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Characterisation of silver surface enrichment in historical silver coins — a method proposal and study of the trace element behaviour

Lecture by PhD Student Mahir Hrnjic (Aarhus University).

Info about event

Time

Tuesday 9 April 2019,  at 16:00 - 17:00

Location

Centre for Urban Network Evolutions (UrbNet), Aarhus University, Moesgård Allé 20, 8270 Højbjerg, Denmark (Building 4230-232).

Abstract

The depletion of Cu from near-surface areas is a common issue in the non-destructive analysis of historical silver coins. Such coins will exhibit higher content of Ag near a surface of a coin, which might lead to the misinterpretation of a coin’s original composition. The study evaluates previously reported non-destructive method for characterising Ag enriched surfaces on coins based on measuring and comparing the intensity ratios of Ag K?/Ag L? peaks between coins and Ag standards with a similar composition. A number of silver coins with different chronologies and origins were analysed with ?XRF in order to measure Ag K?/Ag L? intensity ratios and subsequently compared with the peak ratios of Ag alloy standards with a different composition. In cases where silver surface enrichment was confirmed, coins were sampled and analysed with LA-ICP-MS to measure the content of trace elements in the core and at the surface of coins. The analysis confirmed that the presence of silver surface enrichment was not detected with all Ag silver standards and that multi-standard approach allows for more reliable identification. The results from LA-ICP-MS show that besides Ag and Cu, the silver surface enrichment also influences behaviour of minor and trace elements — Au, Co, Ni, As, Rh, Pt. The content of Au was significantly higher in the enriched areas comparing to the core of coins; while Co, Ni, As, Rh, Pt were depleted from the surface. The study suggests the application of multi-standard approach in identifying the silver surface enrichment in fineness and provenance studies of historical silver coins.