Considerations on Copper Smelting from Fahlores and the Metallurgy of Cu-As Bronzes

Abstract

The copper production in the Alps began three thousand years BC, usually near ore deposits in Austria and Switzerland. For copper smelting, sulphidic ores like chalcopyrite and fahlores were used. Copper produced from fahlores was widely used in the Alps resulting in As and Sb contents in the metallic copper. Such copper alloys were generally referred to as arsenical bronzes. However, in ancient bronze objects, a wide range of arsenic content was observed. One question relates to how much arsenic is transferred into the bronze alloy during smelting. By thermodynamic equilibrium calculations, the roasting and smelting processes are simulated and show that As reacts already to gaseous As compounds before Cu2S is attacked and metallic Cu is formed. In case of Sb liquid, Sb2O3 is formed quickly during roasting and is finally enriched in the slag.These results have been confirmed by the investigation of an ancient copper ingot containing 4 wt. % As and 2.5 wt. % S as well as smaller quantities of Sb, Ni, Fe, and Ag. The main phases apart from metallic copper are Cu3As, Cu2S, and Sb oxide. In a slag sample, an inclusion was characterized containing chalcopyrite, FeO, and Sb oxide This result is in accordance with the thermodynamic calculations where Sb2O3 was obtained.Another interesting point is the As loss of arsenical bronzes during remelting. Thermodynamic equilibrium calculations reveal that Cu3As is very stable and during remelting no evaporation of As is observed. Moreover, during oxidation of metallic Cu-As, the formation of Cu2O is favoured from Cu and not from Cu3As. Consequently during melting of Cu-As for casting, at first Cu2O is formed resulting in an As enrichment in the melt and in the casted object, respectively. These effects are superimposed and, if recycled Cu alloys are used, the starting concentration of As is unknown.