The analysis of antique gold coins by mobile laser ablation and ICP-MS

An investigation on the Kosōn staters from Transylvania

1. Objectives

The project aims at the metal composition of the so-called Kosōn-Stater, an ancient gold coin known to the European scholarly world for 500 years (first mentioned by Erasmus of Rotterdam). Despite intensive efforts, this coin type still puzzles researchers. Even basic questions such as who minted the coins and where the mints were actually located remain unsolved. This is an unique phenomena given that these coins have been minted in relatively large quantities for centuries. The only chance for a breakthrough in the historical contextualization of this enigmatic category of objects is offered by minimally invasive trace element analysis for material classification and, if possible, for determining the origin of the gold. Within the framework of this project, these investigations will be carried out using the innovative mobile laser ablation method in combination with inductively coupled mass spectrometry (pLA-ICP-MS).

Participants: Moritz Numrich, Ernst Pernicka

2. State of research and research question

On the obverse of the Kosōn stater (AV; approx. 8.5 g; 17-20 mm) a consul walking to the left, is displayed; he is accompanied by two lictors. Furthermore, the Greek legend KOΣΩN is to be found on the obverse. On the reverse, an eagle is depicted. About two thirds of the known specimens have a monogram, which appears in two forms. In addition, there are Kosōn types in silver, which were partly made from the same dies as the stater.

Traditionally, the Kosōn coins are associated with the events of the Roman civil war in the wake of the assassination of Caesar (44 BC) and are interpreted as minted on behalf of Brutus for an allied Dacian king named Kosōn. However, this approach is problematic; the most recent contribution to the topic develops a new chronology and contextualization. The task of this project is to examine these approaches as well as a possible existing production context with the posthumous Lysimachos staters found in the same archaeological context by means of scientific analyses, to specify them more precisely and, if necessary, to develop them further.

The analytical investigations of Kosōn coins to date have been based on the X-ray fluorescence method (XRF), which allows the determination of the elemental composition of the coins with regard to the major and only some minor elements. This method works completely non-destructive, however, much less sensitive. Therefore, such analyses are usually not sufficient to answer questions regarding the origin and authenticity of objects. In addition, laser ablation can examine significantly deeper layers of the material (approx. 100-200 µm) compared with the XRF method, as the X-rays penetrate only few microns into the material. This means, that chemical alterations of the surface, e.g. due to storage in soil which lead to a depletion of the surface in copper and silver, influence the pLA-ICP-MS analysis result to a lesser extent. The significance of these analyses for the research questions mentioned here is partly low and partly problematic. There are also studies on individual coins using proton activation analysis (PAA).

3. Method

The systematic analysis of archaeological gold artefacts, including some trace elements, began with Axel Hartmann. However, enormous progress in analysis technology makes it meanwhile possible to analyze a much larger spectrum of trace elements with much better detection limits. The method of choice for about three decades has been mass spectrometry with plasma excitation (ICP-MS). Later, coupling with lasers made it possible to perform almost non-destructive analysis. This combined method is now a standard method of analysis in ancient numismatics. A major disadvantage, however, is the necessity to either bring the object to the laboratory or to take a (very) small sample.

Because of theses limitations, a mobile laser system for sampling gold objects was developed within the framework of an Austrian Academy of Sciences project (Innovation Fund). This innovative method allows for the first time a minimally invasive sampling (i.e. the sampled position will not be visible to the naked eye) wherever the object is stored and a subsequent analysis of the collected material as well as of international reference materials for comparability in the laboratory with ICP-MS. This represents a major advancement to other mobile methods such as XRF. The application of this method is expected to lead to significant new insights into the Kosōn staters.

https://altegeschichte.univie.ac.at/forschung/projekte/die-analyse-antiker-goldmuenzen-durch-mobile-laserablation-und-icp-ms-eine-untersuchung-zu-den-kosoon-stateren-aus-siebenbuergen/