EMF - Abstracts

Silver plating in late Roman coinage: Implications of new evidence.

Constantina Vlachou et al. (Bradford University)

Roman Coinage suffered from severe debasement during the 3rd century AD. By 213 AD the fineness of the Roman silver coin had been reduced to 50% Ag and by 250 AD to just 5% Ag. By that time, the production of complex copper alloy (Cu-Sn-Pb-Ag) coins with a silvered surface, became common practice. One of the most characteristic examples was a new coin, the nummus, introduced by Diocletian, in his monetary reform (AD 293). Previous analyses of this coin did not solve key technological issues and in particular, the silvering process (Cope 1970; La Niece 1993; Anheuser 1996).
The British Museum kindly allowed further research at Bradford to examine coins from Cope's Archive in more detail utilizing XRF, SEM-EDS metallography and LA-ICP-MS. Metallographic and SEM examination of 59 coins, revealed that all the metallographic sections in unetched condition had a main metallographic structure which consisted of the Cu/Sn alloy whereas, Ag and Pb were segregated in the grain boundaries. The silver layer survived in 22 coins. It was very difficult to trace because its thickness was approximately a few microns and in some cases it was present under the corrosion layer (Vlachou, Janaway et al. 2000).
Results derived from the LA-ICP-MS research have demonstrated, for the first time, the presence of Hg in the surface layers of these coins. The optimazation of the method and the effects of the differing matrices densities, and volatility, the high density of the coin metallic core, the surface layer enriched in Hg (volatile) and low density corroded surfaces in the analyses results, are discussed.
Keywords: late Roman coinage, copper-tin-lead-silver alloys, silvering, metallography, SEM-EDS, LA-ICP-MS
Bibliography
Anheuser, K. (1996). An investigation of amalgam gilding and silvering on metalwork. Ph.D. Thesis. Oxford University.
Cope, L. H. (1970). Surface-silvered ancient coins.In (E.T. Hall & D.M. Metcalf, Eds) Methods of chemical and metallurgical investigation of ancient coinage. London: Royal Numismatic Society, pp.261-278.
La Niece, S. (1993). Silvering. In (S.La Niece & P. Craddock, Eds.) Metal plating and patination. Oxford: Butterworth-Heinemann, pp. 201-210.
Vlachou, C., Janaway, R. C. et al. (2000). The metallurgical investigation of the tetrarchic nummi. Archaeometry 2000. Mexico City (in press).