Sampling and combined Pb and Ag isotopic analysis of ancient silver coins and ores

Abstract

The combination of Pb and Ag isotopes in silver coins, artefacts, and ores greatly helps identifying ancient metal sources and detecting alteration such as lead addition, ore mixing, and metal recycling. The present study proposes a new set of procedures suitable for high-precision analysis of lead and silver isotopes in Ag-bearing ores and ancient silver coins. The protocols for combined Pb and Ag extraction and purification are based on multi-step ion-exchange and reversed-phase partition chromatography. In addition to conventional drilling, which irreversibly damages the silver object being sampled, a new method was developed to collect metal from silver coins by etching their edges with a solution of hydrogen peroxide and ammonia (H2O2 + NH4OH), either pure or with added dispersed silicon carbide (SiC) to increase etching efficiency, a technique that only damages the coin patina. This technique can also be applied to silver artefacts other than coins with minor modifications to adapt to different object shapes. Multi-collection inductively-coupled plasma mass spectrometry (MC-ICP-MS) was used to measure the isotopic compositions of both Pb and Ag in ores and coins. Variations in Pb and Ag isotopic compositions are uncorrelated for all the material analyzed, whether ores or silver coins, which demonstrate that isotope variability results from physically different processes. Evidence of mixing between different components is observed for ores, while etched and drilled coins show a conspicuous range of Pb and Ag isotopic compositions. The latter is interpreted as resulting from mass-dependent fractionation during surficial alteration under oxidizing burial conditions, and from patina formation. Silver in patina is isotopically light due to the formation of Ag2S, while the layer below the patina is isotopically heavy. Preliminary patina removal followed by H2O2 + NH4OH etching with suspended SiC may be used as an alternative to coin drilling when the latter is not possible such as for rare and precious coins. The present techniques will further help distinguish primary Pb isotopic variability in artefacts and coins from secondary variations due to underground interaction with the water table, weathering from prolonged burial, and curatorial handling.