The Smelting of Sulfide Ores of Copper in Preconquest Peru

Autor: Dudley T. Easby, Earle R. Caley
Rok vydání: 1959
Předmět:
Zdroj: American Antiquity. 25:59-65
ISSN: 2325-5064
0002-7316
DOI: 10.2307/276679
Popis: Doubts that the Peruvian Indians could smelt copper ores in pre-Columbian times are unjustified, although archaeological evidence for the very effective wind furnaces described by early chroniclers is scanty. Oxide and carbonate ores from shallow deposits are more simply smelted than the sulfide ores, which required roasting of the crushed ore prior to reduction at a higher temperature. New evidence for the smelting of sulfide ore comes from chemical analysis of an ingot from the Province of Ica, and suggests the need for restudy of older analyses and a search for new prehistoric metallurgical evidence. A CRUDE copper ingot or cake (Fig. 1), said to come from "La Legua," Valle Ingenio, Province of Ica, not only raises anew the unresolved question of preconquest smelting, but shows that mining techniques were more advanced than generally believed. More important, it indicates that the man who made it had somehow hit upon the relatively complicated processes involved in smelting sulfide ores of copper, such as chalcocite (Cu2S), covellite (CuS), and the mixed sulfides of iron and copper, chalcopyrite (CuFeS2). These and other chemical formulae used in this article must be understood to refer to "pure" ores. The fact of the matter is that in nature various impurities, such as silver, lead, iron, nickel, bismuth, arsenic and antimony, may be present in copper ores and in metal objects produced from such ores (Nordenskiold 1921: 135-9). Before going on to consider our ingot and some finished Andean copper and bronze objects in which sulfur is present as an impurity, it may be in order to review briefly what is needed in the way of equipment and temperatures to smelt copper ores. It is commonly accepted that man was able to melt native metals long before he could smelt minerals to extract metal from something that was not a metal to start with. Whatever doubts may have been expressed about the smelting of copper ores in the New World before the Spanish conquest, no one has ever questioned the fact that native copper was melted and cast. This is an important fact to bear in mind when doubts are raised whether the early pre-Columbian workers could achieve with primitive means the temperatures necessary to smelt copper ores. To fuse native copper requires a temperature of 1083? C. or 1981.4? F. No smelting process for any copper ore calls for a higher temperature, and for most a considerably lower temperature will suffice. In the formation of the copper-bearing minerals of Peru (Raimondi 1878: 82-3), as indeed elsewhere in the world (IForbes 1954: 585; Thompson 1958: 3), the shallower deposits or secondary ores tend to weather and become oxidized. This accounts for an oxide ore like cuprite (Cu,O), the brilliantly colored carbonates, malachite (CuCO,, Cu(OH)2) and azurite (2 CuCO,, Cu(OH),), a silicate like chrysocolla (CuSiO3, 2 H2O), or an oxychloride like atacamite (Cu2Cl (OH) ). The last mentioned ore is found in coastal Peru and Chile, and Raimondi notes that the presence of chlorine is due to the action of sea water in its formation. Barba (1925: 165), who wrote the great early classic on Andean metallurgy in 1640, calls these secondary ores of copper pacos, "whether they be green, blue, orange, or of whatever color; all those that were not steely or mirror like." As the depth of mining increases, however, the oxychloride, oxide, and carbonate ores disappear, and the sulfide ores of copper mentioned above in the first paragraph begin to appear. These are harder, and have a distinctly metallic luster. Barba calls them negrillos, and describes them as being steely or mirror-like in appearance. In connection with our ingot or cake from "La Legua," it may be significant that Raimondi (1878: 90) reports that chalcocite, covellite, and chalcopyrite, three of the principal sulfide ores of copper, are present in that part of Peru. However, evidence that any of these ores was mined there in preconquest times has not been reported; nor, according to our informants, is any such evidence likely to be found, due to the continuous working of productive deposits. The reduction of oxide and carbonate ores of copper by primitive means presents no serious problem. Thompson (1958: 3), citing the outstanding English metallurgist, Gowland, has stated that the charcoal reduction of copper oxide or cuprite commences at 500? to 600? C., or about 932? to 1112? F. Referring to the famil
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