Hyperfine Interactions 150: 1–5, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Material Analysis in Archaeology

R. GEBHARDArchäologische Staatssammlung München, Lerchenfeldstraße 2, 80538 München, Germany

Abstract. Material finds are the main source of archaeological knowledge. The scientific analy-sis of found objects can help to interpret many aspects of archaeological interest, like productionand manufacturing processes or provenance. Such insights can often be used in further historicalinterpretations.

Key words: material analysis, archaeology, physical methods, Mössbauer spectroscopy.

1. Introduction

The term archaeology is generally used for a special part of the historic sciences.Archaeology tries to reconstruct the culture and history of past societies, especiallyof those on which no or poor written sources exist and for which information onevery day life is scarce. Archaeology, in its broadest sense, studies a past materialculture. In detail it is like a puzzle of very different aspects that is fitted togetherto a picture of the past. Archaeology today uses a whole variety of methods andtools: survey and excavation, environmental analysis with pollen or glacial records,paleobotany and paleozoology, scientific and historical dating methods, historicand iconographic sources, models that were developed from sociology and ethnol-ogy, archaeological experiments and, last but not least, material analysis of foundartefacts [1]. The use of methods borrowed from the natural sciences is nowadayssubsumed under the term archaeometry.

2. Material analysis and technological studies

Material analysis plays a dominant role in archaeometry. As manifold as the ap-plied methods are, as different may be the results. The application of only onemethod is often not enough, and only the application of different methods mayresult in a detailed view of the objects.

Material analysis has two main topics: the characterisation of the material ofwhich objects consist and the characterisation of the technical treatment or themanufacturing of the objects. The first can, for instance, give information aboutthe provenance of the material, while the latter helps to reconstruct ancient tech-niques. The provenance of an object or its raw material can give information aboutresources, trade contacts and economic systems. The ancient techniques used in

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a culture can be a main argument in the discussion of the level of civilisation theculture has. In most cases, highly developed techniques only appear in complex,structured social systems, because experiments and communication between dif-ferent crafts result in technical progress. The application of scientific analysis inthe second half of the 20th century has brought new light into archaeology. Todaymaterial analysis is an integral part of the archaeological work. The secret of suc-cess in such interdisciplinary studies is always a very close collaboration betweenarchaeologists and scientists. It is therefore necessary that both be familiar with theprinciples of the methods used.

3. Analysis of firing techniques by Mössbauer spectroscopy

Processes in which high temperatures were used are of special interest in studies ofancient technologies. The quality of ceramic production or metal working dependson the available firing techniques. Two aspects have to be monitored during a firingprocess: the temperature and the kiln atmosphere. Only a perfect control of bothcan result in a high-quality product. The experience of the craftsmen was widelydifferent in different prehistoric cultures. It depended on the traditions that existedin the society. Smiths, for instance, often represent a very small community withjealously guarded trade secrets. Their work was often highly specialized and en-abled them to live without additional farming. Pottery making, on the other hand,was a part of everyday life in many prehistoric cultures. Generally, however, inhighly developed cultures pottery making and other crafts tended to specialise, andoften the products and techniques became standardised. The changes in the skillsof crafts are of much interest for the archaeologist, since they can give informationon internal technical developments or influences from outside. In this context,processes requiring temperatures near 1000◦C are of special interest since suchtemperatures are difficult to reach and control [2]. Much experience in constructingkilns as well as a thorough knowledge of the properties of the fuel are needed. Thecommand of temperature is essential in the development of metallurgy, of glassmaking and, though to a lesser extent, of pottery. The introduction of charcoal asa fuel in the bronze age went along with enforced ventilation of fires by bellows.This was definitely an evolution that accompanied the spreading of techniques forthe melting of ores. At the same time, kiln and furnace linings began to be used torestrict the loss of heat and to improve the possibility to control the atmosphere ofthe fire.

Mössbauer spectroscopy on 57Fe has turned out to be one of the best methodsfor technological studies of firing techniques [3]. It can be applied to all iron-containing materials involved in the firing process. Typical objects that can bestudied by Mössbauer spectroscopy are all kinds of ceramics, like pottery, tuyeres,moulds and parts of furnaces, as well as non-ceramic materials like slags and glass.

Mössbauer measurements on fired ceramic materials contain information onfiring temperatures, kiln atmospheres and even complex firing cycles. For an in-

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terpretation a comparison with measurements of material fired under controlledconditions in the laboratory is often necessary.

At this point the potential of the method can best be illustrated by an example:In Celtic Europe the adoption of a monetary system was essential for the establish-ment of a specialised economy. Gold, silver and bronze coins were used in internaland external trade. The coins were produced in a standardised process [4]. Forsilver and gold coins, blanks of the desired weight were produced first and mintedafterwards with dies. The blanks for gold coins were melted in clay moulds, whichwere used only once. This facilitates an exact reconstruction of the melting processby Mössbauer measurements on different layers of the moulds [5, 6]. By compar-ison with samples subjected to laboratory firing under controlled conditions, theMössbauer measurements gave three results: (i) The vitrified surface was heated totemperatures above 1050◦C, which are required for melting gold. (ii) The temper-ature of the bottom and the interior of the coin mould never exceeded 500–700◦C,and (iii) the melting process took place in a reducing atmosphere.

As the distance from the top to the interior of the moulds is less than a centime-ter, the firing can have lasted only for a short time and the heat must have beenapplied from the top. These notions obtained from the Mössbauer analysis wereused in conducting a field test in a reconstructed furnace. Embedded thermocou-ples in the replica coin moulds used in the field experiment exactly reproduced theconclusions from Mössbauer spectroscopy. Only the uppermost layer of the coinmoulds attained temperatures above 1000◦C while the core was heated to between400 and 800◦C. The melting point of gold was reached after 2 to 5 minutes. On theother hand, coin moulds from Southern England show uniform firing in a reducingatmosphere, thus telling us that they were used in a different manner [7].

4. Pottery analysis

In the same way as Mössbauer spectroscopy was applied in this example, it canbe used in pottery analysis [8, 9]. During a project dealing with a certain culture,one normally studies a number of different materials, which exhibit a variety ofMössbauer spectra. This can have different reasons, e.g., variations in the claysources, different techniques used by different people or chronological differences.

The interpretation often requires a rather large number of sherds to be analysed.We have observed that only when several dozens of samples have been studied byMössbauer spectroscopy it becomes possible to group the spectra into types. Suchtypes may represent standard production procedures used, for instance, in certainworkshops.

5. Analysis of non-ceramic material

Besides ceramics, Mössbauer spectroscopy allows one to study many other ma-terials, like glass, gold, iron, slags, or phenomena like the corrosion of metals

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and the weathering of non-metals. In glass studies, Mössbauer spectroscopy givesinformation on the chemical state of the iron that is present in quantities up to abouta percent in most antique glasses [10]. Mössbauer spectroscopy can also be used tostudy the chemical state of tin and antimony in glasses or glazes [11]. The colourof glass is mainly caused by metal compounds added to the glass. It depends onthe oxidation state of the metal ions and various factors like the glass compositionand the kiln atmosphere. Mössbauer spectroscopy can give detailed information onsome of these features.

Mössbauer spectroscopy of gold is a useful supplement for other analyticaltechniques. As precious objects from museums normally cannot be sampled, non-destructive analytical techniques are required. These, however, give only the com-position of the outer surface that can be altered during burial. The same is truefor gamma ray backscattering and CEMS Mössbauer spectroscopy, although thesemethods allow to probe different depths, while transmission Mössbauer spectros-copy can give information on the bulk of the gold objects [3, 12, 13].

The analysis of slags is always important for the reconstruction of the metalmelting process and metal production [14]. Mineral phases in the slags can easilybe identified by Mössbauer spectroscopy and can be used to give information onthe furnace temperatures and atmospheres used during the melting process.

6. Corrosion studies

Another point that should be mentioned here is the study of corrosion. This maybecome a major application for Mössbauer spectroscopy in the future, since cor-rosion studies are very important for restoration concepts of both metallic andnon-metallic objects. Many restoration departments of museums have still no pos-sibility of getting the necessary information on the chemical processes leading tosurface corrosion of iron artefacts that they need for an adequate treatment and forconservation. Iron Mössbauer spectroscopy could be developed to a powerful andrelatively cheap routine method in this field.

7. Conclusions

Material analyses and especially Mössbauer spectroscopy today can answer or atleast help to answer a wide variety of archaeological questions. It should, however,not be forgotten that it is not only the scientific aspect that should provoke anarchaeologist, and a scientist to analytical investigations. It is as well the commit-ment to better understand the beautiful objects produced by our ancestors and toconserve the material objects of our cultural heritage for the next generation. Inthis sense Mössbauer spectroscopy will keep its “conservative” place.

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References

1. Renfrew, C. and Bahn, P. G., Archaeology: Theory, Methods and Practice, 3rd edn, Thamesand Hudson, New York, 2000.

2. Gebhard, R., Industry in Celtic Oppida − Aspects of High Temperature Processes, In:G. Morteani and J. P. Northover (eds.), Prehistoric Gold in Europe: Mines, Metallurgy andManufacture, Vol. 280 of NATO ISI Series E: Applied Sciences, Kluwer Academic Publishers,Dordrecht, 1995, pp. 261–272.

3. Wagner, F. E. and Kyek, A., Mössbauer Spectroscopy in Archaeology: Introduction and Ex-perimental Considerations, In: U. Wagner (ed.), Mössbauer Spectroscopy in Archaeology, Hyp.Interact., topical issue, Vol. I, to be published.

4. Moesta, H. and Franke, P. R., Antike Metallurgie und Münzprägung. Ein Beitrag zur Tech-nikgeschichte, Birkhäuser Verlag, Basel, 1995.

5. Gebhard, R., Große, G., Lehrberger, G., Riederer, J., Wagner, F. E. and Wagner, U., WhatMössbauer Spectroscopy Can Tell us about Precious Metal Working in Celtic Times, In: I. Or-talli (ed.), Conf. Proc. Ital. Phys. Soc., Vol. 50, Società Italiana di Fisica, Bologna, 1996, pp.781–784.

6. Gebhard, R., Lehrberger, G., Morteani, G., Raub, C., Wagner, F. E. and Wagner, U., CoinMoulds and Other Ceramic Material: A Key to Celtic Precious Metal Working, In: G. Morteaniand J. P. Northover (eds.), Prehistoric Gold in Europe: Mines, Metallurgy and Manufacture,Vol. 280 of NATO ISI Series E: Applied Sciences, Kluwer, Dordrecht, 1995, pp. 273–301.

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10. Gebhard, R., Riederer, J., Schwabe, R., Wagner, U. and Kossack, G., Coloration of Celtic Glassfrom Manching, In: Y. Maniatis (ed.), Proc. of the 25th Int. Symposium on Archaeometry,Elsevier, Amsterdam, 1989, pp. 207–215.

11. Stievano, L., Bertelle, M., Calogero, S. and Wagner, F. E., The Application of 119Sn MössbauerSpectroscopy to the Investigation of Glass Coatings: Evolution of the Tin Species in Lead-RichWhite Glazes, In: U. Wagner (ed.), Mössbauer Spectroscopy in Archaeology, Hyp. Interact.,topical issue, Vol. I, to be published.

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