughoutthe world, and in tropical areas Sphaeroma spp,

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which are similar in habits but about 15 mm (3/5 in) long, are also found, as well as the relatedExosphaeroma spp. Chelura spp, members of thesub-order Amphipoda, may also be foundinfesting wood; however, they do not appear tocause damage, but simply occupy vacant gribbletunnels.

Shipworm

The molluscan borers are all included within theorder Eulamelli branchiata, but are in twofamilies. The Teredinidae include pile worms,shipworms and cobra. The best known genus isTeredo, the shipworms. The minute larvae swimfreely and eventually settle on wood where theybore holes about 0.5 mm (1/50 in) in diameter.Throughout the rest of its life the Teredo remainswithin the wood, the only visible evidence of itspresence being two syphons projecting from theburrow, which enable water to be drawn in anddischarged. If the burrow becomes exposed tothe air, perhaps during low tide, the syphons arewithdrawn and the hole sealed with the pallet.The Teredo grows, and extends its tunnel inorder to accommodate its enlarging body byusing its shells as cutters, boring principallyalong the grain. The burrows have acharacteristic calcareous lining but the extensivedamage that may be caused in suitable

conditions is almost entirely concealed, unless itis exposed by gribble damage causing the surfaceto break away.

Various species of Teredo are widelydistributed throughout the world, althoughconfined to saline waters which are reasonablywarm; in Europe infestations are generallyconfined to southern and western coasts exposedto the Gulf stream. Bankia spp are generally farlarger than Teredo and are confined to thetropics.

The family Pholadidae, the boring mussels, isof minor importance as the damage caused isrelatively superficial. Martesia is sometimesfound boring in wood, usually after this has beensoftened by decay, but other species are usuallyfound boring in stone or soft mud.

The purpose of this appendix has been toillustrate the wide variety of borers that exist sothat the reader can be aware of the need forpreservation. The identification of adult insectscan be attempted by reference to the descriptionsgiven in the text and the figures, but theidentification of borer damage is rather morecomplex and outside the scope of this book,which is concerned simply with the preservationof wood in order to prevent attack; theidentification of damage is considered in moredetail in the book Remedial Treatments inBuildings by the present author.

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Sapstain

Damp wood is able to support a wide variety offungal infections. In fresh commercial woodsapstain is the first defect that may occur,Ascomycetes and Fungi Imperfecti developing inthe residual moisture of the tree, where both ahigh moisture content and the presence of sugarsare ensured. Sapstain, which, as its name implies,is generally confined to sapwood, results fromdeeply penetrating fungi which causediscolouration through their dark-colouredhyphae or occasionally through staining of thecell walls. The discolouration may be black,green, purple, pink or, very occasionally, brown,but it is most commonly greyish-blue,particularly on softwoods, and is frequentlydescribed as ‘bluestain’. The hyphae that causethis bluestain can, in fact, be seen to be darkbrown when examined microscopically and theblue colouration results from refraction ofincident light by these hyphae. The stained areastend to follow porous routes, spreading alongthe grain and radially to form patches which arewedge-shaped in cross-section.

Most bluestaining appears to be caused byAscomycetes of the genus Ceratocystis(Ceratostomella) but a number of Fungi Imperfectialso cause staining in coniferous wood, the mostimportant being Aureobasidium (Pullularia)pullulans, Hormiscium gelatinosum, Cladosporiumherbarum, Cadophora fastigiata, Diplodia spp andGraphium spp. Sapstain in Scots pine or Europeanredwood appears to be due, in decreasing order of

importance, to Ceratocystis pilifera, C.coerulescens, C. piceae, Aureobasidium pullulansand C. minor. These fungi accounted for 90% ofbluestain hyphae isolated from pine ininvestigations by Professor Henningsson isSweden. In spruce the order was slightly different,but the same principal fungi were involved:Ceratocystis piceae, C. coerulescens, C. pilifera andAureobasidium pullulans.

Mould

Sapstain is almost invariably associated withsuperficial discolouration caused by mouldsforming greenish or black, occasionally yellow,powdery growths, which are easily brushed orplaned away. A very wide range of species is ableto develop in this way on damp surfaces ofwood, these species including common generasuch as Penicillium, Aspergillus andTrichoderma, none of which causes significantdeterioration or deeply penetrating stain. Thesemould growths occur only when the wood isfreshly felled as dampness and sugars are thenboth available.

The superficial darkening of weathered wood isusually caused by Aureobasidium (Pullularia)pullulans, Cladosporium herbarum, Alternaria sppor Stemphylium spp, all of which develop minutedark pustules. Aureobasidium pullulans does notconfine its activities to exposed wooden surfacesbut is also commonly isolated from painted orvarnished surfaces, together with Phoma spp. It is

Appendix C

Wood-destroyingfungi

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often said that this growth results from applyingpaints or varnishes on top of sapstained wood butthis cannot explain how these fungi also occurwhen finishes are applied to metal surfaces. In fact,it appears that these fungi are able to attack thecoating, perhaps forming bore holes and spreadinginto the wood beneath, where they can then cause‘stain in service’ in contrast with the better known‘sapstain on freshly felled wood’.

Bacteria

Generally, the moulds and staining fungi have nosignificant effect on wood, except that thestaining fungi, in common with some bacteria,are able to utilize cell contents, particularlymaterials tending to block the pits, so that theiractivities result in a distinct increase in porosity,particularly in the sapwood of impermeablespecies such as spruce. Some of the Ascomycetesand Fungi Imperfecti can, however, causedamage to the cell walls, resulting in a form ofdeterioration commonly known as Soft rot.

Soft rot

The relationship between this damage and fungiwas established only comparatively recently byMr Savory in England and new species of Softrotting fungi are being progressively identified.At the present time Soft rot is considered to beassociated with wood at very high moisturecontents, perhaps saturated but in aerobicconditions, such as in water-cooling towers,where damage of this type was first identified.The deterioration takes the form of surfacesoftening which becomes progressively deeper,small cuboid cracks developing if the affectedwood is dried. If decay becomes very deep thewood breaks with a distinct brash or cross-grainfracture. It seems likely that Soft rots may beresponsible for unexplained brashness in woodwhich is apparently unaffected by fungal attacks.

Soft rotting fungi are often termed microfungias they are able to progress through wood within

the cell walls, where they are not readily identified,as compared with the Basidiomycetes, whichnormally progress through the cell lumen and pits.As some preservative toxicants are deposited onthe lumen surfaces of the cell walls it is hardlysurprising that many Soft rots are very resistant tosuch preservative systems and are readilycontrolled only by toxicants that penetrate into thecell-wall structure. Although this preservativetolerance is insignificant in softwoods it is a matterof great concern in hardwoods, particularlytropical species, in which Soft rot may progress inground contact conditions, despite very highretentions of otherwise effective preservatives suchas creosote and copper-chromium-arsenic salts.

Basidiomycete classification

The principal wood-destroying fungi areBasidiomycetes, the spores of which are borne onsmall, club-shaped structures known as basidia,which are normally formed in a compact layercalled the hymenium. The wood-destroying fungi,Hymenomycetes, consist of four families, whichdiffer in the form of the hymenium. TheThelephoraceae, a family that includes Coniophora,have the hymenium freely exposed on a flat, skin-like surface. In the Hydnaceae family the hymeniumis on a surface of spine-like outgrowths, whilst thatof the Polyporaceae family, which includes Fomes,Lenzites, Serpula, Polyporus, Coriclis, Poria andTrametes, lines the inside of pores or tubes. TheAgaricaceae, which include Lentinus and Paxillus,are quite distinctive as the hymenium is on plate-likegills underneath a cap-shaped pileus (or mushroom).


If a wood-destroying fungus is producing sporesit is possible to identify its family, but in thewood-preservation industry it is usuallynecessary to rely upon the nature of the decayand the superficial appearance of the hyphae, assporelation is comparatively rare in mostimportant species. One of the most useful identi-

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fication features is the division of theseBasidiomycetes into Brown rots and White rots,depending upon the manner in which theydestroy wood.

In a Brown rot, such as Coniophora puteana,the fungal enzymes destroy the cellulose butleave the lignin largely unaltered so that thewood acquires a distinct brown colour and thestructural strength is almost entirely lost. Asdecay progresses the wood becomes very dry andshrinkage cracks appear both across and alongthe grain, the size and shape of the resultingrectangles often being a useful feature inidentification. In contrast, the White rots, suchas Coriolus (Polystictus) versicolor, destroy bothcellulose and lignin, leaving the colour of thewood largely unaltered but giving a soft felty orstringy texture.

Most of the wood-destroying fungi areconfined to forest situations but those which aredescribed below are often found in structural

wood, particularly in buildings, and occurwidely throughout the world, although certainspecies tend to dominate in particular areas.

Dry rot fungus, Serpula lacrymans

The best known wood-destroying fungus iscertainly Serpula (Merulius) lacrymans, the Dryrot fungus (Fig. C.1). This species is usually foundin buildings, and sometimes in mines, in placeswhere ventilation is restricted and it thus tends todevelop in completely concealed areas. It appearsto have originated as a north European speciesbut it is now found in other parts of the worldwith similar climatic conditions, such as NorthAmerica and parts of South Africa, Australia andNew Zealand. It is comparatively rare in warmerclimates, except where it is associated instructures with condensation caused by the airconditioning system.

The conditions for germination and growth

FIGURE C.1 Advanced decay by Dry rot, Serpula lacrymans, showing typical cuboidal cracking and fungalgrowth. (Cementone Beaver Ltd)

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are extremely critical, requiring a narrow rangeof atmospheric relative humidity and woodmoisture content. Spore germination appears tooccur most readily in acid conditions and it isthese features that perhaps account for the factthat Serpula lacrymans is frequently associatedwith other fungi able to germinate and developin wetter conditions. If wood becomesaccidentally wetted one of the Wet rot fungi,such as Poria vaporaria, may develop but asdrying progresses conditions may arise in whichSerpula lacrymans spores may germinate,encouraged by the acid conditions caused by theprevious Wet rot. Similarly a source ofcontinuing dampness may support a Wet rotsuch as Coniophora puteana but at a furtherdistance from the source the moisture contentmay be optimum for Serpula lacrymans sporegermination, again encouraged by the aciditygenerated at the fringe of the Coniophoraputeana attack.

Serpula lacrymans produces white hyphaewhich are, in fact, very fine tubes or hollowthreads, progressively branching and increasing inlength so that they spread in all directions fromthe initial point of germination, provided that afood source is available. As food is exhaustedsome hyphae are absorbed whilst others aredeveloped into much larger rhizomorphs orconducting strands, which are able to transportfood and water. Thus exploring hyphae finding nonourishment are absorbed to form food forgrowth in more promising directions, giving thefungus the appearance of sensing the direction inwhich to spread towards a food source. Seasonalchanges sometimes inhibit growth, which thenresumes when suitable conditions return. In thisway hyphae contract on drying to form layers ormycelium, each successive layer indicating aseason of growth.

Active growth is indicated by hyphae likecotton wood, perhaps covered with ‘tears’ orwater drops in unventilated conditions, thisbeing the way the fungus regulates theatmospheric relative humidity and accounting

for the name of ‘lacrymans’. Rhizomorphs maybe up to 6 mm (1/4 in) in diameter, they arerelatively brittle when dry and extend forconsiderable distances over and throughbrickwork, masonry and behind plaster,spreading through walls between adjacentbuildings and ensuring a residue of infection,even if all decayed wood is removed; treatmentof adjacent sound wood and replacement ofdecayed wood with preserved wood shouldalways be accompanied by sterilizationtreatment of infected brickwork and masonry.

Mycelium is greyish and later yellowish withlilac tinges when exposed to light, and oftensubsequently green in colour through thedevelopment of mould growth. Sporophoresgenerally develop when the fungus is under stressthrough the food supply being exhaused, thetemperature increasing or the moisture contentdecreasing. Sporophores are shaped like flatplates or brackets and vary from a fewcentimetres to a metre or more across, being greyat first with a surrounding white margin butthen the slightly corrugated hymenium or spore-bearing surface develops to become covered inmillions of rust-red spores which are eventuallyliberated and cover the surroundings with reddust. As fungal growth in buildings is generallyconcealed the sporophore may be the first sign ofdamage, though a characteristic mushroom smellmay be noticed if an infected building is closedfor several days.

Serpula lacrymans can cause severe Brown rotwith pronounced cuboidal cracking, the cubesbeing up to 50 mm (2 in) along and across thegrain. Decayed wood crumbles easily betweenthe fingers to a soft powder. Two importantfeatures of the decay are the fact that it can beentirely internal and concealed in beams, andthat it can spread to dry wood in unventilatedconditions, as the fungus is able to transportadequate water for decay through therhizomorphs. A related species, Meruliushimantioides, is sometimes found causing similardecay in Scotland, Denmark and southern Sweden.

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215

Cellar fungus, Coniophora puteana

The Cellar rot fungus, Coniophora puteana(cerebella) (Fig. C.2), is the most common causeof Wet rot in buildings and elsewhere wherepersistently damp conditions arise through, forexample, soil moisture or plumbing leaks. Sporesgerminate readily and this fungus is likely tooccur whenever suitable conditions arise. Thehyphae are initially white, but growth is not asgenerous as for Serpula lacrymans. In additionthere is little development of mycelium and onlythin rhizomorphs are formed. Theserhizomorphs, which become brown andeventually black, are not so extensive as those ofSerpula lacrymans and never extend far from thewood. The sporophore occurs only rarely inbuildings and consists of a thin skin covered withsmall irregular lumps. The hymenium is initiallyyellow but darkens to olive and then brown asthe spores mature. Wood in contact with a

source of moisture such as brickwork oftenconsists of a thin surface film concealingextensive internal decay. The rotted wood isdark brown with dominant longitudinal cracksand infrequent cross-grain cracks. The easiestmethod for controlling Coniophora cerebella inbuildings is to isolate wood from the source ofdampness.

White Pore fungus, Poria placenta

The White Pore fungus, Poria placenta (Fig.C.3) is common in mines and occasionally occursin buildings. It requires a higher moisturecontent than Serpula lacrymans but, in contrastto Coniophora puteana, it is tolerant tooccasional drying and is therefore the normalfungus associated with roof leaks. Growth isgenerally similar to Serpula lacrymans butstrands remain white, compared with yellow

FIGURE C.2 Advanced decay by the Wet rot, Coniophora puteana, showing dark fungal strands. (CementoneBeaver Ltd)

Appendix

216

or lilac for Serpula and brown or black forConiophora. The rhizomorphs may be up to 3mm (1/8 in) in diameter but are not so welldeveloped as those of Serpula and are flexiblewhen dry. When examined on the surface of apiece of wood or adjacent masonry they appearto be distinctly flattened or to have a flat marginon either side of the strand, and they do notextend far from their source of wood. Thesporophore is rare in buildings but it issometimes observed in greenhouses when severedecay occurs. It is a white irregular plate, 1.5–12mm (1/16 in-1/2 in) thick and covered withdistinct pores, sometimes with strands emergingfrom its margins. The decay damage to wood issimilar to that caused by Serpula but the cubingis somewhat smaller and less deep. Whendecayed wood is crumbled between the fingers itis not so powdery as that attacked by Serpulabut slightly more fibrous or gritty.

Other Poria species—dote

Poria xantha is a similar Brown rot, beingfrequently found in greenhouses but usually withno visible surface growth, although myceliummay be found in cracks, even in the cubingcracks on decayed wood. A thin skin of

yellowish white mycelium occasionally occurs.The sporophore is a thin, yellowish layer ofpores, distinctly lumpy when situated on avertical surface. Poria monticola is sometimesfound both in buildings and as dote on softwoodimported from North America, where it is one ofthe most important wood-destroying fungi. Doteis a form of pocket rot which can develop whenunseasoned wood is close stacked and it is noteasy to detect. It may, however, be visible as faintstreaks or elongated patches of yellowish orpinkish-brown on most softwoods, or of apurplish colour on Douglas fir. When tested withthe point of a knife the wood is found to bebrash within these patches. As decay progressesthe wood acquires a typical brown colour withcracks along and across the grain, as whenattacked by the other Poria spp.

Stringy Oak rot, Phellinus megaloporus

The Stringy Oak rot, Phellinus megaloporus(cryptarum), occurs in Europe on oak in conditionsin which Coniophora puteana or Poria placenta isnormally found on softwoods, such as inassociation with roof leaks or masonry affected bysoil moisture and it is able to resist the relativelyhigh temperatures that frequently occur in roof

FIGURE C.3 Strands of a wet rot, a Poria species. (Cementore Beaver Ltd)

Appendix

217

spaces. As this fungus prefers oak it is largelyassociated with older buildings constructed withthis wood. It is a white rot causing no distinctcolour change in the decayed wood, but thisbecomes much softer, with a longitudinal fibroustexture, and does not powder in the same way aswood decayed by Serpula or other Brown rots.Yellow or brown mycelium is sometimes formedon the surface of wood. The sporophore is a thick,tough plate or bracket, fawn coloured butdarkening as the spores develop. Poria medulla-panis causes very similar decay in oak, particularlyin timber-framed buildings where oak is exposed tothe weather, but there is no practical reason interms of preservation or treatment why these twospecies should be differentiated.

Coriolus versicolor

Coriolus (Polystictus) versicolor is thecommonest cause of White rot in hardwoods,especially in ground contact, but is usuallyconfined to the sapwood in durable species. Itcauses decay on hardwood props in mines andhas been occasionally reported as causing decayin sapwood of softwoods, particularly inexternal, painted joinery (millwork) such aswindow and door frames. The sporophore israrely seen but consists of a thin bracket up to75 mm (3 in) across, grey and brown on top withconcentric, hairy zones and a cream pore surfaceunderneath from which the spores are released.Infected wood initially suffers white flecking andeventually bleaches in colour. Shrinkage is rareand the decayed wood simply appears to belighter and much weaker than sound wood.

Lentinus lepideus

Lentinus lepideus (squamosus) is the principaldecay found in railway sleepers (ties) andtransmission poles, perhaps because it iscomparatively tolerant to creosote and cantherefore develop when treatment with thispreservative is inadequate. A brown cuboidal rot

is caused and the white mycelium, perhaps withbrown or purple tinges, can often be observedwithin the shrinkage cracks. The sporophore is ona stem, woody and brownish with a gill extendingdown the stem when it develops in normal light.In limited light the form is abnormal, perhapslacking the cap or having branched, cylindrical,white or purplish-brown outgrowths.

Lenzites sepiaria

Lenzites sepiaria is comparatively rare in Europebut occurs on imported wood which has beeninfected in the forest and continues to develop ifthe wood is used for fencing, bridging, poles orsituations in buildings where there is anadequate moisture content. The first evidence ofinfection is a pale yellow zone accompanied bysoftening and brashness of the fibres. The woodbecomes progressively darker brown and slightcuboidal cracking occurs. Superficial hyphae arerarely observed but orange-yellow myceliummay occur on decayed wood in concealedsituations. The sporophore is a bracket, up to50×100 mm (2 in×4 in) in size, tough and with ahairy upper surface and distinct gills underneath.It is tawny yellow at first but later dark brownwith a yellow margin and brown gills.

Lenzites trabea—dote

Lenzites trabea appears to have originated inNorth America, but it is now well established incentral and southern Europe. It is sometimesfound in the British Isles and northern Europe onimported wood, frequently as a dote or pocketrot originating as a decay of softwood in theforest. The rot can develop in suitableconditions, causing a brown cuboidal decay. Thesporophore is a thin bracket, tough and withgills on its underside, yellow-brown at first butbecoming darker and then bleaching on theupper surface. This species appears to beparticularly attractive to some termites, such asReticulitermes flavipes.

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218

Paxillus panuoides

Paxillus panuoides causes decay similar to thatcaused by Coniophora cerebella but tends to occurin much wetter conditions. The hyphae develop intofine, branching strands which are yellowish, neverbecoming darker, and a rather fibrous, yellowishmycelium, perhaps with lilac tints, may occur. Thewood is stained bright yellow in the early stages, butdarkens to a deep reddish-brown and shallowcracking occurs. The sporophore has no distinctstalk but is attached at a particular point, tending tocurl around the edges and eventually becomingrather irregular in shape. The branching gills on theupper surface radiate from the point of attachment.The colour is dingy yellow but darkens as the sporesdevelop; the texture is soft and fleshy.

Dote or pocket rot

Dote consists of narrow pockets of incipientdecay and is sometimes described as pipe orpocket rot. The rot in the pocket is brown andcuboidal and hyphae like cotton wool may bepresent. If a pocket is absent, except for theappearance of a brown stain, dote may beconfirmed by brashness when the stained area isprobed with the point of a knife. Dote is observedusually in softwood imported from NorthAmerica; Lenzites and Poria spp have alreadybeen described as two of the possible causes.Another is Trametes serialis, which is usually acause of heart rot in standing trees or closelystacked, unseasoned boards. This fungus willcontinue to develop and the decay will spread ifthe wood remains damp; in South Africa

Trametes serialis on imported wood has beenable to develop locally and is now as serious asConiophora puteana or Poria placenta. Trametesserialis was at one time confused with Poriamonticola, which can cause similar dote; this isthe way in which infections of this fungus aregenerally introduced into Europe. Fomes annosus,a parasite of sickly trees in which it can causeheart rot, turning the wood into long, brown,

fibrous strands, can continue to develop andslowly decay wood after this has been felled if themoisture content is maintained, as in pit props.This species is another form of dote, the decaypocket originating as a black spot and thenenlarging progressively and becoming filled withhyphae which have the appearance of white lint.

North American species

The distribution of individual fungi is moreextensive than that of wood-borers, although inmany cases the fungi evidently originated in oneparticular country and spread to other areaswith wood exports. Certainly the principalwood-destroying fungi in buildings in NorthAmerica have a distinctly different balance tothose found in Europe, but it is clear that tradein both directions is progressively exchangingspecies. At the present time Coniophora puteanais as important in North America as in Europe.Serpula lacrymans is confined to the northernUnited States and Canada, but Poria incrassata,which is similar in many ways and which causessimilar damage, is found only in the southernUnited States. Coniophora arida is sometimesfound causing decay on preserved pine andLentinus lepideus is the major decay fungus onwood treated with creosote, as in Europe. TheLenzites spp are perhaps most prevalent of all,with L. trabea commonly occurring on bothsoftwood and hardwood in service. Poriamonticola is also a serious cause of decay ofwood in service, but it appears that infection isprobably always introduced in standing trees orfelled logs in the forest.

This appendix has attempted to review the fungithat are the most serious causes of decay in wood inservice. However, it will be appreciated that thereare many other forest fungi which may occasionallybe encountered in buildings, though they are notnecessarily associated with wood decay; these latterfungi are described in more detail in RemedialTreatments in Buildings by the present author.

219

AAC, see Alkyl ammoniumabsorption 78–9ACA, see Ammoniacal copper-

arsenicACB, see Acid copper-boronACC, see Acid copper-chromiumacetic anhydride 144acetylation 144acid copper-boron 114–15, 158,

159acid copper-chromium 115, 121–

2, 164acid zinc-boron 114–15, 159, 185ACQ, see Ammoniacal copper-

quaterneryacridine 100Acypetacs copper, zinc 136ACZA, see Ammoniacal copper-

zinc arseniteAczol 114after-glow suppression 121agricultural wood 170Ahic CB, see Wolmanit CBAHIG 164Aldrin 133–4alizarin 10alkyl ammonium 139–40alkyl-benzyl-dimethyl-ammonium

140Allgemaine Holzimprägnierung

GmbH 164alternating pressure process 86aluminium, organo 144Ambrosia beetles 25, 54, 186–7

control of 56Ambrosia fungus 25amines 139Amitermes 32ammoniacal copper-arsenic 114ammoniacal copper-boron 114–15

ammoniacal copper-quaternery114

ammoniacal copper-zinc-arsenic114

ammoniacal preservatives 114–15,124, 158, 159

ammoniacal zinc-quaternary 114ammonium compounds,

quaternary 139–40ammonium salts 146Anabol 132, 149anaerobic deterioration 31Anobids 26–7, 191–6Anobium punctatum 26, 192–3anthracene 10, 100anthracene oil 102Anticimexbolagen 166, 169, 170antimony 107Antingermin 128–9Antinonnin 128–9anti-stain, see Stain controlants 186, 206–7

white, see TermitesAPM, see Alternating pressure

processapplication methods 66–92aqueous preservatives 105–26Armillaria mellea 28Arprocarb 134arsenic (arsenate, arsenite) 115–

17arsenic toxicity 122–3, 163, 171–

2arsine 122Ascomycetes 52Ascu 117–18Auger beetles 189–90Aureobasidium pullulans 54, 149Australian quarantine 57, 91,

170, 172

azaconazole 134, 148AZB, see Acid zinc-boronAZQ, see Ammoniacal zinc-

quaternery

Bacillus thuringiensis 58bacteria 212Bakenfield oil 102bandages, pole 109–10barium naphthenate, see Metal

soapsbark borers 25, 53–4

control of 56Barol 102Basidiomycetes 28–30, 52, 212–

18Basilit A57 110Basilit CFK 111–12, 124–5, 180Basilit U, UA, UAF, UAS 72, 107–

10, 180Baster 115Batson 6bees 186, 207beetles, wood boring 185–209Bell Telephones 117Bellit 107Benlate, see BenomylBenomyl 53, 134, 148, 154benzalkonium 140benzalkyl-trimethyl ammonium

140benzenehexachloride, see LindaneBerrit 110Besemfelder process 88–9Bethell, John 7–8, 11, 77Bethell process 77–8, 84, 86, 87BFCA salts, see boron-fluorine-

chromium-arsenicbichromate, see Dichromatebifluorides 110–11

Index

Index

220

bleeding 89, 162–3bluestain (blueing), see SapstainBMT, see Bodied mayonnaiseboats 169bodied mayonnaise emulsion 141,

150boiling under vacuum 86–7Boissieu, de 6Boliden 85, 115, 166, 169, 170

BIS, S, S-25 113, 115–16,120

CCA, K-33, see K33OPM, see Oscillating pressure

processP50 124, 180

Boracol 126, 150–1, 141borate, see BoronBordenave 6borer control 43–54, 57–8borers

bark 53–4marine 186, 208–9parasites 57–8predators 57–8wood 185–209

Borester 126–7, 141, 150–1boron (borate, boric acid) 53, 55,

89–90, 91, 97, 123–4, 125–6, 130, 139, 141, 145, 146,147–8, 154, 155, 158, 159,161, 180

boron esters 126–7, 141, 150–1boron-fluorine-chromium-arsenic

125Bostrychid beetles 26, 189–90Boucherie process 6–7, 71–2, 110,

112, 113, 114, 124Boulton 3, 7, 9–11Boulton process 3, 7, 86–7BP Hylosan, see HylosanBP Mykocid, see MykocidBreant 11, 77bridges 165broad-leaves trees 15, 16–18bromophenols 147–8Bromophos 134Bryan 106Bub 106buildings 165–9bulking 144–5Buprestid beetles 201–2Burnett, William 7, 102, 112Burnettising 112, 162Burt 11

butane, solvent 83–4

CAA (copper ammoniumadditive), see Copper-ammonia-arsenic

CAB, see Copper-ammonium-boron

CCA, see Copper-chromium-arsenic

CCB, see Copper-chromium-boron

CCZC, see Copperized CZCcalcium naphthenate, see Metal

soapscambium 15capillarity 63Captafol 53, 134, 148, 154Captan 135, 148–9carbamates 134Carbaryl 134carbolic acid 8Carbolineum 102Carbolineum Avenarius 127–8,

131Carbowax 144–5Card process 102–3, 112, 162Caro 10carrier systems 140–2cebuconazole 134, 148Celcure N, F, A, AP, AN, M 115,

117–19, 120–1, 123, 124,125–6, 139, 146, 180

Cellar rot 12, 29Cellon process 83–4cellulose 15–22Cementone-Beaver Limited 168Ceramycid beetles 196–9Ceratostomella spp. 24charring 3–4, 41Chateau 88chemical modification 50–1Chemonite 114chinoline 100Chlordane 91, 133–4chlorine 139chlorinated insecticides 132–4chlorination 50, 102, 127, 129,

131chlorobenzene 132–3, 149

toxicity 175chlorocresol 131chloronaphthalene 131, 149

toxicity 175

chlorophenate, see Chlorophenolchlorophenol 50–1, 53, 97, 127,

128, 129, 130–1, 136, 137,145, 147, 150, 154, 155,180

esters 131toxicity 130, 172–3, 174

chlorophenylphenol 129chlorophyll 13, 20chlorothalonil 134, 148chromated copper arsenate, see

Copper-chromium-arsenicchromated zinc chloride 112Chromel 106chromium 108, 146

toxicity 172coal tar 97, 98coatings

decorative 45, 54, 59–63, 97,142–6

intumescent 64–6Cobra process 90, 110, 163–4Coisne 8, 9Coleoptera 185, 186–209Common Furniture beetle 26,

192–3conifers 15, 16–18Coniophora puteana (cerebella)

12, 29, 48, 52cooling towers 170–1copper 113–15, 117–25, 128,

135–7, 149, 158copper-ammonia-arsenic (copper

ammonium additive) 114copper borates 124copper caprylate 180copper-chlorophenol 180copper-chromium 51, 52, 115,

180copper-chromium-arsenic 51, 52,

71, 91, 106, 115–23, 158,161, 164, 165, 170, 171,180

fixation 121–2retention 121

copper-chromium-boron 71, 123–4, 161, 180

copper-chromium-fluorine 180copper-chromium-phosphorus 180copper, detoxification 48copper 8-hydroxyquinolinolate

137copperized CZC (CCZC) 112

Index

221

copper naphthenate, see Coppersoaps

copper octoate, see Copper soapscopper oxinate 137copper soaps 50, 97, 135–7, 180copper sulphate 6–7, 71, 112,

113, 162Coriolus versicolor 12, 29Cornelisol 104corrosive sublimate, see Mercuric

chlorideCorynetes, see Korynetescrash barriers 164–85Creofixol 104Creosite 104creosol 100creosote 6, 7–11, 97, 98–102,

112, 162, 180arsenical 103fortified 102–3see also Tar

creosote-petroleum mixture 98–9creosote toxicity 172creosoting 7–11Crepin 8Crom-Ar-Cu 119Crustacea 186, 208–9cumene process 129, 150Cumullit 131Cunilate 137Cuprinol 135, 149–50Cuprinol, KP, see KPCuprinol Tryck (KPN) 124, 137,

180Cupristat 137Curculionid beetles 199–200Curtin 113cyclodiene insecticides 133–4Cypermethrin 134CZC, see Chromated zinc

chloride

Dalton, John 9Davy, Huymphrey 6DDT, see Dicophanedead oil 7Death Watch beetle 27, 92–3de Boissieu 6Decamethrin 134decorative treatments, see

Coatingsdegradation, wood 23–42Deltamethrin 134

Dermestid beetles 201–2Derris 149Dessemond process 162deterioration risk 153, 179–84detoxification of preservatives

135Diamond Match 114Diazinon 134dibutyl phthalate 129Dichlofluanid 135, 148–9dichlorobenzene, see

Chlorobenzenedichlorodiphenyl trichloroethane,

see Dicophanedichlorofluoromethylthio

compounds 135, 148–9dichloronaphthalene, see

Chloronaphthalenedichlorophenol, see ChlorophenolDichlorvos 134, 142dichromates, see ChromiumDicophane 92, 133–4, 149, 173dicotyledons 15, 16–18Dieldrin 91, 92, 103, 133–4, 141,

167, 173, 180, 181diffusion 141

double 125treatment 125–6, 150, 158–9

Diffusol 126Difolatan, see Captafoldinitrocresol, see Nitrocresoldinitrophenol, see Nitrophenoldinitrophenolanilin 107dioxins 173dip, see Treatments, superficialdiphenyl mercury 137Diptera 186Diufix 163–4Domtar 114Dote 28double diffusion 125double vacuum processes 82–3,

158Dow process 84, 129, 150Drilon process 83–4Dry rot 4, 29–30, 43, 47, 92,

213–15drying wood 37–8durability, wood 46, 182–3

economics of preservation 1–2empty-cell processes 11, 78–81,

86–7, 149, 154–5, 158, 162

emulsion preservatives 140–1Endrin 133–4energy for impregnation 81–2environmental factors 171–6eramacausis 9Erdalith 117Ernobius mollis 26, 191, 194–5Estrade process 89ethyl mercury compounds 137ethylene glycol, poly 144–5Eucalypts 161–2Eupion 7European House borer, see House

LonghornEuropean redwood 82, 157, 161,

165European whitewood 82, 165

Falck 116, 129Falkamesan 116–17FCAP, see Fluorine-chromium-

arsenic-phenolfences 167Fenchlorphos 134Fenitrothion 134Fenthion 134ferrous sulphate 6fibre saturation point 21–2, 33fibres, hardwood 15fibrils, elementary 19–20Fibrosithe 104finishes, see Coatingsfir, Douglas 89, 157, 161, 181fire 41–2, 64–6, 146

resistance 41retardants 65–6, 121, 146–7

fixation 105, 108, 121–2flame spread 41, 42flies 186, 207floor blocks 144–5Flunax 107Fluoran OG 110Fluorex VS 111–12Fluorfolpet 135, 148–9fluorine-chromium-arsenic-phenol

52, 71, 122, 164, 180fluorine compounds 89–90, 100,

106–12, 147, 154, 159fluorising 110fluorosilicates 107, 111–12Flouxyth 107Flurasil 111–12Folpet 135, 148–9

Index

222

Fomes spp. 28Forestier 8formaldehyde 144fortified creosote 102–3fortified petroleum 105foundations 165–7full-cell process 77–8, 158fumigation 93, 142, 151fungal decay 27–30Fungamin 131Fungi

Basidiomycetes 212–18Brown rot 212–13control 43–54

natural 54Imperfecti 52mould 211–12resistant 52Soft rot 212stain 52–4, 211

control 147–9test 95White rot 212–13wood destroying 211–18

Fungitrol, see FolpetFurniture beetles 26–7, 191–6Fyre Prufe 146

Gammexane, see Lindanegas treatments 93, 142, 151Gay-Lussac 146germanium, organo 137Gewecke process 71–2Gloquat C 140glue-line treatments 91glycols 84Gordon, A 114Gorivac process 76, 81, 83Graebe 10greensalt, see Copper-chromium-

arsenicGreensalt K, S, O 117Gribble 30–1, 58–9, 186ground contact 160Gugel process 72gun stocks 144–5Gunn 115

Häger 116, 124, 136, 137, 145Häger K33, see K-33Halowax 132hardwoods 15, 16–18, 182–4Hartig, Theodore and Robert 9

Hatfield 129hazard, deterioration 153HCH, see Lindanehealth factors 171–6heartwood 13–16, 157–8heavy oil, pentachlorophenol 136Helops coeruleus 27Hennell 10HEOD, see DieldrinHeptachlor, see Chlordanehexachlorobenzene, see

Chlorobenzenehexachlorocyclohexane (HCH),

see LindaneHHDN, see AldrinHickson’s Timber Products

Limited 160, 170high-energy jet process 89–90history of preservation 2–13Hodotermitidae 32Homberg 6, 106Honey fungus 28Hot-and-cold process 72House Longhorn beetle 27, 196–9Houseborer, Common, see

Common Furniture beetleHS-Presser 90Hülsbert 11Hydrasil, see HydrazilHydrazil (Hydrasil) 90, 111–12hydrogen fluorides, see Bifluorideshydroxydiphenol, see

PhenylphenolHylosan 166Hylosan PT 139, 180Hylotrupes bajulus 27, 196–9Hymenoptera 31, 186hyphae 29–30hysteresis 22

ignition 41immersion treatments 158–9impermeable woods, see Resistant

woodsImpreg 144–5impregnation

empty-cell 158energy requirements 81–2full-cell 158plant 73–7pressures 82, 85

Improsol 111, 154incising 89, 161

Industri-og ByggnadsaktiebolagetSuecia 162

injectors 90, 141Wykamol 150

inorganic salts 105–27insect borers 185–209insect traps 56insecticides 97, 131–4

toxicity 173in situ treatment, see Remedial

treatmentintumescent coatings 64–6IPBC 134, 148isocyanate 145isolation preservation 45–6Isopoda 186, 208–9Isoptera 31, 185–6, 202–6isothiazolones 134, 148ITA, see IsothiazoloneIwanowski 129

jet impregnation 89–90Johnson, Benjamin 9joinery 167–8

K-33 86, 116, 119, 120, 122,166, 169, 170, 180

Kalotermitidae 32–3Kamesan 116, 123kilning wood 24–5, 37–9, 53,

63–4Kinberg 106Knowles 7Koch, Robert 9Koppers CCA-B 116, 119Korynetes coeruleus 57–8KP-Cuprinol 127, 136–7, 180KP salt, see KP-CuprinolKPN salt, see Cuprinol TryckKreosot 8Kreosotnatron 103Kresapin process 89Krüsner 88Kulba salt 112Kuntz process 86Kyan, Kyanising 6, 86, 106, 108,

112, 162deep 106mixed 106

laboratory assessment,preservatives 94–5

lacquer, see Coatings

Index

223

Lahontuho K-33see also K-33 180

lamella, middle 19, 21Langwood 119lead, organo 137–8Lentinus spp. 52, 217Lenzites spp. 52, 217Lepidoptera 186, 208leprosy of the house 4Letheby 8Lieberman 10Liebig, Justus 9Lignasan 106lignification 15lignin 15, 22lignite tar 86, 103Limnoria spp. 30, 58–9Lindane 91, 92, 103, 132–3, 141,

167, 173, 175, 180, 181Lister, Joseph 8–9Longhorn beetles 25, 92, 196–9low pressure processes 82–3Lowry process 11, 80, 87, 149,

154–5, 158Lukin 6Lyctid beetles 26, 155, 189–91Lyctids, control 56, 57, 72–3Lyxastan 156

Macbride 6Macrotermes spp. 32Madison formula 145, 149Madurox, see AzaconazoleMalathion 134Malenit 107, 108Malenkovic 107Manalox 144Margary process 6, 113marine borers 30–1, 186, 208–9marine woodwork 169Mary Rose 145Mastotermitidae 32Mayerl process 164Mayfield 10MET 134, 148McMahen 117Melandryid bark-borers 189mercury 6, 106, 108, 162

chloride 137chlorophenate 137organo 53, 137, 147, 154

meristem 13–16Merklen 88

Merulius spp., see Serpula spp.metal soap 143, 149, 180

toxicity 173methyl borate 142methyl bromide 93, 142methylene chloride 84Microcerotermes spp. 32microfibrils 19–20microfungi, see Soft rotMicrotermes spp. 32millwork, see JoineryMinalith 146mineral oil, see Petroleummines 170Minolith 108, 146mites, see ParasitesMoDo 144–5Moll, Franz 7–8Mollusca 186, 208–9monochloronaphthalene, see

Chloronaphthalenemoths 186, 208motorway barriers 164–5movement, wood 33–41Mummies, Egyptian 3mycelium 29–30Mykantin 128–9Mykocid BS 111, 154Mystox LPL 131

NAF salt 110, 112naphthalene 8, 10, 100naphthenates, see Metal soapsnard oil 3Nasutitermes spp. 32Natrum 3Neckal 89nitration 128nitrocresol 108, 128nitrogen, organo 139–40nitrophenol 107, 109, 128No-D-K 103–4Non-Com 146Nordheim process 11, 80–1Nördlinger 102, 128Nuodex 321

Extra 148Nytek GD 137

3.O CCA 121oak 167Oak rot 12, 29Oborex Cu, Zn 135–6

ODB, see ChlorobenzeneOdontotermes spp. 32Oedermerid beetles 200–1oil 143, 144, 145

heavy 128Olimith C20 132olive oil 3open tank, see Immersion

treatments; Hot-and-coldprocess

OPM, see Oscillating pressureprocess

Orben Bois SA 165organic preservatives 49–50,

127–35organic solvents 49–50organoaluminium 144organochlorine insecticides 91,

92, 103, 132–3, 141, 167,173, 175, 180, 181

organogermanium 137organolead compounds 137–8organomercury 106, 137, 147,

154organometal preservatives

135–40, 144organonitrogen 139–40organophosphorus 134organosilicon 143–4organotin 50–1, 97, 137, 143,

144, 150, 180toxicity 173, 174–6

organotitanium 144organozirconium 144oscillating pressure process 85–6Osmol WB4 111Osmolit U, UA 71, 109–10Osmosalts 109–10Osmosar 109–10Osmose 159, 164Osmose process 71, 110

K33, see K-33oxide preservatives 105–27oxine copper, see Copper

8-hydroxyquinolinolateOxylene 146

paint, see Coatingsparasites, borer 202parenchyma 15Paris green 114Parol 131particle board, treatment 91

Index

224

Pasteur, Louis 9paving blocks 165Paxillus spp. 12, 29, 218Payne 7PDB, see Chlorobenzenepeat tar 103–4PEG, see Polyethylene glycolPenicllium brevicaule see

Scopulariopsis brevicaulisPenta, see ChlorophenolPentabor S, SA 130, 147, 154pentachlorophenol, see

ChlorophenolsPenta-Tetra-Copper 136–7Perkin 10Permapruf T 139, 180Permethrin 134, 154–5, 167, 173,

180, 181petroleum 105, 128

fortified 105Phellinus spp. 12, 29phenanthrene 100phenol 100, 129Phenthoate 134phenyl mercury compounds 137phenylphenol 129, 136, 143,

149–50phloem 15Pholadids 186, 209phosphorus 139

organo 134photosynthesis 13, 20Picea spp. 82, 89, 157, 181Pile Guard 164piles 160–3pine

Columbian, see Fir, DouglasCorsican 157, 160, 181Scots 82, 157, 165, 181Shortleaf 160, 175, 181South African 82, 157, 160,

181Southern 157, 160, 181

Pinhole borers 25, 97, 187–8control 154–5

Pinus echinate 82, 157, 160, 181Pinus nigra 157, 160, 181Pinus patula 157, 160, 181Pinus sylvestris 82, 157, 160, 181pitch 2Platypodid beetles 25, 54, 186–8plywood, treatment 91pocket rot, see DotePoiseuille’s formula 66

pole bandages 71, 109–10poles 157–8, 160–3Polybor 126polychloronaphthalene, see

Chloronaphthalenepolyester-styrene 144–5polyethylene glycol 53, 144–5Polyphase, see IPBCPolyporus spp. 212Polystictus spp., see Coriolus spp.polyurethane 145ponding 90–1pore fungi, see Poria spp.pores, hardwood 17–18Poria spp. 12, 29, 52porosity, wood 15posts 157–8, 160–3Poulain process 86, 103, 162Powder Post beetles 26, 97,

155–6, 189–91precipitation from salts 7predators, insect 202preferential wetting 40–1, 142–3preservation

charring 3–4chemicals 97–151chemical modification 50–1economics 1history of 3–12marine 58–9mechanisms 43–66need for 1–2, 156practical 153–76principles 12–13, 153–60requirements 155–6systems 43–96technology 1–22toxic 51USA 98–9wood selection 157–8, 179–84

preservativeapplication 11–12, 49, 66–93bleeding 87–8concentration 82–3detoxification 48evaluation 93–6fixation 47–8, 50injectors 90penetration 66–71, 88–92,

90–1, 179, 182–4permanence 49–50, 179repellant 49, 55retentions 180selection 157–8, 179–84

solvents 140–2toxic systems 46–7, 56–7types 97–8, 179–84

preservativesammoniacal 114–15inorganic 105–27, 180new 96organic 127–35, 180organic solvent 97organometal 135–40salt 6–7tar oil 97, 98–105, 180toxicity 171–6water borne 97

preserved wood, uses of 160–71pressure and vacuum processes

73–89pressure-stroke process 84–5pressure units 76–7Preventol A3, see FluorofolpetPreventol A4, see DichlofluanidPringle, John 6propiconazole 134, 148Pseudotsuga menziesii 89, 157,

181Pullularia spp. see Aureobasidium

spp.Punk 28Pyresote 146pyrethroids 134, 154–5pyridine 100pyridyl mercury 137Pyrolith 146

quarantine, Australian 57, 91,170, 172

quaternary ammonium 139–40

Raco 128–9railway sleepers 163–4Randall, James 6rays 15redwood, European (Baltic) 82,

180, 181, 182Reilly 104relative humidity, atmospheric 22,

38–9remedial treatments 57, 91–3,

149–52, 168–9toxicity 173–6

Rentex 111Rentokil 132, 149resins 143, 145

Index

225

phenolic 144resistant fungi, see Tolerant fungiresistant woods 88–9, 182–4resistence, fire 41Reskol 149–50retentions, typical 180–1Reticulitermes spp. 31–2Rhinotermitidae 32rhizomorphs 30, 43Richardson 106Richardson & Starling Ltd 149Ridsol 132road

blocks 164–5works 164–5

rosinamine 131esters 129

rotBrown 28, 43Cellar 12, 29Dry 29–30, 43, 47Soft 30Stringy Oak 12, 29Wet 29, 48White 28, 43see also Fungi

Rotenone 132, 133, 149Rottier 8round wood 157–8

preservation 184Royal Navy 4–5Royal process 136–7, 145–6Royal Society of Arts 5–6ruberythric acid 10Rüping process 11, 78–80, 86,

87, 158, 162Rütgers, Schwammschutz

(fungicide) 107Rütgerswerke AG 11, 87, 102,

107Runge 8

3.S CCA 121S25 85salt preservatives 6–7, 105–27Saprocresol 89Sapstain 24–5, 211, see also Stain

controlsapwood 13–16, 157–8Saug Kappe process 71–2sawn wood preservation 157–8,

184

Schwammschutz Rütgers 107Scolytid beetles 25, 54, 186–8Scopulariopsis brevicaulis 122–3,

171–2Scots pine 82, 157, 181, 182seasoning wood 24–5, 37–8secondary cell wall 15Serpula spp. 4, 29–30, 43, 47,

213–15service records 93shale oil 104–5Shipworm 30–1, 58–9, 186Shothole borers 187–8Sikkuid 111–12silanes 143–4silica 55silicofluoride, see fluorosilicatesilicones, see SilanesSirex spp. 25Siricid wasps 25sleepers, railway 163–4smoke, fire 42smoke, treatments 141, 151sodium ortho phenylphenate, see

Phenylphenolsodium pentachlorophenate, see

ChlorophenolSoft rot 52softwoods 15, 16–18, 181, 182–4soil poisoning 165–7Solignum 102solvent recovery 83–4solvents 140–2

organic 128, 149–50, 168splitting, wood 35spores 29spray, see Treatments, superficialSpringer-Presser 90spruce, 82, 89, 157, 161, 165,

181spruce treatment, see Resistant

woodsstabilizers, wood 142–5staddlestones 167Stag beetle 27stain, see SapstainStain control 147–9, 154stain in service 148–9stains, see CoatingsStalhane 115Stockholm tar, see Wood tarstructural design to avoid decay

44–5styrene-polyester 144–5

surface tension, water 63Svenska BP Aktiebolag 166

5-T 74–6, 85, 90–1Tanalith C, CCA, CT106, Plus,

CA, NCA 117–18, 120–1,139, 160, 180

Tanalith CBC 123–4, 180Tanalith U, Un, K 108–10, 117,

180Tancas C, see Tanalith CTancas CC 124, 180tar oils 7–11, 97, 98–107, 162,

180fortified 127–8toxicity 172

Taylor Colquitt process 88–9TBTO (tributyltin oxide), see

OrganotinTC oil 129–30TCMTB 134, 148Terebrionids 201Teredinids 30, 58–9, 186, 208–9termites 31–3, 92, 185–6,

202–6control of 165–7Damp Wood 32Dry Wood 32–3Moist Wood 32shields 55, 165–7Subterranean 32–3

Termitidae 32–3Termopsidae 32test methods, preservatives 48–9tetrachloronaphthalene, see

Chloronaphthalenetetrachlorophenol, see

ChlorophenolTetraset process 102–3, 112, 162Thanalith, see Tanaliththiazoles 134, 148Thiram 148thixotropic preservatives 69–70Thriolith, see TriolithTidy 8, 9–10ties, rail, see SleepersTimber 126, 158–9, 180Timber rods 141, 150–1tin, see Organotintitanium, organo 144TMB, see Trimethyl boratetoxicity, preservatives 171–6tracheids 15

Index

226

Trametes spp. 28, 218Transote 104treatment

diffusion 70–2gas 93glue line 91immersion 70penetration 66–71, 85pressure 67, 85remedial 91–3, 149–52superficial 49, 67–70, 91–3temperature 67thioxopic 69–70time 66–7

tree structure 13–22trialkyltin, see Organotintrialkyllead, see Organoleadtriazoles 134, 148tributyltin, see Organotintrichlorobenzene, see

Chlorobenzenetrichloromethylthio compounds

135, 136, 148–9trichloronaphthalene, see

Chloronaphthalenetrichlorophenol, see ChlorophenolTrichoderma spp. 54, 147triethyltin, see Organotintrihalomethylthio-compounds

134–5, 145, 148–9Triolith 103, 108–10trimethyl borate 126trioctyltin, see Organotintrimethyltin, see OrganotinTrioxan U, UA 108triphenyllead, see Organoleadtriphenyltin, see Organotintripropyltin, see Organotintrixylyl phosphate 129Turkey Red dye 10Tuffbrite 134Tuffgard 134Turski 129Tyndall 9

units, pressure and vacuum 76–7

vacuum and pressure processes73–89

vacuum process, double 158vacuum units 76–7Vac-Vac 82–3varnish, see CoatingsVasa (Wasa) 145vessels, hardwood 17–18Viczol 114von Wolniewicz 135

Wade, Thomas 6–7warping, wood 39Wasa 145wasps 186, 207–8Wassermann 78water-borne preservatives 97water gar tar 104water repellants 60–4, 142–6waxes 143weather resistance 95–6weevils, wood 199–200wetting, preferential 40–1, 142–3Wharf borer 200–1White ants, see Termiteswhitewood 82Williams, Greville 10Witwatersrand 113Wohler 10Wolman 103, 123, 125, 164

CCA 119preservatives 52, 86, 90, 97,

106, 107, 128–9Wolmanac CCA 119Wolmanit 90, 108–10

U, UA, UAR 180CB 123–4, 180FCAP 180TS 71, 164TSK cartridges 164

Wolniewicz, von 135wood

annual rings 18borers 185–209cell walls 19–20chemical structure 20–1degradation 23–41durability 182–3effects of water 21–2formation 13–16, 19–20kilning 24–5, 37–8

moisture content 24, 33–41movement 21–2, 182–3panel products 36–7porosity 15properties 182–4quality 23–4resistance to penetration 67seasoning 24–5, 37–8shrinkage, swelling 21–2stabilization 50–1, 63–4,

142–6structure 16–20tar 103–4treatability 182–4weevils 199–200

Woodtreat 141, 150Wood wasps 25, 56, 57Wykamol injectors 141, 150Wykamol, PCP, Plus 132, 149,

150Wykemulsion 150, 168

Xestobium rufovillosum 27,193–4

Xylamon 132, 149xylem 15xylenol 107xylol 100

ZAA, see Zinc-ammonia additiveACA, see Zinc-chromium-arsenicZFD salt 113ZFM salt 113zinc 50, 107, 110, 112–13, 135–

7, 143, 149, 158zinc-ammonia additive 114zinc-ammonia-arsenic 114zinc borate 124, 148zinc chloride 6–7, 146, 162zinc-chromium-arsenic 113zinc meta-arsenite 113zinc naphthenate, see Zinc soapszinc octoate, see Zinc soapszinc oxychloride 7zinc soaps 97, 135–7, 180zinc stearate, see Zinc soapsZirconium, organo 144ZMA, see Zinc meta-arsenite

Wood Preservation, Second Edition - Barry a.richardson

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