-
Per. Minerai. (2002), 71, SPECIAL ISSUE: Archaeometl)' and
Cultural Heritage, 1-16 http://go.to/permin
PERIODICO di MINERALOGIAestablished in 1930
An Intemational JOllmal of
MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS,
PETROLOGY, VOLCANOLOGYand applied topics on Environment,
Archaeometry and Cultural Heritage
Mapping and characterization of stone materials and
theiralterationldeterioration products in the historical center of
Palermo (Italy)
ROSARIO ALAIM0 1, ETTORE AZZAR0 1, RENATO GIARRUSS02, PIETRO
MARESCALCHI3 and GIUSEPPE MONTANA I *
l Dipartimento di Chimica e Fisica della Terra ed Applicazioni
alle Georisorse e ai Rischi Naturali (C.F.T.A.),Università di
Palermo, Via Archirafi, 36, 1-90123, Palermo, Italy
2 CEPA s.r.l., Via Fonderia Oretea, 23 1-90139, Palermo, Italy3
Dipartimento di Rappresentazione, Università di Palermo, Viale
delle Scienze, 1-90lO0, Palermo, Italy
ABSTRACT. - In the last years, natural and man-made building
materials from several importanthistorical palaces and/or churches
of Palermo (Italy)and their alteration/deterioration products
started tobe colIected and analyzed.
In this paper are reported the relief and digitaImapping of
stone material' s typology andconservation state of the Baroque
palaces Lungarini(XVII century), Alliata di Villafranca
(XVIIIcentury), Ugo delle Favare (XVIII century) and thechurch of
Santa Maria dei Miracoli (XVI century),alI located in the
historical center of the city. Theresults of
mineralogical-petrographic analysisperformed on the originaI
building materials andtheir alteration/deterioration products are
alsodiscussed.
Graphic relief and materials mapping of the threeBaroque palaces
were realized starting from aclassical graphic procedure which has
beensuccessively digitalized by means of standardcommerciaI
software. The main façade of the churchof Santa Maria dei Miracoli,
on the contrary, wasrelieved by the «total station»
photogrammetricmethod, consisting of an electronic
theodoliteequipped with a laser-pointing diastimeter.
Graphicrestitution has been carried out via commerciaIsoftware.
Mineralogical-petrographic analysis, XRD, thinsection microscopy
and SEMIEDS alIowed to obtainthe compositional and textural
characterization ofnatural stones (Pleistocene biocalcarenite
andMesozoic welI cemented limestone) and air lime
Con'esponding author, E-mail: [email protected]
plasters as well as their alteration/deteriorationproducts.
According to previous studies, thealteration/deterioration
pathologies affecting bothbiocalcarenites and limestones are
represented bybrownish-blackish encrustations and
sub-efflorescences of soluble salts (mainly gypsum andhalite).
Cyclic crystallization of soluble salts (mainlycomposed of
magnesian sulphates such as epsomite,hexahydrite and kieserite) is
the major cause ofdeterioration of air lime plasters. The
predominanceof magnesium sulphates is to be related to
thewidespread use of magnesian lime in themanufacture of Palermo's
plasters from the XVII tothe XIX century.
The acquisition of a thematic cartography relativeto the most
important natural or artificial buildingmaterials used in the
monumental constructions ofthe historical center of Palermo supply
a usefulinstrument for programming restorationinterventions. The
future realization of a databaseregarding compositions and
mechanisms ofdegradation will concur to choose the besttechnologies
and products.
RIASSUNTO. - Negli ultimi anni, i materialilapidei naturali ed
artificiali utilizzati nei piùimportanti edifici storici, civili e
religiosi, diPalermo sono stati oggetto di una sistematicacampagna
di campionamento ed analisi.
Nella presente nota sono riportati i rilievi e lemappe digitali
che riguardano la tipologia deimateriali lapidei ed il loro stato
di conservazione,relativamente ai seguenti edifici: Palazzo
Lungarini(XVII sec), AZZiata di Villafranca (XVIII sec), Ugo
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2 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
delle Favare (XVIII sec), chiesa di Santa Maria deiMiracoli (XVI
sec). Vengono commentati i risultatidelle indagini
mineralogico-petrografiche effettuatesui materiali originali,
finalizzate alla lorocaratterizzazione ed alla identificazione dei
prodottidi alterazione e degrado.
I rilievi e le mappature dei tre palazzi barocchi,sono stati
realizzati a partire da una classicaprocedura grafica che è stata
successivamentedigitalizzata per mezzo di un software disponibile
incommercio. Diversamente, per la rappresentazionedel prospetto
della chiesa di Santa Maria deiMiracoli è stato impiegato il
metodofotogrammetrico della "Stazione Totale" compostada un
teodolite elettronico corredato didistanziometro a puntamento
laser. La restituzionegrafica anche in questo caso è stata ottenuta
tramitel'utilizzo di un software commerciale.
Le analisi mineralogico-petrografiche, (XRD,microscopia ottica
su sezione sottile, SEM/EDS)hanno consentito di caratterizzare, dal
punto di vistacomposizionale e tessiturale, gli elementi lapidei,
gliintonaci ed i loro prodotti d'alterazione.Conformemente a quanto
ricavato da studiprecedenti, le patologie degenerative che
interessanosia le biocalcareniti che i calcari compatti
simanifestano attraverso la formazione diincrostazioni bruno
nerastre e sub-efflorescenze disali solubili (costituite
principalmente da gesso ehalite). La cristallizzazione ciclica di
sali solubili(maggiormente rappresentati da solfati di
magnesioquali epsomite, hexahydrite e kieserite) è laprincipale
causa di degrado degli intonaci. Inquest'ultimo caso, la presenza
di solfati di magnesioè da porre in relazione al largo utilizzo di
una calcemagnesiaca per la realizzazione degli intonacipalermitani,
soprattutto durante il periodo compresotra il XVII e il XIX
sec.
L'acquisizione di una cartografia tematica relativaai più
importanti materiali lapidei naturali edartificiali impiegati negli
edifici monumentali delcentro storico di Palermo fornisce un utile
strumentoper la programmazione degli interventi di restauro.La
futura realizzazione di una banca daticoncernente composizioni e
meccanismi di degradoconcorrerà alla valutazione dei prodotti e
tecnologiedi restauro più idonei.
KEY WORDS: Sicily; Palermo; building materials;alte ra tion/dete
rio ration; res to ration;Archaeometry.
INTRODUCTION
In the restoration of monumental buildings itis necessary to
know in a comprehensive waythe composition of the employed
stonematerials, their functionality in thearchitectonic structure
as well as the state ofconservation.
In the particular case of the recovery of thehistorical center
of Palermo, the demand for agood scientific knowledge concerning
naturaland man-made building materials, traditionallyused in the
architectural practice of the pastcenturies, increased more and
more. In fact,lacking of this kind of data in support ofrestoration
projects has often induced to chooseinadequate technological
procedures, like as,for example, the use of integration materials
orconsolidating products scarcely compatiblewith the earliest stone
substratum. It is wellknown that every kind of stone material has
gota quite typical pattern of alteration/degradation(also depending
on micro-environmentalconditions) and it is therefore necessary
toknow in detail its nature to be aware of themechanisms and
provide for suitablerestoration tests.
On the base of the above generaIconsiderations it has been made
active, since afew years, an organic and systematic studyof the
building materials used in thehistorical architecture of Palermo,
in order toobtain:
- mineralogical, petrographic, chemical andpetro-physical
characterization of numerousvarieties of lower Pleistocene
biocalcarenitecropping out in the Palermo' s surroundings,whose
utilization is documented in severalbuildings of great
historical-artistic relief(Alaimo et al., 1998);
- review of lime production techniques in theold city area
during past centuries (Montana,1997-a);
- mineralogical, petrographic and chemicalcharacterization of
mortars and plasterscollected from monumental buildings of
thehistorical center (Alaimo et al., 2000-b);
- mineralogical, petrographic and chemical
-
Mapping and characterization ofstone materia/s and their
alterationldeterioration ... 3
characterization of decorative stucco-worksin several Baroque
oratories andchurches (Montana, 1997-b; Montanaand Ronca 2001).
The present contribution concerns with thecompositional
characterization of natural andman-made building materials as well
as theproducts of alteration/degradation which havebeen identified
in the main façades of threeimportant Baroque palaces and a church,
alllocated in the historical center of Palermo:
- Ugo delle Favare palace dates back to thefirst years of the
XVII century. The main gateas well as the lateral ones with
polygonal archare decorated with mythological figuresrealized in
manneristic style.
- Lurzgarini Palace was built up towards thehalf of XVII the
century The wall surface iscoated by two different layers of
plaster. Themost recent can be dated back to the XVIIIcentury; it
covers an older layer (XVII century)which is decorated with a
painted «diamond»ashlar-work.
- In its current appearance the Alliatadi Vill[~franca palace
goes back to 1753,due to the transformations that becamenecessary
after the 1751 earthquake, whichproduced notable damages to the
originaIstructure (La Duca, 1994; Chirco, 1996).Its wall structure
is covered by two layers ofplaster respectively manufactured in the
XVIIand XVIII century. Both the two gates aredecorated with
monolithic limestone columnsand stucco medallions of
valuableworkmanship.
- The church of Santa Maria dei Miracoliwas founded in 1547.
Within a Renaissanceoutline the main façade (constituted of
porouscalcarenite) introduces several successiveelements in Baroque
style.
This study is also expected to acquirepreliminary information on
the frequency ofemployment of originaI lithotypes in thehistorical
center of Palermo and it comprises afirst attempt of realization of
a thematiccartography concerning with the monumentalbuildings of
the city.
METHODs, TECNIQUES AND SAMPLING
Mineralogical-petrographic analysis
Mineralogical and petrographiccharacterization of the collected
materials andof their alteration/deterioration forms has
beencarried out according to Normal ICR/CNR10/82, 12/83, 14/83,
16/84 and 23/86recommendations. AlI the samples wereinitially
studied by visible light microscopy.Plaster samples have been
preventivelyimpregnated with epoxy resin under vacuum, inorder to
realize polished cross sections and thinsections suitable for the
study of stratigraphy.Abundance of aggregate components has
beenestimated by routine quantitative pointcounting procedures.
X-ray powder analyses(XRD) were carried out with a Rigaku D/maxIIlc
diffractometer (CuKa 40 KV, 20 mA,graphite monochromator). SEM/EDS
analysishas been performed on carbon-coated samplesby a Leica
Stereoscan 440 (Link Isis OxfordInstruments, Pentafet Si-Ge
detector), usingboth secondary electron (SE) and back-scattered
electron (BSE) imaging mode.
Reliefof the façades
For the representation of the façade of theSanta Maria dei
Miracoli church it has beenemployed the method of the
photogrammetricrelief, by an instrumentation composed of
anelectronic theodolite equipped with a laser-pointing diastimeter.
The theodolite was alsoprovided with a display that supplies
dataaccording to Cartesian coordinates. This «totalstation»
methodology makes available theselection of points of the façade as
well as themeasurement of the angle between the line ofview and the
optical axis of theinstrumentation. The sequence of measurementcan
be summarized as folIow: 1)photogrammetric relief and reading of
the data;2) photographic survey of the prospect of thechurch by
means of Rollei nutrieinstrumentation (semi-metric, with
calibratedlens and precision micro-grooving); 3) Roto-translation
of the relieved points; 4) image
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4 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
flattening (correction of the distortion); 5)C.A.D. plotting; 6)
mapping of natural andman-made materials and
alteration/degradationforms.
Graphic relief and materials mapping of thethree Baroque palaces
were realized startingfrom a classical graphic procedure which
hasbeen successively digitalized by means ofstandard commerciaI
software.
Sal11plìng
Up to 26 samples considered fullyrepresentative of the employed
natural andman-made materials and of the most
diffusedalteration/degradation products have beencollected from the
main façades of thepreviously cited monumental buildings in orderto
be characterized by mineralogical-petrographic methods.
RESULTS AND DISCUSSION
Mapping of facades: employed l11aterials andtheir conservation
state
The three Baroque palaces resultedcharacterized by the employ of
the similarbuilding materials. Figures l a and l b show, asexample,
the digitalized relieves referred toLungarini and Alliata di
Villafranca palaces.The load-bearing walls are composed ofsquared
blocks of the local porous Pleistocenebiocalcarenite. They show
only slightmacroscopic differences, from pIace to pIace,which can
be easiIy explained considering theuse of the same lithotype
although exploited indifferent quarries of Palermo' s territory.
Thewall structure is constantly covered by one ortwo layers of
plaster, whitish or light pinkish incolor. The biocalcarenite is in
full sight only inthe balcony where constitutes brackets,members
and various decorative sculptures.Monolithic columns decorating the
main gateand their plinths are manufactured, in most ofthe cases,
with a local hard gray limestone(breccias) of Mesozoic age, locally
namedpietra di Billiemi. More or less recent
integrations made by bricks and cement alsooccur.
The forms of alteration/degradation affectingthe studied
monumental buildings have beenmapped and described on the basis of
therecommendation ICR/CNR Normal 1/88. Themost important and
diffuse are:
- black crusts: are mostly concentrated onthe surfaces of the
building repaired from thewashing-action of rain waters (fig. 2a).
Theyshow variable thickness and morphologiccharacteristics
according to the nature of thestone to which they adhere
tenaciously. Whenspontaneously detached the substratum appearsto be
strongIy disaggregated.
- brovvnish-blackish fill11s: show smallthickness and
preferentially develop oncompact materials (fig. 2b). They can
beremoved from the substratum which generallyremains integraI.
- biological patinas: are thin, soft andhomogenous, adherent to
the stone surface,with color from dark green to blackish.
- diffe rential degradati0l1: is in closerelationship with the
compositional or texturalheterogeneity of the building material,
resultingparticularly evident in the calcarenite (fig. 2c).
- decohesion: manifests itself by theseparation of grains under
minimal mechanicalsolicitations.
- pulve rization: is represented by thespontaneous fall of
powdered material orgrains.
- erosion: mechanical removal of materialfrom the surface due to
various processes.
- gaps: are due to loss of parts of the surfaceplaster which
brings to light the inner stonesubstratum (fig. 2d).
The main prospect of the chm"ch of SantaMaria dei Miracoli has
been realized with awhitish Pleistocene biocalcarenite,
veryprobably exploited in the north zone ofPalermo. The whole
façade is covered by anocher film. The decorative elements are
alsomade of calcarenite, with the exception for acoat of arms
realized in marble (fig. 3). It hasto be noticed that the upper
part of the buildinghas been realized in a successive age
(XVIII
-
Mapping and characterization ofstone materials and their
alterationldeterioration ...
Lungarini Palace
10 ml
5
Plast(?r (XVIII ccnt.)
Plaster (XVII cent.)
BiocaJcarenite
Bricks integrations
Concrete integrations
Recent reconstructions
Plaster (XVIII cenI.)
Plaster (XVII cenI.)
Concrete integration
Biocalcarenitc
Bricks intcgration
Alliata di Villqfi"anca Palace
8 mI.
Fig. 1 - Mapping of natural and man-made building materials
employed in the main façades of Lllngarini and Alliata
di\lillafranca palaces.
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6 R. ALAIIVIO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
Fig. 2 - (a) Ugo delle Favare palace (XVII century): solllble
salt efflorescences and black crusts; (b) Alliata di
Villqfrancapalace (XVIII century) brownish-black films on Billierni
limestone; (c) Alliata di Villafranca palace (XVIII
century)examples of black crusts and differential degradation in
the biocalcarenite; (d) Lungarini Palace: detachment of the
XVIIIcentury plaster bringing to light the older one (XVII century)
decorated with a painted «diamond» ashlar-work.
century) and probably the used calcareniteshould be of different
type from the one used inthe Iower order. Unfortunately this
aspect, atpresent, cannot be supported by Iaboratoryanalysis due to
difficulty of sampling.
In this building, from the examination of thestate of the
defects emerges a generaI picture inwhich the hardest degradations
are related tohumidity (capillary infiltrations) and thereforeto
the formation of soluble salts, as well as toenvironmental
pollution and biological factors.Recognition of the defects by
thephotogrammetric relief has been processed in agraphical table
according to therecommendation ICR/CNR Normal 1188,allowing an
immediate and synchronic reading(fig. 4). In generaI, most of the
pathologies arevery similar to those already described for the
baroque palaces. Nevertheless, some othersalteration/degradation
forms have been noted:
- patina: all the wall surface of the mainfaçade is covered by
an ocher patina whichalters the originaI color of the stone
material.This is evident in the parts subjected towashing
processes, in which it can be noted theoriginaI color of the
stone;
- loss of material: it occurs in some parts ofthe façade, mainly
in those jutting out of thecornices and moldings of the
columns;
- washing-effect: Draining of l'ain water fromthe cornices and
the protruding parts causes thewashing away of the ocher coloring
in the wallsurface;
- efflorescences: this expression ofdegradation is caused by the
evaporation of thewater which permeates the squared blocks of
-
Mapping and characterization oJstone materials and their
alteration/deterioration ... 7
Fig. 3 - Image of the main façade of the church of Santa Maria
dei Miracoli (XVI century) showing the different types
ofbiocalcarenite (upper and lower orders) and the ocher film.
Santa Maria deiMiracoli
.... EffiorescenceSuperficial deposi t
Biological patina
g:~ Black crust
• VegetationLime mortar
Concrete
Ilmll Integrations
----Lesions
Metallic pins
Fillet gaps
Scuppers
Gaps
Washed areas
Fig. 4 - Graphic restitution of the photogrammetic relief of the
church of Santa Maria dei Miracoli showing the state ofdefects.
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8 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
biocalcarenite, with consequent crystallizationof salts towards
the external surface; it is foundin some zones under the cornice of
thearchitrave, over the two pilasters and in theframe of the left
window.
Natural stones: notes on traditional use andcompositional
characteristics
The Pleistocene biocalcarenite, in antiquitynamed Kiddan by Arab
conquerors (that is tosay «golden stone»), is the most used
buildingmaterial in the monumental architecture ofPalermo and of
the whole western Sicily. Thisextensive employment strongly
depended onthe trouble-free quarrying and shaping of thelithotype
as well as on the large distribution ofits outcrops, generally
located along the coastalareas. In the plain of Palermo these
porousrocks, with a thickness of 20-30 m, cover byunconformity
Mesozoic limestone anddolostones as well as tertiary flyschoid
deposits(Caflisch, 1966; Ruggieri, 1973). Theirpeculiar
depositional process is clearlyresponsible for the variability in
color, grainsize, degree of cementation, abundance ofdetrital
accessory minerals and rock fragments.The heterogeneity of the
textural, structuraland, in some extent,
compositionalcharacteristics is sometimes found even in thesame
quarry, both laterally or vertically alongthe stratigraphic
sequence. Different qualitiesof building stone were therefore
exploited to beadapted to the various architectural employs. Inthe
urban Palermo, at present days, are rarelyvisible the remains of
ancient quarries whichactually were dozens and dozens
(Cipolla,1929; La Duca, 1964; Todaro, 1988; Alaimo etal., 1998;
Montana and Scaduto, 1999). Sincethe first half of the XVI century
started theexploitation of the areas located in the extraurban
territory which continued unceasinglyuntil the first decades of the
XX century. Thefirst phase of cultivation of a calcarenite
quarrygenerally took pIace by means of undergroundgalleries (or
mucate, according to the Arabicdenomination) in order to go after
theproductive rock-Iayers. Then, this system ofexploitation was
followed by cultivation under
the open sky, which allowed lower costs andmaterial waste
(Todaro, 1988). The lands to beexploited were subdivided in smaller
fields ofrectangular shape, whose traces are still visiblein the
territory (fig. 5). Mining and the shapingof the blocks were
carried out during summerin order to allow the porous stone to
naturallydry. The squared blocks were usually leftexposed two years
to the atmospheric agentsfor being able to recognize and discard
themost vulnerable ones (Cipolla, 1929).
As already seen in the previous paragraph, inthe four studied
monumental buildings, thelocal biocalcarenite constitutes by far
the mostrepresentative lithotype, used both for theprospects and
for ornamental elements.Nevertheless, the presence of diffuse
blackincrustations or films and patinas interfere withthe appraisal
of the macroscopic differences(color, degree of cohesion and
prevailing grainsize) between the stone' s variety used in thewalls
and the one employed in the carved parts.
After the observation of thin sections underthe polarizing
microscope, the calcarenitesused in Lungarini palace, Ugo delle
Favarepalace and in the church of S. Maria deiMiracoli are alI
classifiable as grainstone(Dunham, 1962). They showed
quitecomparable compositional and texturalfeatures, (fig. 6). The
mean/prevalent grain sizeranges between 0,4 mm (Lungarini palace)
andl,O mm (Ugo delle Favare); maximum grainsize is comprised
between l,O mm (S. Mariadei Miracoli) and 3,0 mm (Ugo delle
Favare).Concerning with the bioclasts, in thesevarieties, the
fragments of calcareous algaeresulted always prevailing, followed
byrelati vely less quantities of foraminifera,lamellibranch,
bryozoan and echinoderms;fragments of algae are often impregnated
ofiron oxides which are responsible of the slightyellowish color of
the stone. Detritalminerals/lithoc1asts resulted quite
sporadic,represented by Mesozoic limestones,dolostones, with
micritic or microspariticstructure and mono-crystalline quartz.
Primaryfine grained matrix (micrite mixed with c1ayminerals) is
poorly represented (only scarce
-
Mapping and characterization ofstone materials and their
alterationldeterioration 0.0 9
Fig.5 View ofthe remains an ancient quarry ofbiocalcarenite in
the surroundings ofPalermo (Aspra).
residues enclosing or filling the spaces betweenthe larger
particles). An interstitial cementmade of sparry calcite is
relatively moreabundant even though it cannot be consideredcopious.
It is also characterized by crystalsshowing the typical acute
scalenohedral formand by small druse aggregate, filling the
largerpore spaces. The total macroporosity has beenvalued to be
around the 20%. Intergranularpores deriving from the selective
dissolution oforiginaI bioclasts are prevalent. The walls ofthe
pores often result covered by a thin layer ofmicrite.
This composition has been confirmed by thepowder XRD patterns
which showed the clearpredominance of calcite with only
smallquantities or trace of dolomite, quartz and clayminerals.
The variety of biocalcarenite used in themain façade of Alliata
di Villafranca palaceresulted quite different. It can be classified
as
packestone (after Dunham, 1962). Therefore,the microcrystalline
primary matrix isrelatively more abundant with respect of theabove
described varieties (fig. 7). Theprevalent grain size has been
recognizedaround 0,2 mm (fine sand). Moreover, it isricher in de
tritaI mineralsllithoclasts likelimestone fragments,
mono-crystalline quartzand chert grains, up to 10% (area).
Finally, some words should be spent aboutthe well cemented and
hard Mesozoiclimestone, locally named grigio di Billienti,which has
been often used for plinths andcolumns. It belongs to the lower
Lias - upperTrias and crops out in the mounts surroundingPalermo
toward south. It is characterized by arich fossil fauna and a dark
gray color withplagues of fine black or yellowish material (fig.8).
Whitish calcite veins and concretions arealso frequento The more
famous andappreciated variety, above all in baroque age,
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lO R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and (
Fig. 6 ~ Palarizing microscape image af the biacalcarenitetype
used in the façade af Lungarini palace (crossed nical;scale bar =
0,3 mm).
Fig. 8 Macrascapic appearance af the brecciated varietyafthe
Billiemi gray limestane.
shows a typical brecciated texture, constitutedof elements with
variable dimensions from acentimeter to various decimeters (fig.
9). Theparticular durableness and resistance to theatmospheric
agents, if considered together withthe nice appearance (in the
several tonalities ofgray) and, above all, the thickness of the
stratawhich favored the exploitation of enormousmonolithic blocks,
have privileged theextraordinary diffusion of this lithotype in
thebuilding practice (specially as decorative stone)of Palermo in
the past centuries (G. Montana &V. Gagliardo Briuccia,
1998).
Plasters: local tradition, raw l1wterials andcomposition
X-ray diffraction, optical microscopy andSEM/EDS analyses
allowed to define the
Fig. 7 - Palarizing mi,type used in the façadl(crossed nical;
scale b,
Fig. 9 - Transmittedvariety af the Billiel71i !
nature of sand astudied plastencomposition satialready
determibuildings of PalelThe same sequencobserved in thewhich
resulted pmore ancient la:composed by anstone substratumaround 10
mm,abundant and poo60%); it follows aof thickness) corraggregate
(40-60Ssorted, with medilmm); the sequenc
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Mapping and characterization (~lstone materials and their
alteration/deterioration 0.0 Il
level (1-2 mm), containing a relatively smalleramount « 5-20%)
of fine aggregate grains «0.2 mm). Only few compositional
differenceshave been established between plastersmanufactured in
the XVII and XVIII century.AlI the analyzed samples showed a
similarcomposition of the sandy aggregate. In generaI,a slight
predominance of siliceous materials(up to 50-65 % of total
aggregate) has beenoted with respect to components of
carbonatenature (45-50%). The siliceous constituentsare:
policrystalline and mono quartz(prevailing), chert, feldspar,
quartzarenite andradiolarite grains (fig. 10b and fig.
10c).SporadicalIy fragments of powdered potteryhave also been
noted. The carbonatecomponents are lithoc1asts deriving from
thePleistocene biocalcarenite as well as Mesozoiclimestones and
dolostones. Even as regardscomposition of the binder matrix
stronganalogies between plaster manufactured indifferent ages can
be pointed out. By X-raypatterns and SEMIEDS imaging is evident
thata magnesium-rich lime has been used (fig.10d). It has been
c1early manufactured fram thecalcination of magnesian carbonate
rocks (bothdolomites and dolomiti c limestones), ischaracterized by
the presence, together withcalcite, of magnesite and,
secondarily,hydromagnesite. Brucite-composed white c10tshave been
frequently observed (up to severalmillimeters in size), which are
consistent with abad carbonation. In fact a magnesium richcarbonate
rocks if heated at about 900°C, willproduce CaO together with MgO,
the most partof which is sintered. In this case, the
hydrationreaction (in the course of lime slaking) takespIace with
lower efficiency in comparison withpure CaO.
The comparison of the mineralogical-petrographic data with the
lithology of therocks cropping out in the surroundings ofPalermo
and the consultation of ancientmanuscripts, books and notarial
acts, let todeduce the exploitation areas of raw materials.The
composition of the aggregate essentiallyreflects the contribute of
the Numidian Flyschformation (Upper Oligocene-Lower Miocene)
as regards siliceous fragments, while thecalcareous component
derives from Mesozoicrelieves and Pleistocene biocalcarenites.
Sandaggregate compositional sorting put forward afluvial sand,
therefore the supplying sitesshould be chosen between the
watercourses ofthe Palermo surroundings whose drainagebasins draw
on the above mentioned geologicalformations. In a notarial act
dated back to the1654 is evidently requested that the sand
formaking up the plaster must come from theSperone, a locality
along the shoreline whichthree centuries ago was only about 3
kilometersfar from the city walls. However, the finalstretch of the
Oreto, another stream nearby theold walls of Palermo, should be
considered assand supplying site.
As regard the binder, its composition isconsistent with the
Mesozoic dolostones(Fanusi formation) widely cropping out in
therelieves which surround the western side of thecity. Moreover,
chemical analysis of this roc1e(Montana, 1997-a) demonstrate a very
lowconcentration of Si02 and A120 3 (both around0.2 wt%) and,
consequently, an hydraulic index(weight % Si02+A1203+Fe203/CaO +
MgO)even lower than 0.1, therefore appropriate foran air lime
binder. In these lands, which in timepast belonged to the
Benedictine abbey of S.Martino, it developed a widespread
craftproduction of lime. Ruins of several ancientlime kilns are
still visible and dozens oforiginaI documents, notarial acts of the
17th and18th centuries expressly require the «SantoMartino» lime
for the manufacture of plasters,mortars and stucco works (Montana,
1997-a).
Alteration/degradation products
Precipitation affects stone primarily in twoways: dissolution
and alteration. In exposedareas of the buildings roughened surfaces
canbee observed, due to removal of material andloss of carved
details. Thealteration/deterioration pathology affectingboth the
descri bed biocalcareni te andlimestones consists of
brownish-blackishencrustations and films. Both showed a
similarmineralogical composition: calcite and gypsum
-
12 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
Fig. lO (a) Reflected light micrograph of the XVIII century
plaster from Lungarini palace showing a typical
stratigraphysequence composed of three layers: (A) inner and
coarser level; (B) intermediate level; (C) thinner finishing level
(scale bar= 2.0 mm); (b) polarizing microscope image of the inner
layer of the XVIII century plaster (crossed nicol; scale bar =
0.6mm); (c) polarizing microscope image of the intermediate layer
of the XVIII century plaster (crossed nicol; scale bar = 0.5mm);
(d) SEM micrograph of the XVIII century plaster from Alliata di
Villafranca palace showing the adherence of thebinder to the
aggregate grains; qualitative chemical composition carried out by
EDS point analysis (upper right) is referredto the binder
matrix.
-
Mappìng and characterìzatìon ojstone materìals and theìr
alteratìonldeterìoratìon .,. 13
turned out to be the most representative phasesby XRD and
SEM/EDS analyses. This blackcrust, although it can form anywhere
onexposed carbonate stone surfaces; nevertheless,it only remains on
protected surfaces which arenot directly washed out by rain-water.
Gypsumcrystals form networks that trap partic1es of dirtand
pollutants, so the crust looks black. Sooneror later the black
crusts swell up and thecrumbled stone become exposed.
SEMinvestigations revealed some ultra-structuraldifferences between
the outer and the innerparts of the crusts. In the latter single
grains arecoalescing and porosity is reduced, while in theexternal
part, where particulate matter depositsprevails, the aggregate is
always more porousand, specialIy in the thickest black crusts, it
isheterogranular. Soluble salt, such as gypsumand halite (the
latter deriving from marineaerosols) are mainly present in
sub-efflorescences (fig. 11). Their cyc1ic growth,carried by
humidity and water migration in
pores space produce a strong disaggregatingeffect. They
preferentially form under thecornices which are subjected to
stagnation andevaporation of water infiltrating from the upperpart.
Sometimes alkaline soluble salts mayform due to the use of
incompatible recentintegration materials (Portland cement
basedmortars).
Intergranular decohesion due to thecrystallization of soluble
salts inside pores is themain deterioration effect in the studied
plasters.In some cases the effects of degradation areparticular1y
heavy and interest the entirestratigraphic sequence, above alI in
the lowerparts of the masonry (capillary action). Thewater
imprisoned in the porous stonesubstratum migrates more slowly
towards theoutside due to the smaller permeability of theplaster,
favoring soluble salt crystallization and,consequently,
deterioration phenomena(detachments). XRD and SEM/EDS
analysesestablished what has been already seen on other
Fig. Il - SEM micrograph of the black crust developed on the
biocalcarenite from Ugo delle Favare palace; EDS areaanalysis is
also showed (upper right).
-
14 R. ALAIMO, E. AZZARO, R. GIARRUSSO, P. MARESCALCHI and G.
MONTANA
external plaster samples collected from othershistorical
buildings of Palermo (Alaimo et al.,2000-b). Magnesium sulphates
(epsomite,hexahydrite and kieserite), calcium sulfate(gypsum) and
sodium chloride (halite) are thepredominant mineralogical phases
(fig. 12).Gypsum seems to slightly prevail in respectwith magnesium
sulphates in the sub-efflorescences. The process which
originatesuch a deterioration effect starts from thedeposition of
atmospheric particu1ate matter andsea spray on the external wall
surface coveredof plaster. As well known, either acid pollutantsor
salts become chemically reactive when incontact with precipitation
and/or condensationwaters. The main effect are dissolution
andsulphation of the calcareous binder matrix aswell as formation
of soluble salts efflorescenceand subefflorescence. It should be
emphasizedthat, in the specific case of Palermo, the sourceof
sulphate ions seems to be only to a lesserextent of anthropogenic
origin (e.g. emission of
S02) but has to be related to the great influenceof sea spray
and to windborne gypsum particlescarried by the southern winds
(Alaimo et al.,1989). The predominance of magnesiumsulphates is to
be related to the widespread useof magnesian lime in the
manufacture ofPalermo's plasters from the XVII to the XIXcentury,
as already underlined. In fact, [S04F-enriched percolation water
reacting withmagnesium carbonate in the binder matrix areliable to
precipitate calcite and solublemagnesium sulphates (soIubiIity of
epsomite =720 glI at 20°C). In fact calcite is by far IesssoIubIe
than magnesite, respectiveIy 0,014 glI(20°C) and O, Il glI.
CONCLUSIVE REMARKS
Acquisition of data on the natural or artificialbuilding
materiaIs used in the monumentalconstructions of the historical
center of
Fig. 12 - SEM micrograph showing deliquescent crystals of NaCl
(halite) growth in the binder matrix (composed ofmagnesite and
calcite) of the XVIII century plaster collected from the façade of
Lungarìnì palace; EDS area analysis isshowed in the upper right
part of the image.
-
Mapping and characterization ofstone materials and their
alterationldeterioration o •• 15
Palermo with the relief of the state ofconservation and a
thematic cartographyrelative to the most important prospects,
isexpected to supply a useful instrument forprogramming
restorations. The realization of adatabase regarding the
compositionalcharacteristics and types of degradation of
usedbuilding materials in the architecture practiceof the ancient
city would concur to theplanning of restoration interventions,
withtechnologies and products custom-made for thespecific
requirements. At the same time, thestudy of the mechanisms of
alterationldegradation, for every single case study, hasthe scope
to adjust the research towardsspecific procedures and materials to
be used forsubstitutions, integrations or reconstructions.
Like already pointed out in the introduction,one of the more
arduous problems to solve inthe course of the planning of an
architectonicrestoration consists in the quantitativedefinition of
the «compatibility» between theoriginaI materials and those to be
used in theintegrations. In some cases, the choice ofproducts
technologically considered in theforefront, which are not, however
(in thespecific case), the most suitable ones, mightseriously
compromise the restorationintervention (especially in terms of
duration)and also cause worsening. In the cases studied,concerning
with the historical center ofPalermo, it has been shown that the
stonesubstratum on which the most ancient layers ofplaster adhere,
it is constituted of the localbiocalcarenite. This rock type
generallypossesses a very elevated open-porosity,ranging from the
25% to beyond 40% (Alaimoet al., 2000-c). Being Palermo a coastal
citywith a temperate humid climate andcharacterized by an intense
vehicular traffic,problems associated with soluble saltsdeposition
and migration (essentiallyalkaline/earth-alkaline sulfates and
sodiumchloride), should be checked, specially inpresence of such a
porous lithic substratum.
Therefore, the possibility to determine thepetrographic and
physical characteristics ofnatural and man-made building materials
as
well as to replicate the receipts of to ancientplaster and, if
it is the case, improve them inthe light of current knowledge, it
seems toassume a great importance in the safeguard ofcultural
heritage.
ACKNOWLEDGMENTS
This research has been financially supported by1999 MURST-Cofin
grant.
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