A New Interpretative Approach to the Chemistry of Copper-Alloy Objects: Source, Recycling and Technology
Bray, P. J., Pollard, A. M., Antiquity
Archaeometallurgy has often been criticised for not considering the human and environmental reality behind the chemical analyses--for being far too concerned with documenting technology, at the expense of considering the social context of the manufacture and use of metal. This paper shows how a new procedure uses the chemical composition of bronze objects in north-western Europe to deduce their histories; not only where they originated, but how they were made by observing how often the bronze had been recycled and reworked.
The conceptual basis of the model
It has long been accepted that the composition of copper-alloy objects is, in part at least, dictated by the mineral assemblage of the ore from which it is smelted (e.g. Friedman et al. 1966). Thus, native copper, oxide ores, and sulfide ores will give rise to copper alloys with distinctive proportions of other elements, depending on the exact mineralogy of the ore and the smelting process(es) involved. The highly influential Studien zu den Anfangen der Metallurgie (SAM) project (Junghans et al. 1960, 1968, 1974) systematised this approach by producing a series of decision trees based on concentration levels of trace elements in order to group objects by chemical variation. Northover (1980) modified this by noting the presence of key elements when creating his A to H alphabetic system to denote metal types. More recently this work continues through the linking of chemistry, geology, isotopes, archaeological objects and mines (Ambert 1991; Ambert & Barge-Mahieu 1991; Krause & Pernicka 1996; Ixer & Budd 1998; Needham 2002; Ixer & Pattrick 2003; Niederschlag et al. 2003).
A clear consensus has emerged through this and similar work that low levels (rarely exceeding 5%) of arsenic (As), antimony (Sb), silver (Ag) and nickel (Ni) show characteristic variation in prehistoric copper objects, and that similarities in these patterns of variation can often be interpreted as indicating a common ore source. This composition will obviously then be influenced by the precise ore beneficiation and smelting processes applied. We argue that, additionally, the final element signature of a given object can be affected by a range of post-smelting processes that are central to how people actually used metal: melting and casting, smithing, recycling and alloying.
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Re-melting copper alloys results in a loss of certain of the included elements through oxidation, particularly arsenic and antimony (McKerrell & Tylecote 1972; merkel 1982; Pernicka 1999; Earl & Adriaens 2000), a process illustrated by an Ellingham diagram (Figure 1). This plots, amongst other information, the affinity of an element for oxygen at different temperatures, and is the foundation of modern chemical metallurgy (Beeley 2001). Alongside the underlying driving force of oxygen affinity, the precise rate of oxidation is also affected by the degree of agitation of the molten metal, mutual solubilities, activities, volatilisation and partial pressures (Merkel 1982: 30; Beeley 2001: 31). These properties are exploited by modern industry in order to oxygen-refine (i.e. remove unwanted levels of other metals) the copper produced by smelting (Copper Development Association 2011). Consequently there is an extensive literature on oxidation effects in molten metals that is consistent with the archaeological experience: a bronze object will be depleted in certain of its metals depending on how many times it has been reheated, and to what temperature (e.g. Hampton et al. 1965; Charles 1980; Pickles 1998; Beeley 2001: 497; Tanahashi et al. 2005; Lee et al. 2009).
Summarising the causes of chemical variation, Pernicka (1999) concludes that a very limited range of trace elements are directly related to the provenance of the ore (most significantly gold [Au], silver [Ag], bismuth [Bi], iridium [Ir] and nickel [Ni]), whilst a larger number of the others (including tin [Sn], providing it is present at less than c. …