Objects from Wanka I (AD 1000-1350) contain little or trace arsenic, and undetectable or trace amounts of the other possible alloying element, tin. The objects mar be regarded as having been made of copper with trace contaminants (Pb, Fe, S, As, Ag)o None of the objects found in Wanka II (AD 1350-1450), the immediately pre-Inca period, contained more than a trace amount of tin. On the other hand, the average arsenic content of the objects is 1.5 weight percent. About hall of the objects contain less than 1.5 weight percent arsenic (see Figure 1). According to Budd and Ottaway (Budd 1990; Budd and Ottaway 1989), below this concentration the presence of arsenic does not appear to improve the work hardenability of copper. Northover (1989) puts the mark slighdy higher, at 2 weight percent arsenic. The objects analyzed were predominantly needles or fragments of needles. It is equally likely for a needle to have more or less-than 1.5 weight percent arsenic. The question is whether the Xauxa understood and exploited arsenical copper for its hardness. The evidence at this point is equivocal. They might have been ignorant of the advantages of arsenical copper, or they might have recognized them, but been unable to control the arsenic content. The situation is further complicated if one allows for the possible reuse and recycling of metals, since each melting operation could cause any initial arsenic content to decrease, regardless of the intentional nature of its incorporation in the first place.

Another reason for the deliberate use of copper-arsenic alloys mar have been to take advantage of the formation of a silvery surface due to macrosegregation. Budd and Ottaway (1989) have observed change in the color of chill cast ingots with 4 weight percent or more arsenic. Currendy there is no information on whether observable macrosegregation will take place in cast objects with less than 4 weight percent arsenic and a much smaller volume-to-surface area ratio than the ingots used in prior experiments.

Figure 1: Arsenic content of Wanka II objects analysed b y ICP-ES

Figure 2 : Tin vs. arsenic content of Wanka III objects analysed using ICP-ES

ICPt is clear that tin bronzes were not present in contexts preceding the Inca occupation of the Mantaro valley. Instead, copper based artifacts were made of relatively puree copper or arsenic containing copper. At this point the evidence for the intentional incorporation of arsenic into copper is scant and it is likely that the presence of arsenic was due to its inclusion in or with the ores and fluxes used in smelting.

The average arsenic content of all objects from the Inca period (Wanka III), including both arsenical coppers and tin bronzes is 1 weight percent, compared to the 1.5 weight percent of pre- Inca objects. Qn the other hand, the average tin content of all Inca period objects is 3 weight percent. Almost half of the objects contain less than 1.5 weight percent arsenic and less than 1 weight percent tin (see Figure 2).

A further 12.5 percent of the objects contain less than 1 percent tin but more than 1.5 percent arsenic. All but one object in this group is an ingot, and mayor mar not have been the product of intentional alloying. Either way, a significant 57.5 percent of objects contain virtually no tin and represent the persistence of the products of local or regional pre- Inca technology. There are several possible explanations for this persistence of tin less copper. One possibility is that pre- Inca metal smelting technologies continued to be used after the Inca conquest of the region. Another is that objects manufactured prior to the Inca conquest continued to be used, and were eventually deposited in Wanka III contexts together with later, tin­containing artifacts made under the Inca.

Under 40 percent of Wanka III copper objects analyzed contained enough tin to perceptibly alter their properties. In the subsequent Spanish colonial Wanka IV period, however, over 80 percent of the analyzed objects contained enough tin to affect their properties. Thus the period of Inca occupation appears to be one of technological transition. The nature of transition is hard to determine because one cannot necessarily consider that all objects found in a certain context were manufactured in the period of deposition.One of the following three scenarios mar characterize the technological transition:

1.The transition was a gradual matter of changing preferences, taking place as the Inca established systems of distribution capable of providing tin from distant sources, and as local metalworkers independently adopted new and additional procedures.

2. The transition was relatively fast, perhaps due to clear advantages attributed to tin alloys, but the curation and/or recycling of earlier objects obscures the rapid adoption of the new technology.

3.The transition was essentially instantaneous, as might be expected if the Inca state imposed technological norms or took control of some or all metal production, but once again, the change is disguised in the archaeological record by curation and/or recycling.

Although the nature of the transition at this point is still uncertain, it has been demonstrated that in the Mantaro, the adoption of copper-tin alloys clearly began during the Inca occupation. Nevertheless, a sizable proportion of copper based objects without tin were also in use under the Inca, even if they were not necessarily all made during this time.

The comparison of the compositions of ingots with objects that have les s than and more than 1 weight percent tin gives us some clues as to the manufacture of tin bronzes (see Figure 3). The arsenic, lead, and antimony content of ingots is significantly greater than that of worked pieces and fragments whether or not they contain tin. If we assume that the ingots examined represented an early stage in the manufacture of the objects, then the decrease in the concentration of arsenic, lead, and antimony mar suggest a manufacturing step wherein the ingots were melted in an oxidizing atmosphere and cast into rough blanks with were then further worked and annealed. McKerrell and Tylecote (1972) have documented the loss of arsenic under such conditions. Lead and antimony would also easily become oxidized and removed from the molten copper by forming a slag with the crucible material (Craddock 1995, 202-203). The fact that none of the ingots contain tin suggests that tin bronze did not circulate as a raw material. One possibility is that tin metal or ore was exchanged, and tin bronze objects were prepared locally by procuring tin and adding it to molten ingots of copper or arsenic containing copper. Another possibility is that tin bronze objects were not made locally at all, and arrived in the region as finished artifacts.

Figure 3: Comparison of average composition for Wanka III ingots and worked objects

Why were tin bronzes so thoroughly adopted in the Mantaro? One possibility is that tin bronzes were noticeably better than tinless alloys in some way. As discussed earlier, less than 1.5 weight percent arsenic in copper does not appear to offer advantages over the use of pure copper. If alloying with arsenic was in fact practiced in the Mantaro valley, then the process was not consistent. In contrast, the addition of tin to molten copper would afford metalworkers greater control over the alloying process. This argument that using tin bronze afforded metalworkers more consistent alloy composition and hence was preferred by the metal producers makes sense only if metalworkers actually took advantage of the properties of tin bronzes. The specific uses of tin bronzes does seem to suggest that metalworkers exploited the advantages of the new material. Needles that contain tin are generally longer or thinner with a sharper point than unalloyed needles. The tin content would have facilitated the fabrication of sharper points that dulled less than their copper counterparts. Furthermore, the tin content would have made long needles less likely to deform under stress. The exploitation of the superior hardness of tin bronzes is perhaps best demonstrated by edge tools. Only three such objects have been found in Inca contexts: two chisels and an ax. They all contain tin and their edges are work hardened. The ability of tin bronzes to take on and retain a reflective surface appears to have been used in the manufacture of tupu pins worn by women. The addition of tin lowers the melting point of copper and improves its casting behavior. Since the only cast objects were found in Inca contexts, and they all contain tin, it is likely that the above properties of tin bronzes were indeed exploited in the making of cast objects. In summary, it is very likely that the properties afforded by tin bronzes were recognized and exploited for both worked and cast objects. We are unsure whether the toxicity of arsenic was a factor in abandoning the manufacture of arsenical copper pieces.

The use of tin bronzes for prestige purposes presupposes knowledge of, exploitation of, and preference for its mechanical properties or the formal characteristics that those properties make possible. Some of our material evidence supports the existence of this knowledge and these preferences. For a given type of artifact to function as a prestige good, access to that type of artifact must also be at least somewhat restricted. Owen found precisely this sort of preferential elite access to silver objects, and to a les ser extent to copper-based objects in general (1986). The question now is whether there is a similar pattern of preferential elite access to objects that contain tin, as compared to copper-based objects that do not. Fortunately, the UMARP data set allows us to evaluate this. In Wanka III contexts 33 percent of commoner objects as opposed to 50 percent of elite objects contains more than 1 weight percent tin. However, with our small sample sizes, the difference between 33 and 50 percent is not statistically significant. These data indicate that the elite mar have had somewhat more access to tin bronzes, but the degree of preferential access to tin branzes enjoyed by the elite, if it existed at all, was minimal in comparison to their greater access to silver and to copper objects in general. This suggests that tin bronze, as a material in contrast to other copper alloys, was not particularly preferred for purposes of prestige or status legitimation.

Tin bronze technology was clearly intraduced into the Mantara valley by the Inca. Either it was adopted gradually, coexisting with copper and arsenical copper production for a transition period, or a large number of old, pre-Inca and pre-tin objects continued in circulation. In either case, the copper based objects of all types in use under the Inca included both tin bronzes and copper materials without tin, diluting any practical effect of the technological change. It appears that the properties of the new tin bronze materials were known and exploited. However, the lack of distributional differences so far indicate that as a prestige good, tin bronze was not valued significantly more highly than copper or arsenical copper.