Copper Production from Polymetallic Sulphide Ores in the Northeastern Balkan Eneolithic Culture

There is a general view in the archaeological literature that Eneolithic metallurgy in general, and that of the brilliant Eneolithic cultures in the Balkans in particular, was based solely on the smelting of oxide ores. The beginning of sulphide ore processing is usually attributed to the Bronze Age. To test this view a survey of Eneolithic metal and crucibles with smelting residues from the Gumelnitsa and Varna Cultures of Bulgaria was made. These cultures can be dated from the second half of the 5th to the beginning of the 4th millennium bc (cal.). A group of 30 finds with a high content of copper and iron sulphides was the subject of a detailed study using metallography and X-ray microanalysis. The objects included ornaments and small tools from settlements at Dolnoslav, Hotnica, Karanovo, and Ruse, and from the Durankulak cemetery. The sulphide content varied from 0·45 to 5% by volume and the copper plus iron to sulphur ratio for these inclusions was found to be 5:3; this figure is similar to that for “bornite matte” from modern sulphide smelting. The use of sulphides as part of the initial charge can also be confirmed by the presence of lead sulphides and wustite. It was possible to determine the nature of the process by analysis of the solid layers of residue on the walls of the crucibles found at Dolnoslav and Chatalka (Gumelnitsa Culture). According to the mineralogical, petrological, metallographic and microprobe analyses the layers contain chalcocite and malachite, together with zinc and lead sulphide inclusions. In several places fayalitic slag inclusions were found with copper prills surrounded by chalcocite. These residues should be considered as the products of polymetallic sulphide ore smelting, possibly mixed with malachite. The ore was roasted and then smelted in reducing conditions at 1100 to 1200°C. It is also possible that Eneolithic metallurgists used a single stage process for copper production, consisting of the simultaneous burning of sulphides, ignition and reduction of oxides.