X-ray Fluorescence analytical criteria to assess the fineness of ancient silver coins: Application on Ptolemaic coinage

The application of X-ray Fluorescence (XRF) analysis in a non-invasive manner on ancient silver coins may not provide reliable bulk compositional data due to possible presence of a surface, silver enriched layer. The present work proposes a set of three complementary analytical methodologies to assess and improve the reliability of XRF data in such cases: a) comparison of XRF data on original and cleaned micro-spots on coin surface, b) Ag K/L ratio test and c) comparison of experimental and theoretically simulated intensities of the Rayleigh characteristic radiation emitted from the anode. The proposed methodology was applied on 82 silver coins from the collection of Ioannes Demetriou, donated to the Numismatic Museum of Athens in the 1890s. The coins originate from different mints and are attributed to the first five Ptolemaic kings' reign (321–180 B.C.). They were analyzed in-situ by using a milli-probe XRF spectrometer. The presence of an Ag-enriched layer was excluded for the majority of them. The silver fineness was found to be high, with very low concentrations of copper and lead. The composition data provide important information about possible sources of silver during the Ptolemaic period and indications of a gradual coinage debasement after 270 B.C. due to economic or technical reasons.