Evidence for the widespread use of dry silver ore in the Early Islamic period and its implications for the history of silver metallurgy

Abstract

Over much of its history, silver production has been tied to lead metallurgy. For the early Middle Ages in particular, it is widely accepted that galena, a lead sulfide, was the dominant source of silver. Since galena can be associated with silver in the range of 0.1–0.5%, rarely more, this implies that tons of lead must have been laboriously processed to extract kilograms of silver. While all extant physical evidence from mines, slag and the metal itself point to this being true, this study has found evidence that extremely rich silver ores must have played a key role in one of the major silver-using polities in the 8th and 9th centuries AD: the Early Islamic Caliphate. Metallography of 26 coins revealed that matte inclusions (silver-copper sulfides) are widely found in Umayyad and Abbasid dirhams, renowned for their exceptionally pure silver. Since matte preserved in the coins could not survive the strongly oxidising refining process required to separate lead from silver, this silver cannot have been produced solely from lead ore or through the use of lead. A new paradigm for the understanding of early medieval extractive metallurgy is required. ‘Dry’ silver ore consisting of nearly pure silver minerals were processed without lead and made a vital contribution to the Early Islamic silver supply. The results of this study have major technological and economic implications and overturn long-standing views on the history of silver metallurgy. They also have important consequences for provenance studies and the interpretation of elemental and lead isotope data.