OXALID

Interpretation of lead isotope data

The identification of ore deposits that were used for production of metals and pigments in antiquity is based on a direct comparison, sample by sample, of available lead isotope data for appropriate minerals from different mines and the data obtained on samples of ancient artefacts.

 The archaeometallurgical surveys of ancient mines in Europe, Turkey and the Near East have been a central topic of research by archaeologists and archaeometallurgists for more than 30 years. Surveys of lead, silver and copper depositsin Greece, Turkey, Serbia, Bulgaria, Iran, Jordan, Israel, Switzerland, Germany, Austria, Britain, Spain, Italy and Southern France brought much information on this topic. Also, to a certain extent the published results of research in geochronology of ore deposits can supply lead isotope data relevant to archaeometallurgical provenance studies. The lead isotope data for ores from the Western Mediterranean has recently been much enlarged by researchers from Spain, Sardinia and France. Ideally the web-based lead isotope database should be extended by adding all these published data to the data obtained in the Isotrace Laboratory at Oxford to provide a complete tool for researchers.

The interpretation of the LI data obtained for archaeological artifacts has evolved over the years from comparing the lead isotope ratios of different samples of ores and artefacts by simply plotting them on two-dimensional diagrams and is currently done by using a three-stage procedure:

·         Firstly, the Euclidean distances in the three-dimensional space with axes defined by the three LI ratios are calculated between each of the artifact’s LI ratios and all currently available LI data points for ore and slag samples. Software called TestEuclid sorts out the data in the order of increasing Euclidean distances. The LI ratios of the artefact and an ore sample are regarded as identical if all three ratios for both are within the analytical error for the each of the three LI ratios.

·         Secondly, the geochemical, geographical and historical (archaeological) information is considered.

·         Finally, the data points are compared in two-dimensional graphical plots of LI ratios of the artefacts and ores selected in the previous two steps.

Usually at the end of these procedures all but one or two ore sources can be eliminated.

The lead isotope plot below gives an example of the interpretation of data obtained on some 5th c BC Greek silver coins from Athens and Thasos. The ores from Chalkidiki and Thasos have indistinguishable lead isotope compositions, so the dating of silver extraction in these two locations in the North Aegean is crucial for determining the origin of the silver used to make the Thasian staters, as are other factors discussed by Gale et al. 1988. In contrast the use of silver from Lavrion to make the Athenian ‘Owl’ tetradrachms is proved unequivocally by the lead isotope analyses.   

References:

Gale, N.H.,  W. Gentner, W. and Wagner, G.A. 1980. Mineralogical and Geographical Silver Sources of Archaic Greek Coinage. In  D.M. Metcalf, (ed.)  Roy. Num. Soc. Special Publication No.13, Metallurgy in Numismatics I,  London, 3-50.

Gale, N.H., Picard, O. and Barrandon, N. 1988. The archaic Thasian silver coinage. In G.A. Wagner and G. Weisgerber eds. Antike Edel- und Buntmetallgewinnung auf Thasos. Der Anschnitt Beiheft 6, Deutschen Bergbau-Museums, Bochum. 212-223.

Using the database

At this stage the OXALID database does not provide the  tools for interpretation of lead isotope data. The data published here is limited to the sets of lead isotope data in the digital format which can be copied from the database into an Excel sheet and then can be used for graphic or numerical comparisons.

• Go to the page 'The database'
• Click on the required geographical area or group
• Select the sub-group and open the file
• Copy and paste