Arsenic Bronze: Dirty Copper or Chosen Alloy? A View from the Americas

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Maney Publishing is collaborating with JSTOR to digitize, preserve and extend access to Journal of Field Archaeology.Archaeologists and historians of metallurgy have attempted to explain the gradual abandonment of arsenic bronze in favor of tin bronze in the ancient Old World by making comparisons between the mechanicalproperties of the two bronzes. These comparisons purport to show the superiority of copper-tin alloys over alloys of copper and arsenic, despite an absence of data on the physical properties of the copper-arsenic system. The study reported here presents the results of mechanical tests carried out on experimental samples of both types of bronze over a broad range of alloy compositions. Hardness, tensile strength, and elongation determinations were made on cold worked and hot worked (forged) material. Whereas tin bronzes can be work hardened more extensively than arsenic bronzes, the fargreater ductility of arsenic bronze makes it a desirable alloy for the manufacture of thin metal sheet. The widespread use of low-arsenic copper-arsenic alloys in the Americas, especially in the Andean culture area, is attributable in part to the tradition there ofsheet metal production in the elaboration of three-dimensional forms. From Copper-arsenic to Copper-tinIn both the Old World (Europe and western Asia) and the Americas, copper-arsenic alloys were produced over a vast area, from Russia to Great Britain and from Chile to Mexico. This production was made possible by the relatively large number of metallic mineral species that contain arsenic, by their geological co-occurrence with ores of copper, and by the widespread association of these ores in the earth's crust (Lechtman 1991; Rapp 1988). In both the Old World and the New, the manufacture of copper-arsenic bronze preceded the development of copper-tin alloys. Beginning in the late fifth and first half of the fourth millennia B.C., peoples in the Near East and Europe worked with arsenic bronze for almost two millennia before tin bronze became a significant competitor (Eaton and McKerrell 1976; Muhly 1988; Chernykh 1992). At Tepe Yahya in the Zagros mountain region of Iran, Heskel (Heskel and Lamberg-Karlovsky 1980) excavated and analyzed early fourth millennium artifacts of copper-arsenic alloy which he argues were produced from native copper and two of the mineral arsenides of copper, domeykite (Cu3As) and algodonite (Cus5.2-8As). All three ores occur together in the Talmessi-Anarak mine region, roughly 800 km to the north, and were likely exploited there by ancient miners.Northover's study (Shalev and Northover 1993) of the Chalcolithic (fourth millennium B.C.) hoard of copperbased metal objects from Nahal Mishmar, in the J...

A Re-evaluation of inverse segregation in prehistoric As-Cu objects

“…However, since As-Cu alloys have no commercial applications they are seldom studied by modern metallurgists, except in regard to specific archaeological questions (e.g. Budd 1991a; Lechtman, 1996;McKerrell and Tylecote, 1972;Meeks, 1993;Northover, 1989). Following convention from the fields of metallurgy and materials science, we have chosen to use the terms "arsenical copper" and "arsenical bronze" for arsenic content <0.5 wt% and !0.5 wt%, respectively.…”

“…The alloys might have been produced by: 1) combining native copper with copper-arsenide minerals (Budd et al, 1992); 2) smelting copper ore rich in arsenic (e.g. Fahlore); 3) the reduction of roasted copper sulfarsenides such as tennantite and enargite, which would have resulted in the production of toxic arsenous oxide fumes and the loss of much of the arsenic present in the ores (Lechtman, 1996); 4) the co-smelting of oxidic and sulphidic ores such as malachite and arsenopyrite (Lechtman and Klein, 1999); or 5) the melting of speisses (iron arsenides) and copper (Thornton et al, 2009;Rehren et al, 2012). There is currently no evidence for the cementing of arsenic vapor with copper, or the intentional addition of arsenic-rich minerals to copper or copper ores (e.g.…”

Metallurgical analysis of copper artifacts from Cahokia

“…However, SEM-EDX revealed the presence of arsenic in the grooved region. The 4.5 wt% arsenic concentration in the groove is similar to that of some arsenic bronzes, which are significantly harder than pure copper (Lechtman, 1996). This would explain the high Knoop hardness measured along the groove.…”

Non-destructive characterization of archeological Cu-based artifacts from the early metallurgy of southern Portugal

“…7a). It is important to highlight that, although the introduction of arsenic starts to produce a beneficial effect in the final alloy product at concentrations of about 0.5 wt.% (Lechtman 1996), a marked improvement in the mechanical properties of the alloy mainly occurs only at As values of around 3.0-4.0 wt.% (Rovira 2004;Subramanian and Laughlin 1998). Accordingly, within the metals analyzed in this study, only three objects, i.e., the saw ETAR-61 (4.0 wt.% As), the knife ETAR-65 (5.0 wt.% As), and the spearhead EST-1275B (8.0 wt.% As), appear to contain As in sufficient amounts to benefit from an appreciable increase in hardness by mechanical treatment, while most artifacts show an As content lower than 1.0 wt.% (Fig.…”

“…This topic is still a matter of dispute and different arsenic thresholds have been adopted by a number of researchers to distinguish between alloys in which As could be considered as a deliberate addition or an accidental occurrence Guerra 1997/1998;Craddock 1995;Hunt-Ortiz 2003;Lechtman 1996;Northover 1989;Rovira 2004). Thus, as it is evident, to interpret the low and erratic arsenic content is not an easy task, even because several plausible causes can explain this variability.…”