Material properties of pyrotechnical ceramics used in the Bronze Age Aegean and implications on metallurgical technologies

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BEIhEft 26

Archaeometallurgy in Europe III

Andreas hauptmanndiana modarressi-tehrani

BEIhEft26


EditorsAndreas HauptmannDiana Modarressi-Tehrani

Bochum 2015

Archaeometallurgy in Europe IIIProceedings of the 3rd International ConferenceDeutsches Bergbau-Museum Bochum

June 29 – July 1, 2011

DER ANSCHNITT

Herausgeber: Vereinigung der Freunde von Kunst und Kultur im Bergbau e.V.

Vorsitzender des Vorstands: Prof. Dr. Karl Friedrich Jakob

Vorsitzender des Beirats: Bergassessor Dipl.-Kfm. Dr.-Ing. E.h. Achim Middelschulte

Geschäftsführer: Museumsdirektor Prof. Dr. rer. nat. Stefan Brüggerhoff

Schriftleitung: Dr. phil. Andreas Bingener M.A.

Editorial Board: Prof. Dr. Stefan Brüggerhoff, Dr. Lars Bluma, Dr. Michael Farrenkopf, Prof. Dr. Rainer Slotta, Dr. Thomas Stölllner

Wissenschaftlicher Beirat: Prof. Dr. Jana Gerslová, Ostrava; Prof. Dr. Karl-Heinz Ludwig, Bremen; Prof. Dr. Thilo Rehren, London; Prof. Dr. Wolfhard Weber, Bochum

Anschrift der Geschäftsführung und der Schriftleitung:Deutsches Bergbau-Museum Am Bergbaumuseum 28 - D-44791 Bochum Telefon (02 34) 58 77-0Telefax (02 34) 58 77-111

Einzelheft 9,– €, Doppelheft 18,– €; Jahresabonnement (6 Hefte) 54,– €; kostenloser Bezug für die Mitglieder der Vereinigung (Jahres-Mitgliedsbeitrag 50,– €)

Montanhistorische Zeitschrift

Der ANSCHNITT. Beiheft 26

= Veröffentlichungen aus dem Deutschen Bergbau-Museum Bochum, Nr. 202

Cover

Domus Vettiorum / Casa dei Vettii, Pompeii (Campania, Italy, 63-79 BC), which was excavated in 1894. Section of a Pompeii-style scenic fresco showing Erotes and Psyches in a gold assay laboratory. In the left corner, scales for weighing gold are put on a table. Next to it, one of the Erotes is working with a small hammer on an anvil. On the right side, an assay furnace is shown. Ano-ther of the Erotes is holding a small crucible with pincers with the right hand while using a blowpipe with his left hand, supplying the fire with air. The large bellow for the assay furnace is driven by the third of the Erotes.

RedaktionDiana Modarressi-Tehrani, Andreas Hauptmann

LayoutRolf Krause

TitelgestaltungKarina Schwunk

DruckGrafisches Centrum Cuno GmbH & Co. KG

Bibliografische Informationen der Deutschen Bibliothek

Die Deutschen Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http/dnd.ddb.de abrufbar.

ISBN 10: 3-937203-74-5ISBN 13: 978-3-937203-74-4

The conference Archaeometallurgy in Europe III was supported by

Keyence

Analyticon

MLS GmbH

Zeiss

Thermo Scientific

Springer Verlag Berlin Heidelberg New York


Scientific Advisory BoardGilberto Artioli, Universitá di Padova, Italy

Roland Gauß, Fraunhofer-Institut für Silicatforschung, ISC, Alzenau

Alessandra Giumlia-Mair, Merano, Italy

Gert Goldenberg, University of Innsbruck, Austria

Sabine Klein, J.W. Goethe University of Frankfurt/Main, Germany

Marcos Martinon-Torres, University College London, United Kingdom

William O’Brien, University of Galway, Ireland

Vincent Serneels, University of Fribourg, Switzerland

Standing Committee Yannis Bassiakos, Athens, Greece

Alessandra Giumlia-Mair, Merano, Italy

Andreas Hauptmann, Bochum, Germany

Ivelin Kuleff, Sofia, Bulgaria

Susan LaNiece, London, United Kingdom

Ignacio Montero, Madrid, Spain

Local Organizing Committee Michael Bode

Andreas Hauptmann

Diana Modarressi-Tehrani

Michael Prange

Ünsal Yalçın

This volume comprises a range of articles, which were submitted and selected from all the presentations given on the International Conference ”Archaeometallurgy in Europe III”, held from the 29th of June to 1st of July 2011 at the Deutsches Bergbau-Museum Bochum, Germany.

The present volume is the third in the series “Archaeo-metallurgy in Europe” , capturing the spirit of the suc-cessful series of international conferences on this special theme of research. The first conference “Archaeometal-lurgy in Europe” had been organized by the Associazi-one Italiana di Metallurgia and took place in Milano, Italy, from the 24th to the 26th of September 2003. The second conference was held in Aquileia, Italy, from the 17th to the 21st of June 2007. It was also organized by the Associazione Italiana di Metallurgia.

The splendid idea to launch this conference series, a scientific series of meetings limited to the countries of Europe, came from the late Prof. Dr. Walter Nicodemi, formerly President of the Assoziazione Metallurgia di Italia. Thanks to the efforts of Dr. Alessandra Giumlia-Mair, Merano, these conferences have developed into increasingly productive events with a high scholarly qua-lity. Since then three conferences have taken place and the fourth meeting is at an advanced stage of prepara-tion and will take place in Madrid, Spain, from the 1st to the 3rd June 2015.

The title of the conference series covers a research field which is a distinctive part of archaeometry, and which so far was usually included as one of the topics in the program of the “International Symposium on Archaeo-metry” (ISA), organized every third year at different lo-cations in Europe and in the United States. However it is our opinion, that in the last decade archaeometallurgy has developed as a very important research field, and we are observing a large number of scholarly activities all over the world. We are convinced that such an im-portant topic needs to be organised and presented in conferences specifically dedicated to this field. Therefo-re the topic of this conference is the history of metals and metallurgy primarily in Europe, but it also includes other regions of the Old World.

The future prospects of the conference series are pro-mising, especially because “Archaeometallurgy in Euro-pe” constitutes an extremely useful broadening and a regional counterpoint to the well-established and suc-cessful conference series “The Beginnings of the Use of Metals and Alloys” (BUMA), which was launched in

1981 by Professors Tsun Ko, Beijing, China, and Robert Maddin, then Philadelphia, USA. The focus of the eight BUMA conferences held so far (the last one was held in Nara, Japan, in 2013) lays on the development of metallurgy in South-East Asia and the Pacific Rim. We firmly belief that the two conferences complement each other very effectively and should therefore continue to exist side by side.

With this special volume of Der Anschnitt, we are de-lighted to publish a selection of the lectures presented at the conference at the Deutsches Bergbau-Museum Bochum in 2011. Many of the authors contributed with very instructive and informative papers, which finally resulted in this volume.

We are very much obliged to all these authors who, with patience and persistence, cooperated with us and helped to shape this volume. We would also like to thank the reviewers who decisively contributed in the improvement of the scientific level of this volume.

Our thanks go first to all those colleagues and friends who helped to organize the conference in 2011. The former director of the Deutsches Bergbau-Museum, Prof. Dr. Rainer Slotta, and the present director, Prof. Dr. Ste-fan Brüggerhoff encouraged and promoted our efforts to organize this scholarly meeting. Dr. Michael Bode, Dr. Michael Prange, and Prof. Dr. Ünsal Yalçın supported the conference planning and realization in every aspect. Many colleagues of the staff of the Deutsches Berg-bau-Museum, and many of the students working in our research laboratory offered their assistance and help.

Finally, our thanks go to Mrs. Karina Schwunk and Mrs. Angelika Wiebe-Friedrich who performed the editorial work, design, and layout for this volume.

Andreas HauptmannDiana Modarressi-Tehrani

Contemporaneously to the conference in 2011 a volume with abstracts on every lecture given and every poster presented was published:

2011 HAUPTMANN, Andreas, MODARRESSI-TEH- RANI, Diana & PRANGE, Michael (eds.), Archaeometallurgy in Europe III. Abstracts. METALLA, Sonderheft 4, 2011.

Editorial

Table of contents

Early mining and metallurgical innovation stages in Europe

Hans Anderssson Iron – a driving force in early urbanisation 13

Florence Cattin, Matthias B. Merkl, Christian Strahm & Igor Maria Villa Elemental and lead isotopic data of copper finds from the Singen cemetery, Germany – a methodological approach of investigating Early Bronze Age networks 19

Guntram Gassmann, Sabine Klein & Gabriele Körlin The Roman mines near Ulpiana, Kosovo 33

Marc PearceThe spread of early copper mining and metallurgy in Europe: an assessment of the diffusionist modelA key-note lecture 45

Ignacio Soriano The earliest metallurgy in the north-eastern Iberian Peninsula: origin, use and socioeconomic implications 55

Thomas Stöllner Humans approach to resources: Old World mining between technological innovations, social change and economical structures. A key-note lecture 63

Simon Timberlake, Tim Mighalll & Thomas Kidd Newresearch into Roman metal mining in Britain 83

Regional studies in Europe and beyond

Lucile Beck, Elise Alloin, Anne Michelin, Florian Téreygeol, Claire Berthier, Dominique Robcis, Thierry Borel & Ulrich Klein Counterfeit coinage of the Holy Roman Empire in the 16th century: silvering process and archaeometallurgical replications 97

Maryse Blet-Lemarquand, Arnaud Suspène & Michel Amandry Augustus’ gold coinage: investigating mints and provenance through trace element concentrations 107

Velislav Bonev, Boika Zlateva & Ivelin Kuleff Chemical composition of fibulae from the Iron Age in Thrace (Bulgaria) 115

Carlo Bottaini, Claudio Giardino, Giovanni Paternoster The Final Bronze Age hoard from Solveira (northern Portugal): a multi-disciplinary approach 125

Jennifer GarnerBronze Age tin mines in central Asia 135

Alessandra Giumlia-Mair, Susan C. Ferrence & Philip P. Betancourt Metallurgy of the copper-based objects from Gournia, east Crete 145

Elisa M. GrassiRoman metalworking in northern Italy between archaeology and archaeometry: two case studies 155

Babara Horejs & Mathias MehoferEarly Bronze Age metal workshops at Çukuriçi HöyükProduction of arsenical copper at the beginning of the 3rd millennium BC 165

Rüdiger KrauseNew horizons: archaeometallurgy in eastern Europe and beyondA key-note lecture 177

Janet LangThe Anglo-Saxon cemetery at Dover Buckland, Kent, UK and the technology of some of the iron artefacts 185

Lene MelheimLate Bronze Age axe traffic from Volga-Kama to Scandinavia? 193

Alicia Perea, Patricia Fernández-Esquivel, Salvador Rovira-Llorens, José Luís Ruvalcaba-Sil, Ana Verde, Oscar García-Vuelta & Fabián Cuesta-Gómez Prehistoric gold metallurgy: the Arqeomeb research project 203

Irina Ravich & Mikhail TreisterThe mirrors of the early nomads of the foothills of south Urals: a complex archaeo-technological study 211

Irina Segal, Miryam Bar-Matthews, Alan Matthews, Yehudit Harlavan & Dan AsaelProvenance of ancient metallurgical artifacts: implications of new Pb isotope data from Timna ores 221

Béla Török, Árpád Kovács & Zsolt GallinaIron metallurgy of the Pannonian Avars of the 7th - 9th century based on excavations and material examinations 229

Frank Willer, Roland Schwab & Kati BottLarge Roman Bronze statues from the UNESCO World Heritage Limes 239

Vladimir I. Zavyalov & Nataliya N. TerekhovaThree-fold welding technology in the blacksmith’s craft of Medieval Rus’ (concerning Scandinavian innovations) 247

Reconstructing ancient technologies

David Bourgarit & Nicolas ThomasAncient brasses: misconceptions and new insights 255

Vagn F. BuchwaldOn the characterization of slags and ancient iron artefacts applying the slag-analytical method 263

Joseph Gauthier, Pierre Fluck, Alessandre Disser & Carmela ChateauThe Alsatian Altenberg: a seven-hundred-year laboratory for silver metallurgy 271

Anno Hein, Ioannis Karatasios, Noémi S. Müller & Vassilis KilikoglouMaterial properties of pyrotechnical ceramics used in the Bronze Age Aegean and implications on metallurgical technologies 279

Silviya Ivanova, Veselina Rangelova, Deyan Lesigyarski & Ivelin KuleffObservations on the technology of Bronze Age copper and copper alloy finds from Bulgaria 287

David KillickArchaeometallurgy as archaeologyA key-note lecture 295

Steffen Kraus, Christian Schröder, Susanne Klemm & Ernst PernickaArchaeometallurgical studies on the slags of the Middle Bronze Age copper smelting site S1, Styria, Austria 301

Matthias Krismer, Gert Goldenberg & Peter TropperMineralogical-petrological investigations of metallurgical slags from the Late Bronze Age fahlore-smelting site Mauken (Tyrol, Austria) 309

Matthias B. MerklSome thoughts on the interpretation of the elemental composition of Chalcolithic copper finds from central Europe 319

Nerantzis NerantzisExperimental simulation study of prehistoric bronze working: testing the effects of work-hardening on replicated alloys 329

Barbara S. OttawayExperiments in archaeometallurgy A key-note address 337

Alessandro PaciniThe Lombard fibula of the Arcisa: a substitution? 347

Salvador Rovira, Martina Renzi, Auxilio Moreno & Francisco ContrerasCopper slags and crucibles of copper metallurgy in the Middle Bronze Age site (El Argar Culture) of Peñalosa (Baños de la Encina, Jaen, Spain) 355

Sana Shilstein & Sariel ShalevComparison of compositional variations in modern European bronze coins with variations in some ancient coins 363

Elena Silvestri, Paolo Bellintani, Franco Nicolis, Michele Bassetti, Siria Biagioni, Nicola Cappellozza, Nicola Degasperi, Marco Marchesini, Nicoletta Martinelli, Silvia Marvelli & Olivia Pignatelli New excavations at smelting sites in Trentino, Italy: archaeological and archaeobotanical data 369

Maria A. Socratous, Vasiliki Kassianidou & Gaetano Di PasqualeAncient slag heaps in Cyprus: the contribution of charcoal analysis to the study of the ancient copper industry 377

New approaches, new technologies in archaeometallurgy

Gilberto Artioli, Matteo Parisatto & Ivana AngeliniHigh energy X-ray tomography of Bronze Age copper ingots 387

Elisa Barzagli, Francesco Grazzi, Francesco Civita, Antonella Scherillo, Alessio Fossati & Marco ZoppiCharacterization of ancient Japanese sword hand guards through time-of-flight neutron diffraction and scanning electron microscopy 391

The authors 401

279

Summary

The present study concerns material properties of pyro-technical ceramics, which were used in Bronze Age metallurgy in the Eastern Mediterranean. On the basis of three case studies microstructures of smelting furnac-es and crucibles are evaluated, in terms of porosity and pore size distribution. For this purpose, scanning elec-tron microscopy (SEM) and mercury porosimetry are applied. Apart from non-plastic inclusions, which were increasing the heat resistance of the ceramics, pores and voids, intentionally generated by organic tempering, are an important characteristic of pyrotechnical ceram-ics, such as smelting furnaces and crucibles. Through porosity the thermal conductivity was controlled and thus the heat transfer or heat loss into the environment, re-spectively. This relation is demonstrated with measure-ments of the thermal conductivity of furnace fragments. The effect of decreased thermal conductivity on the thermal efficiency of a smelting furnace is discussed using results of a computer model.

Introduction

Pyrotechnical ceramics constitute a major part of the archaeological evidence for past metallurgical activities, since functional ceramics played an important role in various metallurgical processes. Ceramic materials have been used for smelting furnaces, but also for tools such as crucibles, moulds or tuyères. Apart from the investi-gation of finds which are directly related to the metallur-gical process, such as slags or metal remains, therefore, the study of a metallurgical site usually involves also the investigation of the pyrotechnical ceramics (Tylecote 1982; Freestone 1989; Tite et al. 1990). Technological studies of metallurgical ceramics revealed an interesting picture, identifying diverse approaches for producing ceramics able to withstand the extreme temperatures occurring during metallurgical processes. Parameters that are of importance for a ceramic structures’ or objects’ suitability to be used in pyrotechnical processes include the raw material selected, clay tempering with various

non-plastic inorganic and organic materials, as well as the design of the installations (Hein & Kilikoglou 2011). It has been found that high concentrations of non-plas-tic temper increase the refractoriness of the ceramics, in terms of reducing the reactivity of the material at high temperatures. At the same time the thermal conductivi-ty of the ceramics is reduced in a material with a high porosity, which can be introduced for example by organ-ic temper (Hein et al. 2007). In this way the heat trans-fer within a ceramic is suppressed, so that heavy reac-tions of the ceramics’ matrix are restricted to regions directly in contact with the heat load. The effect of the pore structure on the thermal conductivity was assessed with two-dimensional computer simulations (Hein & Ki-likoglou 2007).

Another objective of studies of pyrotechnical ceramics has been the estimation of the temperatures, which de-veloped in the ceramic body during the metallurgical processes, in order to reconstruct operating conditions (Kingery & Gourdin 1976; Tite et al. 1990). This can be achieved through recording a temperature profile through a wall or base fragment for example, by establishing the degree of vitrification in different distances from the ce-ramics’ surface. More recently, a two-dimensional com-puter model of a smelting furnace was developed and evaluated with the finite element method (FEM) in order to consider various kinds of heat transfer and heat loss into the environment (Hein & Kilikoglou 2007). Refine-ments of computer models, however, require the knowl-edge of the material properties of the ceramics. In this context, we will present results from an ongoing study, of pyrotechnical ceramics from different Bronze Age sites in the Eastern Mediterranean Region. This encompass-es the examination of the metallurgical ceramics for their thermal conductivity, heat capacity, density, porosity and pore size distribution, and based on the results obtained on the archaeological material, an assessment of differ-ent approaches of clay paste modification and adaptation to the functionality of pyrotechnological tools. This as-sessment is expected to contribute to a deeper under-standing of the manufacture and use of early pyrotech-nical ceramics in general and of technical ceramics applied in Aegean metallurgy in particular.

Material properties of pyrotechnical ceramics used in the Bronze Age Aegean and implications on metallurgical technologies

Anno Hein, Ioannis Karatasios, Noémi S. Müller & Vassilis Kilikoglou

Anno Hein, Ioannis Karatasios, Noémi S. Müller and Vassilis Kilikoglou

280

Studied materialMetallurgical ceramics from three regions within the Eastern Mediterranean were analysed. The first set of material comprises furnace fragments from three differ-ent Early Cycladic copper smelting sites on the island of Seriphos, namely Avessalos, Kephala and Fournoi. The ceramics found among the copper slags appeared to belong to furnaces and furnace linings, the exact shape of which, however, could not be reconstructed yet. Furthermore, fragments from the Late Bronze Age copper smelting site Politiko-Phorades in Cyprus (Hein et al. 2007) were also examined. In this case, the great number of large base and wall fragments found at the site was allowed reconstructing a standardized shape of the freestanding smelting furnaces. This shape has been employed in investigations on operating conditions using computer models (Hein and Kilikoglou 2007). Fi-nally, fragments of Neopalatial and Postpalatial crucibles from the Late Minoan settlement at Palaikastro on Crete (Evely et al. 2012) were included in the study. The rel-atively small sample sizes, however, did not allow com-plete testing, particularly the measurement of the thermal conductivity was not possible for these fragments.

Analytical methods

Scanning electron microscopy

The ceramics’ microstructure was examined on polished sections. Fragments were cut perpendicular to the sur-face, embedded in epoxy resin and ground and polished. After examination under the optical microscope the pol-ished sections were carbon coated and their microstruc-ture was studied under the scanning electron microscope (SEM), using a FEI, Quanta Inspect D8334 scanning electron microscope, coupled with an attached ener-gy-dispersive X-ray spectrometer (SEM-EDS). The sam-ples were examined over the whole cross section both in secondary element mode and in backscattering mode. Particularly the pore structure and the distribution of coarse inclusions were of interest because these textur-al parameters are known to affect the material properties of the ceramics, such as strength or thermal conductiv-ity (Hein et al. 2007; Hein et al. 2008). Then, using high-er magnification, the microstructure was examined to assess the degree of vitrification, i.e. the development of the glassy phase. In this way the temperatures to which the ceramics were exposed to during use, in dif-ferent distances from the inner surface, could be esti-mated (Tite et al. 1990). Temperature profiles were thus compiled, which were expected to provide information about the operating conditions.

Mercury porosimetry

Mercury porosimetry was used to obtain an estimation of pore volume and distribution within the ceramics. The material was infiltrated with mercury under a controlled applied pressure. This pressure can be related to the pore size, assuming cylindrical pores. In a pore of cir-cular cross-section or radius r, the surface tension of liquid within the capillary acts to force the liquid back, and is applied along the line of contact with the edge. The surface tension tends to force the liquid out of the capillary. The magnitude of this force is given by 2πrγ, where γ is the surface tension force per unit area, and in the direction of the axis of the capillary by 2πrγ cosθ, where θ is the contact angle. In equilibrium, this force is balanced by the pressure forcing the liquid into the pore, πr2p. Balancing these forces gives the Washburn equation:

r = − 2π cosθ

p

To measure the pore volume intruded at an applied pres-sure, it is necessary to place the sample to be tested within a sealed pressure vessel into which mercury is forced. The volume of mercury intruded is determined by measuring the change in resistance of a wire sus-pended in a capillary tube leading into the mercury. An example where the technique has been employed to study archaeological building materials was recently presented by Karatasios et al. (2009).

Thermal conductivity

The thermal conductivity of selected furnace fragments was measured with a modified Lees’ disk setup. Disks with a diameter of c. 60 mm were cut from the fragments and ground to a thickness of 7 to 15 mm with even and parallel surfaces. The ceramic disks were in contact with a heating plate which was held at a stable temperature on one side, and with a brass disk on the other side. The heat loss of the brass disk, depending on its tem-perature, had been determined beforehand. The heat loss of the brass disk corresponds to the heat flux through the ceramic disk. Therefore, at steady state, the tem-perature difference between heating plate and brass disk provides the thermal conductivity of the ceramic mate-rial. The method has already been successfully em-ployed to estimate thermal conductivities of typical cook-ing pot materials (Hein et al. 2008).

Computer modeling

In recent years, computer simulations have been devel-oped in order to examine the influence of ceramic mi-crostructure on heat transfer processes and to investi-


281

gate heat transfer in furnaces. The digital models are evaluated with the finite element method (FEM) applying simulated loads and examining the resulting heat flux, considering apart from thermal conduction also the heat convection in the ambient air (see, e.g., Hein & Kilikoglou 2007). In order to do so, the two- or three-dimensional model is subdivided in discrete regions of small but finite size which are connected amongst each other via so-called nodes. In this way, the original non-trivial problem, the response of the entire system to the applied thermal loads under specific constraints, is converted into a large system of small but solvable problems, corresponding to the interaction and response of the specific regions, the so-called finite elements. When examining heat transfer in furnaces, transient solutions of the models allow for the simulation of operating conditions in terms of temperature development over time, in relation to material properties. Comparing these results with tem-perature profiles estimated on the basis of the degree of vitrification in archaeological samples, i.e. the furnace base and furnace wall, provide information about pro-cess temperatures and operating time.

Finally, the further evaluation of the heat flux provides estimations of the thermal efficiency. Smelting furnaces with insulating ceramic textures for example provide a better heating efficiency η, which is the ratio between the heat energy required for smelting Esmelt and the heat energy equivalent to the applied fuel, i.e. the combustion energy of the charcoal:

η =Esmeltmfuel ⋅Hc

where mfuel is the mass of the applied fuel and Hc the respective fuel value. In a recent study, computer sim-ulations of a smelting furnace indicated that with a suit-able ceramic texture the consumption of charcoal could be reduced in the range of 10 % (Hein & Kilikoglou in press).

Results and discussionMicrotexture of the ceramics

The SEM examination of the samples revealed micro-structures compatible with known Bronze Age pyrotech-nical ceramics. All samples presented a quite coarse microstructure with frequent non-plastic inclusions, which were either part of the original raw material or intentionally added to the ceramic paste, and which in-crease the ceramics’ heat resistance. Noticeable are, particularly in the case of the Late Bronze Age furnaces and the Late Minoan crucibles, characteristic voids, which were generated by the addition of organic temper that would burn out during use. Figure 1 shows two SEM

micrographs of sections of Minoan crucibles with differ-ent types of organic temper. Indeed, there appeared to have been different strategies of organic tempering dur-ing different periods, the functional implications of which remain to be investigated (Evely et al. in press). In the case of the furnace fragments from Seriphos and the crucible fragments from Crete, SEM micrographs in backscattering mode were evaluated by image process-ing in order to estimate the porosity of the ceramics (Table 1). The crucible fragments generally showed a higher porosity than furnace fragments. There seemed to be also a correlation to the different organic temper which was used, animal hair in the case of the Neopa-latial crucibles PK4349 and PK4363 and vegetal fibres in the case of the Postpalatial crucibles PK4614 and 4627.

Selected samples were also measured with mercury porosimetry, presenting a quite similar picture in terms of total porosity (Table 1). The Late Bronze Age samples in general showed a higher porosity, also the smelting furnaces from Cyprus. Discrepancies between the po-rosities measured with the mercury porosimeter and those estimated by image processing can be explained with smaller pore sizes which are still considered with mercury porosimetry and the rather subjective selection of SEM micrographs.

The measured pore size distributions provided to some extent information of the kind of temper material, par-ticularly in the case of the crucible fragments. The veg-etal fibres obviously generated larger voids than animal hair (Figure 2). Also, in the case of the furnace fragments there appears to be a trend towards larger voids in the Late Bronze Age.

Thermal conductivity

The thermal conductivities of the furnace samples were measured at two different temperatures, 100 °C and 200 °C (Figure 3). Conductivity increased with temperature. This is expected for porous ceramics because of the thermal expansion of the ceramic matrix and the related decrease of pore sizes. There is also a clear correlation of thermal conductivity and bulk density, which was de-termined for the ceramic disks as ratio of dry weight and volume. Noticeable is that the Seriphos samples have a clearly higher bulk density, which is largely due to their lower porosity. Therefore, the thermal conductivities of the Late Bronze Age smelting furnace fragments are significantly lower than the thermal conductivities of the Early Cycladic smelting furnace fragments. Even though this has to be investigated with more samples from oth-er sites, based on these two case studies the techno-logical choices in the manufacture of Late Bronze Age ceramics seem to be more clearly directed at reducing heat transfer.


282

Total Porosity [%] Estimated Porosity [%] density [g/cm]

average stddev

Seriphos

Ave A1

Ave A3

Ave A4

Fou A1

Fou A3

Fou A5

Kef A1

Kef B1

Kef B2

6.7

11.7

11.7

10.2

13.0

16.9

8.2

15.7

9.1

10.9

17.5

7.8

11.2

14.7

8.0

9.2

10.5

12.7

5.7

3.7

1.3

1.7

2.8

1.5

1.0

1.3

1.6

1.48

1.66

1.72

1.62

1.64

1.77

1.80

1.66

1.61

Cyprus

Pho 19

Pho 20

Pho 26

Pho 33

Pho 36

16.9

21.9

18.7

32.7

22.0

19.3

23.0

28.8

22.7

18.4

1.7

1.3

4.2

5.0

1.9

1.39

1.22

1.38

1.10

1.49

Crete

PK4349

PK4363

PK4614

PK4627

19.8

13.6

25.1

27.7

17.7

10.7

22.9

18.2

2.0

1.0

4.8

3.6

n.d.

n.d.

n.d.

n.d.

Table 1: Total closed porosity as measured with mercury porosimetry, porosity estimated by image processing of SEM micrographs and density as determined with the ration of dry weight and volume of the measured ceramic disks. n.d. = not detected. stdev = standard deviation.

Figure 1: SEM micrographs in backscattered mode of twocrucible fragments with different type of organic temper: PK4349, LM I crucible tempered with animal hair(left); PK4627, LM III crucible tempered with vegetal fibres.


283

Figure 2: Pore size distribution of crucible fragments (left) and furnace fragments (right) as determined with mercury porosimetry. Presented is the intruded pore volume down to a pore size of 10 μm. The total porosity measured for each sample is given in pa-rentheses.

Figure 3: Thermal conductivity of the furnace fragments in relation to their estimated density measured at 100°C ( Politiko-Phorades, Seriphos) and at 200°C ( Politiko-Phorades, Seriphos).


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Influence on thermal efficiency

The determined values for density and thermal conduc-tivity were evaluated on the base of a digital model of a smelting furnace (Hein & Kilikoglou 2007). Using this model the thermal efficiency of a furnace can be as-sessed, which is basically the ratio of the energy used for the actual metallurgical process and the energy in-troduced to the system. In order to increase the efficien-cy the heat loss through the furnace wall and furnace base has to be reduced. While the heat loss through the furnace wall is dominated by heat convection at the ex-ternal surface into the ambient air the heat loss through the furnace base occurs through thermal conduction into the ground. The heat flux through the base decreas-es during the operation of the furnace due to the heating of the solid soil. The heat flux at the external surface of the walls, however, reaches equilibrium and remains at a rather high level being apparently the main reason for heat loss (Figure 4). With a temperature dependent ther-mal conductivity of the ceramics in a range of 0.4 and 0.8 W/(mK) and an external furnace wall with an area of 0.5 m2 the heat loss was estimated with 21 MJ per hour, which corresponds to approximately 0.7 kg char-coal per hour or 3 kg for a whole smelting operation (Hein & Kilikoglou in press). The furnace fragments from Phorades presented a lower thermal conductivity com-pared to the model, starting in some cases with values below 0.3 W/(mK), while some of the Seriphos fragments presented even a higher thermal conductivity.

The total fuel amount for a smelting process can be estimated with approximately 20 kg charcoal based on experiments. Therefore, the control of thermal conduc-tivity of the ceramics can contribute to fuel economy in the range of 10 %. This might have become a more im-

portant issue in Late Bronze Age when the copper pro-duction for example in Cyprus reached a large scale level.

Conclusions

The results achieved so far revealed that the technolog-ical investigation of the pyrotechnical ceramics is essen-tial in order to comprehend the metallurgical processes, providing complementary information to what can be learnt from the analysis of slags or metal remains. There are some indications that the ancient metallurgists were trying to improve their ceramics. One possible motivation for these developments and improvements appear to be the thermal efficiency, at least in the case of furnaces and crucibles. The thermal efficiency was controlled by the thermal conductivity of the ceramic material and the design of the ceramics. It had a direct effect on the ability of the tools to achieve and maintain high temper-atures and thus, improving the thermal efficiency allowed for developing and enhancing metallurgical processes. Furthermore, fuel economy probably was a major issue when extending metallurgical activities towards large scale metal production.

Acknowledgements

The present study is funded by the Institute for Aegean Prehistory (INSTAP).

Figure 4: Simulated heat flux through the internal surface of the wall and the base of a smelting furnace model against thermal con-ductivity. The simulated heat flux through the wall corresponds to equilibrium, which is reached after ca. 30 min., while the heat flux through the furnace base corresponds to one hour and two hours operation time, respectively (Hein & Kilikoglou in press).


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Material properties of pyrotechnical ceramics used in the Bronze Age Aegean and implications on metallurgical technologies

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