Metal analysis

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We live in interesting times. Ingenuous new equipment has enabled non-destructive metal analysis. New data are becoming available. They turn the science of numismatics around. The new data provide insight in purity. The course of inflation becomes clearer and that means we are starting to learn more about the consequences of the policies of the leaders we now know mainly for their military actions. We are learning more about how people lived.

In practical terms, studies of individual coins are giving way to better analysis of hoards. Collections of coins by type are succeeded by observations and measurements of coins spread out in museums and other (semi) public collections around the globe. As more and more coins are brought online with full data, statistical analysis can be widened and old assumptions questioned.

My aim for this thread is to collect some studies that rest on metal analysis of coins. Please feel free to contribute.

Peter

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Eeva Jonsson, Metal analyses of Viking-Age coins. In: Metal analyses of coins 2018:1. Stockholm Numismatic Institute

In the present study, the silver content of 155 Viking-Age coins was analyzed. The aim was to re-exam the traditional interpretations of the silver content of different coinages during the Viking Age, and to offer an overview on the subject. The main question is what level of silver purity was considered as fine silver in everyday transactions in the Northern Lands and what variation was tolerated? The second aim of the study was to find possible inconsistencies in the silver content.

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FIB-FESEM and EMPA results on Antoninianus silver coins for manufacturing and corrosion processes by María Teresa Doménech-Carbó, Francesca Di Turo, Noemí Montoya, Fiorenzo Catalli, Antonio Doménech-Carbó & Caterina De Vito.

A set of ancient Antoninianus silver coins, dating back between 249 and 274 A.D. and minted in Rome, Galliae, Orient and Ticinum, have been characterized (...)

The results revealed that, contrary to the extended belief, a complex Ag-Cu-Pb-Sn alloy was used. The use of alloys was common in the flourishing years of the Roman Empire. In the prosperous periods, Romans produced Ag-Cu alloys with relatively high silver content for the manufacture of both the external layers and inner nucleus of coins.

This study also revealed that, although surface silvering processes were applied in different periods of crisis under the reign of Antoninii, even during crisis, Romans produced Antoninianus of high quality. Moreover, a first attempt to improve the silvering procedure using Hg-Ag amalgam has been identified.

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Trial Metallurgical Analysis of a Silver Coin of Bengal Sultan: Explore through methods and Interpretation by Syed Ahsan et. al.

The paper is a report of experimentation with two methods of non-destructive analysis. The sample used is too small for any conclusions, but it shows the potential of the equipment used. Good for a first orientation in non-destructive metal analysis.

Peter

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H. Gitler, M. Ponting and O. Tal, Metallurgical Analysis of Southern Palestinian Coins of the Persian Period, Israel Numismatic Research 3 (2008)

"By means of inductively coupled plasma atomic emission spectrometry (ICP-AES), metallurgical analyses of southern Palestinian coins of the Persian period were performed. The main group of analyzed coins consists of dome-shaped quarter sheqels (“drachms”), which were struck from worn, recut and repolished obverse dies that based on their circulation were defined as Edomite. In addition, several Philistian coins were analyzed as a reference group. Our results suggest that much of the silver bullion used for striking the Edomite and Philistian coins originated in the Greek world, most probably from Athenian ‘owls’ and that Edomite coinage was probably produced by a central Philistian minting authority based on identical silver content."

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Here is a great analysis of using different non-destructive technologies on relatively modern coins that had become unreadable. Its conclusion that museums must maintain large collections is of course highly debatable, as it denies the importance of co-operation between museums as well as cooperation with large private data bases. Somewhat naively, the author enthuses that these technologies can also be used on other metallic items.

If, in the end, no respectable museum can go without this equipment, its price will fall, perhaps to the point where the technology can be made available to collectors. May that happen sooner, rather than later.

Peter

This thesis demonstrates the application of a number of analytical techniques on a selection of silver coins from the Western Australian Museum. Results of analysis are used to appraise the applicability of surface analytical techniques on samples that have corroded. Analysis has also been used to determine when, where and how coins too heavily corroded to visually identify were minted. Further, the importance of maintaining large collections and assemblages of objects in museum collections is demonstrated, and further applications of the techniques used are discussed.
Four hundred silver coins and a selection of silver artefacts were provided for analysis from the collection of the Western Australian Museum.

The coins and artefacts were recovered from the following shipwrecks; the Batavia wrecked 1629; the Vergulde Draeck, wrecked 1656; the Zuytdorp, wrecked 1712; the Rapid, wrecked 1811; and the Correio da Azia, wrecked 1816. All of the ships were wrecked off the coast of Western Australia. The coins represent 22 mints in Spain, Spanish America, the United Netherlands, Germany, and the Spanish Netherlands, and 19 European sovereign issuers from Spain, Saxony, the Holy Roman Empire, Denmark and Norway, the United Netherlands, Holstein Gottorp, and Saxe-Coburg Saxe-Eisenach. The coins were minted between 1560 and 1816.

This research gives new information about economic networks, including trade between the Americas, Europe and the Far East during the 16th, 17th and 18th centuries, the height of the great maritime empires, and more specifically, the procurement, manufacture and trade of silver as a global commodity at this time. Further, the techniques used in this study are applicable to many other items of cultural heritage significance for future analysis.

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Here is an interesting paper in the same field as that of Eeva Jonsson above: Merkel, S., Hauptmann, A., Hilberg, V. and Lehmann, R., 2015 Isotopic analysis of silver from Hedeby and some nearby hoards In Viking Worlds. Things, spaces and movement, edited by M.H. Eriksen, U. Pedersen, B. Rundberget, I. Axelsen and H. Berg,  pp. 193-210. Oxbow, Oxford. While the Jonsson study concentrated on silver content standard, this one looks at the "elemental signature" of the metal used, finding a surprising absence of silver coming from the British isles in Viking coins. This would change the nature of Viking plundering, to put it crudely, from robbery to feed the folks back home to violent taxation.

Peter

Silver played an important role both as a material of status and as a medium for exchange in the Viking Age. Hedeby was at the frontier between the monetized kingdoms of the West and the hacksilver/bullion economy of Scandinavia and the Baltic. Fueled by the influx of newly mined and recycled silver from the Middle East, Central Asia, and Europe, mints were irregularly maintained at Hedeby and across Denmark in the 9th–11th centuries. A diachronic study was undertaken to examine the flow of silver as a raw material at Hedeby from the 10th–11th centuries with the use of elemental and lead isotope analysis. Sampling of coins was done by Laser Ablation Inductively Coupled Plasma Mass Spectrometry, allowing for precise and accurate analyses with limited damage to the objects. The minting campaigns at Hedeby provide an excellent chronologic mirror to the changing sources of silver.

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Doctoral students are using the technology also. Archaeometallurgical analyses of pre-Islamic artefacts from ed-Dur (Emirate of Umm al-Qaiwain, U.A.E.) by Parsival Delrue is an extensive (600 pages) report on the analysis of finds of an expedition of the University of Ghent. Coin collectors will find Chapter 7 of most interest. Personally, I am also happy with chapters 2 - Historical, geographical and archaeological context and 3 - Trade. Technicians will like chapter 4 - Analytical methods.

The conclusions on the coins:

2. The analyses of the collection of SE-Arabian coins.

The coins are treated as a separate group. They were analysed by SEM-EDX for their chemical composition. The majority of the coins were made from silver or a copper-silver alloy. One copper coin was attested and one specific group contained small amounts of tin. True bronze coins were not attested, and it can be questioned if bronze coins were ever produced. It also shows that the intuitive determination of alloys is often wrong. The same ‘mistake’ was made with the brass artefacts and as seen in this study the correct interpretation of the material can lead to new and additional insights. The compositional data of the coins was projected on the typology of the coins and there is a certain relation between the alloys used and the type of coins. Evidence was brought forwards for a pickling process used to enrich the coin surface with silver in an artificial way. It is also suggested that the coin blanks may have been hot struck. The hypothesis that the SE-Arabian coins were made locally also implies that the coining metal may be alloyed locally. This can be true, since the alloys used in the (limited amount of) foreign coins analysed in this study, are completely different. If the production hypothesis is accepted then an additional circumstantial clue is found to address the local technological level. As stated several metallurgical technique are attested amongst the coins and although the depiction level never reached that of large monitory systems, a process of pickling does suggest a certain technological level. In any case the analysis of the coin collection is a valuable addition to the numismatic study of the coins of the region. The trace elemental and lead isotopic analyses performed on 18 coins were not evaluated in this study and are kept for a future publication. The data is however not very willing to release its secrets and can be seriously biased by recycling of the coin metal. Therefore the analytical results have to be approached with the necessary caution.

Peter

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A paper that is a few years old now (2005), when metal analysis was still a new thing. Metallurgical Analysis of Chinese Coins at the British Museum, by Helen Wang, Michael Cowell, Joe Cribb and Sheridan Bowman. If you collect Chinese cash coins, you must have this 99 page paper in your library.

The aim of this publication is to bring together the results of metallurgical analysis on Chinese coins undertaken at theBritish Museum during the last 15 years. The largest projectlooked at the metal content of Chinese cash coins over a periodof more than 2,000 years. Although the results of the survey  were summarised and published (Bowman, Cowell and Cribb, 1989), full details of the survey and photographs of the coins tested are presented here for the first time, along with an introduction by Joe Cribb and comments by Michael Cowell.

Peter

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Who Minted those Owls? by M. Ponting, H. Gitler and O. Tal is a magnificent illustration of how metal analysis can rectify informed, but false guesses.

This paper uses the analytical results from inductively-coupled plasma atomic emission spectrometry (ICP-AES) and lead isotope analysis (Q-ICP-MS) of Athenian-style tetradrachms found in excavations in Israel, in order to investigate their origins. Some of these coins have been classified as Eastern imitations based on style, but the analysis suggests that many of these coins may actually be authentic Athenian issues. This is because they were in all probability produced from bullion that came from the silver mines of Laurion in Attica. Given the stylistic variability of the Athenian(-style) tetradrachms found in Israel, we can assume that they are representative of the ‘owls’ circulating in the East in Achaemenid times.

Peter

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New Perspectives on the Roman Coinage on the Eastern Limes in the Late Republican and Roman Imperial Period by Kenneth K. A. Lönnqvist. In this book, metal analysis leads to genuinely new information.

The focus of this work has been to interpret aspects of the ancient coinages of Syria-Palestine and the coinage alloys, as well as put the results in a defined political, social and economic context in the Roman period.

One of the major contributions is the analytical study of the chemical constituents of the early provincial coinage of Judaea that brought new approaches to the interpretation of the regional coinages and coinage alloys. The author imagines much progress will be made in this field in years to come, as some of the presently on-going archaeo-metallurgical research will be published.

Trace element level profiles for the early provincial coinage of Judaea suggest that the first Roman governors may have been responsible for removing the stock of pre-Roman bronze coins from circulation and initiating a process of their liquidation in A.D. 6. The most noteworthy changes in the chemical composition of the provincial coinage of Judaea occurred under Emperor Tiberius (A.D. 15-37), which in turn has prompted ideas about changing the mandate periods of some of the Roman governors.

Furthermore, the new interpretation of the coin data from Qumran indicates that the site had an advanced monetary economy in the Roman period similar to contemporary sites in Israel. The important archaeological conclusion emerging from the study of the Qumran silver, as far as the settlement chronology is concerned, is that the burial date of most of the silver caches does not support the chronology suggested by the original excavator for the main occupational periods. The earliest possible burial date for one or more of the Qumran silver coin hoards is in the light of this study conclusively not 9/8 B.C. but ca. A.D. 52/53-66, either before or during the First Jewish Revolt (A.D. 66-70). However, the extraordinary manner of seemingly simultaneous archaeological burials in mostly unknown ceramic vessel types, in the mind of the author points to a burial date for all the three hoards possibly in the post A.D. 210 period.

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The paper LA-ICP-MS ANALYSIS OF SPANISH SILVER COINS Analysis of Coins Found in Association With Four Ships Wrecked Off the Western Australian Coast by Liesel Gentelli uses metal analysis to come up with details that are not on flan or worn away.

As we move to a situation where the metallurgical identity of coins is known, it will be possible to identify the date of more coins. This is important because coins found on an archeological site are often the best way to date other finds on that site.

Three hundred and six silver coins were provided from the collection of the Western Australian Maritime Museum, Perth, Western Australia. The coins represented four Spanish metropolitan mints and five Spanish American mints. The coins were recovered from four shipwrecks, the Vergulde Draeck, wrecked 1656, the Zuytdorp, wrecked 1712, the Rapid, wrecked 1811 and the Correio da Azia, wrecked 1816. All four ships were wrecked off the coast of Western Australia. The sample coins span the years between 1627 and 1816.

Samples were analysed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), a technique which facilitated multi-element analysis without causing significant damage to the coins themselves. Fifty seven elements were determined in triplicate in each coin, thereby providing an elemental “fingerprint” for each coin. Data were interpreted using linear discriminant analysis (LDA) which allowed the coins to be grouped into easily identifiable sub-groups. Using LA-ICP-MS plus LDA it was possible to use the trace and minor elemental composition of the coins to identify chemical signatures which were specific to the mints of origin, a discovery which has also formed a potential platform for the identification of the year in which the coins themselves were minted.

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Here is a chapter from the book Treasures of the Gupta empire by Sanjeev Kumar. The author says it better than I can:

Chapter 9 covers the metal analysis of the gold coins of the Kushanas (from the British Museum Collection), the gold & silver coins of the Gupta Kings, as well as the silver coins of the Kshatrapa kings (prior to, and upto their conquest by Kumaragupta I). The data provided herein references both the specific gravity analysis done by various scholars in the past, as well as new data based on an extensive XRF spectrographic analysis study for the gold and silver coins of the Gupta kings as well as the silver coins of the Western Kshatrapa kings.

The results shown from the analysis here, totally negate the theories put forth by past scholars and historians that starting with Skandagupta a debasement of the Gupta gold coins occured, caused invariably by invasions by three foreign powers - i.e., the Sassanians, the Kushana Śakas and the Hūṇas (Jayaswal 1934:34), a false premise, derived from wrong assumptions, leading to wrong conclusions which are then treated as 'facts' and continue to get repeated and quoted time and again by later scholars.

Peter

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While analysis is fun, data base building is a dull, but necessary investment into future research. This paper develops data for a narrow, range of coins that is so under-studied that Cayón does not cover them in his benchmark catalogue of Spanish coins.

Analysis of the metal of the coins of Ebusus and Northeastern Spain (3rd – 1st c. B.C.E.)

by Alejandro G. Sinner, Giacomo Pardini, Anna Candida Felici, 2017, XV International Numismatic Congress Taormina 2015 Proceedings

During the late 1980s, X-ray Fluorescence (XRF) spectroscopy played an important role in developing our understanding of the composition of ancient bronze coins in the Iberian Peninsula, but the studies involved addressed only individual mints and issues and did not aim at developing a wider picture. Between 1995 and 1999, Abascal and Ripollès, building on previous work, undertook a series of metallographic analyses, also using XRF spectroscopy, published in four papers . Their goal was to produce an initial databank of information on the composition of coinages minted in the Iberian Peninsula during the Republican period and the early Empire, and map this. This remains the only systematic treatment of this material so far completed, though some further analyses were undertaken by other scholars.

The aim of this paper is to add yet more data to the existing databank of metallographic analysis of Iberian coinages, and present the results of an analysis and comparison of the composition of the bronze coins of the Punic mint of Ebusus and the Iberian mints of NE Spain, during the 3rd-1st c. B.C.E.: 339 coins were analyzed using XRF spectroscopy. In the case of Ebusus, the sample was large enough to make possible a diachronic study of the metal composition during the life of this mint.

Peter

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Metal analysis has found its way to Bangladesh, which its rich history. Mohammad Abu Al Hasan, Researcher in the department of Archaeology of Jahangirnagar University, Savar, Dhaka wrote Archaeometallurgy of the Coins of Bengal Sultans: An Unexplored Way to the Study of Political, Social and Economic Life of Mediaeval Bengal saying:

Archaeometallurgy has become a significant branch of Archaeology. Coins are the most important and frequently found artifacts of the sultani period of Bengal. The political history, chronology and geographical area of the kingdom of Bengal sultans have been exposed by the direct help of coin. But no archaeometallurgical analysis has been ever done before on any sultani coin of Bengal. Metal content of sultani coin can tell us much information such as economic, political and social history of that period by proper and in depth archaeometallurgical analysis. As we know nothing about the metallurgy of the coins of Bengal sultans, it can be an adventurous and exciting step to research on the unknown facts of sultani period of Bengal. The 6-page article is an enthusiastic introduction to the promise of metal analysis but does not describe an actual study.

Peter