Smelting Experiments with Chalcopyrite Ore based on Evidence from the Eastern Alps

Abstract

In order to assess the fractionation of copper isotopes during smelting under reconstructed conditions, smelting experiments with chalcopyrite ore were conducted in built furnaces based on archaeometallurgical evidence from the Bronze Age Eastern Alps and ethnographic examples from Nepal. Two experimental series, S2 and S4 were chosen for analysis. Each series consisted of a number of roasting and smelting experiments with different experimental parameters, and both series yielded metallic copper. Each type of experiment, their outcomes, and observations made during them are described in detail to facilitate future experimental work. Both series differ significantly in their outcome. XRD analyses and chemical analyses were carried out to reveal the reasons for the observed differences.  The chemistry of the obtained matte shows that roasting is pivotal for a successful smelting process and that two cycles of matte roasting and subsequent smelting can be sufficient to remove most of the sulphur and iron from the matte. Furthermore, different conditions in the shaft furnaces resulted in a more efficient oxidation of iron in series S4. During the subsequent smelting of the matte in the pit furnace, it was possible to extract larger amounts of metallic copper and sponge copper, as well as to produce a thin well-melted plate-like slag. The pit furnace did not always show clear traces of metallurgical activity and thus might not be identifiable in the archaeological records without chemical analysis of the pit lining and surrounding soil. Although more trials are needed to replicate the process, these experiments give a strong hint towards the reconstruction of the matte smelting process in the Bronze Age alpine area.