Large scale smelting of speiss and arsenical copper at Early Bronze Age Arisman, Iran

We report analyses of different slag types found at the Early Bronze Age site of Arisman in North-West Iran. Here, an estimated 20 tons of slag provide evidence of sustained and large-scale production of metal, using both furnaces and crucibles. The results show that both speiss, an iron-arsenic alloy, and arsenical copper were produced, apparently side-by-side at the same site but using different ores and processes. We offer a tentative outline of the metallurgical processes involved in the production of these arsenic alloys and an explanation why speiss needed to be produced from arsenopyrite in a separate step, instead of using the mineral in its native form as a source for arsenic. We suggest that the speiss was then added either to secondary copper ore or to separately smelted copper metal, in order finally to produce arsenical copper in a regular and well-controlled process.

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► Demonstrated evidence for smelting of iron arsenide (speiss) during EBA.
► Argue that speiss was added to copper ore in order to smelt arsenical copper.
► Explain how iron and arsenic behave differently during arsenical copper production.
► Speiss possibly long-distance trade item for arsenical copper production elsewhere.