Issue 74
The Importance of Silver Standards in Archaeology
Using certified silver standards produced by the LBMA, researchers at the Goethe University have undertaken analysis of the composition of silver coin dating back to 500 BC in order to understand the origins of the silver bullion used to mint the coinage and also the changes caused by key historical events over the last 2,000 years or more.
A Spartan silver Didrachm (7.85 g) dating to circa 380-340 BC from the Greek colony Taras, which became the ancient Roman city of Tarentum in Apulia (southern Italy). Scale in centimetres.
A comprehensive investigation of ancient silver coinage is underway at the Goethe University in Frankfurt. As part of the interdisciplinary research, some 200 silver coins from across the Western Mediterranean are being analysed for their chemical composition and lead isotope signatures. The aim is to discover the origins of the silver bullion used to mint these coins and to monitor the continuity or changes caused by key historical developments during the period 500- 100 BC. One of the main hurdles encountered, however, was finding appropriate silver reference materials for the purpose of analysis and to assess the reliability of the results. Only very recently were two certified silver standards produced by the LBMA that can be used in archaeological science. These standards play a key role in our research project.
Coinage and the dynamics of power
The research project, called ‘Coinage and the dynamics of power: the Western Mediterranean 500-100 BC’, is funded under the Lichtenberg scheme by the Volkswagen foundation. It is based at the Goethe University, Frankfurt am Main (Germany) and involves two PhD projects, two post-doctoral projects and a principal investigator.
In 500 BC, some Greek cities in the Western Mediterranean had just started to mint coins, mainly of silver, with widely varying weights, sizes and designs. By 100 BC, the entire West Mediterranean had been incorporated into the unified currency system of the Roman Empire, for which coins were issued on a fixed standard.
These coins circulated widely and were used in everyday transactions.
The project focuses on the intervening process. Based on an interdisciplinary approach, the aim is to find out how coinage was adapted and used by the various peoples inhabiting the Western Mediterranean. The working theory is that the ability to mint coins, and to use them for various purposes, was greatly influenced by the constantly changing balance of power in the area. In the struggle over hegemony in the region, coinage was an important instrument to finance the various bids for power, to display developing identities and shifting loyalties, and to facilitate control over subjugated areas.
There is a close relationship between the production of coins and the control over resources – there are no coins without metals. Access to resources was continuously changing throughout the period under discussion. Mining was obviously an important source for metal, yet metal could also be obtained by melting down other objects or foreign coins that could be gained through trade networks or as booty or tribute. In order to understand how coinage affected the discourse of power in the Western Mediterranean and vice versa, the project aims to trace the sources available to the different peoples in the area and how this changed over time.
The variation in the composition and isotopic signature of the coins can be related to that of geological sources, namely ore deposits. This information can be used to establish, or refute, an ore source (provenance) for the silver metal used to mint the coins. To this end, we are sampling and analysing a comprehensive and representative selection of silver coins issued between c. 500 and 100 BC in the Western Mediterranean. This is an entirely new approach.
“There is a close relationship between the production of coins and the control over resources – there are no coins without metals.”
Figure 1: A Bronze Aes (coin denomination) being sampled from the Poseidonia mint, a Greek colony in southern Italy (circa 350-290 BC)
To drill or not to drill?
Destructive analysis of some of the first coinages is a sensitive issue, where conservation has to be reconciled with scientific value and perceived improvements in scientific practice. The 200 silver coins selected for analysis in this study are to be spread across multiple universities and museums to avoid overburdening any single collection. The project has just recently completed the sampling and analysis of the first 40 coins, all from the Tübingen University collection. Three drillings are made into the rim of each coin, using a 0.5-0.8 mm diameter drill-bit and a hand-held rotary tool, collecting the turnings (around 30 mg or more) for analysis (though omitting the initial surface). The analysis of the unadulterated ‘heart’ metal is preferred, as the surface of the coin can be corroded and artificially enriched in some elements. Surface analysis techniques may be non-destructive (or micro destructive) but are not necessarily representative of the whole coin composition – results can be misleading.
Lead isotope provenancing
A small portion of the collected turnings are dissolved in acids and prepared for lead isotope analysis, a technique used to determine the origins of a metal. The ratios between different lead isotopes help identify particular geological regions. This allows for a lead isotope signature of a coin to be matched to a geological source (provenancing). Lead isotope provenancing relies on having a reference collection of known signatures. Thanks to a great body of work built up by previous scholars, there is an extensive collection of lead isotope analyses of metal-bearing ores across Europe and in the Mediterranean especially (for example, in Greece, the Aegean Islands, Italy and Spain). A few islands and regions within the Mediterranean, however, have overlapping lead isotope compositions that cannot easily be distinguished from one another. Fortunately, the composition of the silver itself, namely the trace elements, can be used to help separate these isotopically similar deposits. One problem with lead isotope provenancing is the issue of mixing that results from the recycling of coins for new issues, or from melting differently sourced bullion. In these cases, no single source can be found because it does not exist; however, multiple samples may infer the different sources that were used.
Figure 2: An X-radiograph of a Euro cent coin after test drilling showing the variation in hole sizes made using different drill bits (0.5-0.8mm diameter). The diameter of a Euro cent is 16.25 mm, and the thickness 1.67 mm.
The chemical composition of silver
The remainder of each sample is placed in epoxy resin and flatly polished as a metallographic block, a common form of sample preparation in materials science. This allows each sample to be examined under a microscope, or even to be subjected to further analysis by various beam techniques. Our project utilises two complementary beam techniques to determine the full chemical composition of each coin. The first beam technique (electron probe microanalyser) is a routine method for determining the bulk composition of the coin in terms of atomic weight percent. This makes it possible to determine the purity of the silver (i.e. 99.84 %), as well as quantify any other minor (or alloying) elements, such as copper or lead. The bulk composition alone may be sufficient in identifying debased coinage, perhaps relating to times of economic stress.
The second analytical technique (laser ablation inductively coupled plasma mass spectrometry) is then used to accurately quantify the trace element composition of the metal (everything amounting to less than 0.1%). The first technique is capable of detecting down to 100 parts per million (0.01%), whereas the second technique can detect much smaller amounts, for example, less than 1 part per million (0.0001%). For those coins with indistinct lead isotope signatures that relate to more than one ore deposit, the trace element composition can be exceptionally useful in further refining the provenance of a coin to one source in particular. The trace element composition can also help characterise coinage produced from recycled metal, providing a chemical ‘identity’ for that group, even though it may be impossible to provenance the group accurately.
Certified reference materials
In order to obtain reliable results from analytical instruments, it is necessary to use control materials, often referred to as ‘standards’. These are used to calibrate analytical instruments, as well as test their precision and accuracy. By analysing a standard, it is possible to see how the actual results of an instrument compare to the ‘real’ values reported for the standard, giving a measure of the instrument’s accuracy. The ability of the instrument to repeat such accurate measurements provides an indication of its precision. The accuracy and precision tests, therefore, help provide a measure of the quality and reliability of the results being produced. The process of accuracy and precision testing, however, depends heavily on the reliability of the standards being used in the first place.
Standards come in two forms. A ‘reference material’ (RM) is often produced and analysed by a single organisation, which may or may not be an accredited National Calibration Laboratory. A ‘certified reference material’(CRM) is one that has been analysed by multiple independent accredited laboratories (usually six or more) and issued with a peer reviewed certificate of the composition. The degree of confidence, or certainty, of the composition directly relates to the number of independent analyses, which is why CRMs are preferred, even though they are less abundant than RMs.
Finding appropriate silver standards containing trace elements has long been a challenge for archaeological science. Although there are RMs available, the production of CRMs was only recently met by the LBMA, which produced two silver standards (AgRM1 and AgRM2). These two pure silver standards (99.95%) contain a comprehensive set of 21 trace elements (mostly transition metals). The confidence of the certified values is impressive, with an overall average (median) error of around 10% in trace elements (often less than 50 parts per million) for each silver standard.
Comparability of results
One of the principle aims of our analytical research is to make the results comparable with future datasets. Aside from preserving the integrity of the data, the aim is to minimise further sampling of the same coins in future studies. The use of the LBMA standards for the analysis of silver coins promotes the comparability of datasets produced by different research institutions (using the same standards), so that errors can be quantified and directly compared. This is a pertinent issue for coin studies, where thousands of coin compositions have been published, though many of the datasets are largely incomparable. This is not solely due to the adoption of different analytical techniques, but due to the use of different RMs.
The future of coin studies and reference materials
In recent years, a huge effort has been made to produce and promote RMs that can be useful for researching materials in archaeology and aiding the comparison of datasets. Many RMs and CRMs exist for various copper alloys (as well as pure copper) containing trace elements, but silver alloys are an outstanding issue. The LBMA CRMs are essential and set a benchmark in quality, as well as being a practical asset to archaeological research. There are coin series, however, that are not pure silver, which have been deliberately debased for reasons owing to the socio-political and economic climate. Antique counterfeits were also made using silver alloys (with concealed cores). To remedy the absence of silver-copper alloy standards, six research institutions (led by the Goethe University) have commissioned a new set of silver alloy standards that will be used to study silver coins primarily debased with copper (and also containing appreciable amounts of lead and gold). The co-operation between universities and museums engaged in archaeological research has been paramount to establishing the manufacture of relevant standards. The efforts made by the LBMA to produce a set of silver CRMs have been appreciated not only by the metal industry, but by archaeologists. It is hoped that such efforts will stimulate further preparation of more silver-based RMs to be used in archaeological research for studying ancient silver artefacts and coins.
Dr Thomas Birch
Thomas Birch is a postdoctoral researcher at the Goethe University, Frankfurt am Main. Having received his MSc in the Technology and Analysis of Archaeological Materials from the Institute of Archaeology (University College London), he went on to specialise in iron provenancing of weaponry from Iron Age Scandinavia (University of Aberdeen). Upon finishing his doctoral degree he accepted a postdoctoral position at the Goethe University in 2013, where he is the principle archaeometric investigator researching the provenance of bullion for antique coinage.
Prof. Dr Fleur Kemmers
Fleur Kemmers is Lichtenberg-professor for Coinage and Money in the Graeco-Roman World at the Goethe University, Frankfurt am Main.
Having received her MA in European Archaeology from the University of Amsterdam, she specialised in numismatics during her PhD research on roman coin finds from a major roman legionary fortress in the Netherlands at the Radboud University Nijmegen. Upon finishing her doctoral degree in 2005 she continued as postdoctoral research at the same university, simultaneously running her own archaeological consultancy business. In 2010 she was granted the Lichtenberg-professorship in Frankfurt am Main.