Platinum-group elements and gold in silver coinage and the issue of salt cementation

Abstract

It has been proposed that gold purification by cementation could account for the low gold content of ancient Greek coinage from Attica and the Cyclades. In order to place new constraints on this suggestion, the concentrations of platinum-group elements (PGEs) and gold have been measured in 72 silver coins mostly from the Greek Archaic and Classical periods, but also from Rome, India, medieval Europe, and colonial Spanish Americas, by inductively coupled plasma mass spectrometry. A novel technique allowing these concentrations to be determined in silver coins is described. Variations are consistent with element position in the periodic table. The volatile elements Rh and Os are commonly at or below the detection level, which may reflect evaporation during smelting and cupellation. Ruthenium and Ir, which binary phase equilibrium experiments show to be insoluble in solid silver and gold, and soluble Pd and Pt, show variations in coinage consistent with these properties. The dichotomy of Ir/Au ratios is not consistent with Ir loss in gold during salt cementation (parting) and is better explained by the contrast between Au-rich and Au-poor ore districts. This contrast is suggested to reflect either regional differences or the variability of conditions during ore genesis, such as hydrothermal solution chlorinity.