Abstract : During the analysis of ancient silver coins, compositional results will vary depending on the depth of the analysis taken. The variable composition of the coins mainly occurs as a result of depositional conditions. Therefore a reliable analysis of these artefacts requires the removal of the silver-enriched surface layer to reveal the unaffected alloy. The alloy can be characterised by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which generates concentration profiles to estimate the evolution of the composition throughout the depth of the coin. By this way, the silver-enriched surface layer can be removed to analyse the unaffected alloy for the major elements present. Minor and trace elements also benefit from the main features of LA-ICP-MS: the large number of elements that can be measured and the low detection limits provide a complete characterisation of these alloys. Complementary analytical methods have been used to validate the LA-ICP-MS protocol developed.
The silver coins minted by Carolingian rulers are the first to have benefited from the improvments provided by the application of LA-ICP-MS to these objects. The evolution of the fineness of coins minted from 751 to 864 AD has been studied and scrutinized regarding the numismatic and historical data. Some geographical disparities regarding the silver content of the alloys have appeared. The study of the concentrations of minor and trace elements present within the coins has revealed differences in the fingerprint of the silver used for coin striking in specific mints, notably at Venice.