Although arsenical bronze occurs in the archaeological record across the globe, the earliest artifacts so far known, dating from the 5th millennium BC, have been found on the Iranian plateau. Arsenic is present in a number of copper-containing ores (see table at right, adapted from Lechtman & Klein, 1999), and therefore some contamination of the copper with arsenic would be unavoidable. However, it is still not entirely clear to what extent arsenic was deliberately added to copper and to what extent its use arose simply from its presence in copper ores that were then treated by smelting to produce the metal.
Reconstructing a possible sequence of events in prehistory involves considering the structure of copper ore deposits, which are mostly sulphides. The surface minerals would contain some native copper and oxidized minerals, but much of the copper and other minerals would have been washed further into the ore body, forming a secondary enrichment zone. This includes many minerals such as tennantite, with their arsenic, copper and iron. Thus, the surface deposits would have been used first; with some work, deeper sulphidic ores would have been uncovered and worked, and it would have been discovered that the material from this level had better properties.
Using these various ores, there are four possible methods that may have been used to produce arsenical bronze alloys. These are:
* The direct addition of arsenic-bearing metals or ores such as realgar to molten copper. This method, although possible, lacks evidence.
* The reduction of antimony-bearing copper arsenates or fahlore to produce an alloy high in arsenic and antimony. This is entirely practicable.
* The reduction of roasted copper sulfarsenides such as tennantite and enargite. This method would result in the production of toxic fumes of arsenous oxide and the loss of much of the arsenic present in the ores.
* The co-smelting of oxidic and sulphidic ores such as malachite and arsenopyrite together. This method has been demonstrated to work well, with little in the way of dangerous fumes given off during it, because of the reactions together among the different minerals.
Furthermore, greater sophistication of metal workers is suggested by Thornton et al. They suggest that iron arsenide was deliberately produced as part of the copper-smelting process, to be traded and used to make arsenical bronze elsewhere by addition to molten copper.
Artifacts made of arsenical bronze cover the complete spectrum of metal objects, from axes to ornaments. The method of manufacture involved heating the metal in crucibles, and casting it into moulds made of stone or clay. After solidifying, it would be polished or, in the case of axes and other tools, work-hardened by beating the working edge with a hammer, thinning out the metal and increasing its strength. Finished objects could also be engraved or decorated as appropriate.