This interesting plaque, weight 87.80 grams, measures 84 mm x 51mm, and including the height of the human figures, is 84mm x 76.5mm. It is shown in Figure 21. The plaque is about 2mm thick and dates from the period around 650 AD- 850 AD, probably from the La Aguada cultural region. Gonzalez (1975) states that these types of bronze plaques are rare. A surface examination of the plaque was carried out using the binocular microscope at a magnification of x40 . The surfaces of the plaque are corroded and covered in a well developed patina which has all the appearance of a natural corrosion crust. Within the crevices of the line ornamentation which form the long bodies of the two human figures, there is a complex assemblage of mineral material associated with the burial environment. The plaque has been cast by the lost-wax process: none of the surfaces show any sign of working of the metal, instead the plaque is skillfully modeled in wax and then cast in bronze.

A non-destructive X-ray fluorescence analysis was carried out on this plaque on the reverse. The elements detected were iron, copper, arsenic, tin and antimony. This is of considerable interest because antimony is not commonly mentioned in pre-Hispanic materials but the ores of the Bolivia Argentina area are complex and high nickel or arsenic or antimony concentrations accompanying the extraction of the copper before alloying with tin would certainly not be unexpected.

The composition of the plaque was estimated quantitatively to be: 88% copper, 0.8% arsenic, 8% tin, 2.7% antimony and a trace of iron.

In fact, this surmise is borne out by some previous analyses from Argentina: for example Alberto Rex Gonzalez (1975) mentions a semicircular copper fragment , number 5550, from Grave 11 of the Corral Blanco cemetery, Laguna Blanca, Belen which was analyzed by Fester (1962) and had composition of: copper 92.33%, arsenic 3.40%, zinc 1.22% tin 2.05% and antimony 0.42%., from the Condorhuasi cultural period.

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Figure 20a : Unetched cross-sectional view of part of the silver sheet of the spoon with a gold inlay over part of the surface. As polished, magnification x80.

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Figure 20b : Etched microstructure showing the diffusion interface at the surface between the gold foil inlay and the silver substrate. The junction appears double because corrosion has only attacked the underlying alloy to a certain depth below the surface, diffusion of gold into the topmost silver layer has resulted in enhanced resistance to corrosion. The silver grains clearly show discontinuous precipitation of copper at the grain boundaries. Etched in acidified potassium dichromate, magnification x320.

A flat axe with socket, number 11549 associated with the Cienaga cultural area also contained antimony. An analysis by Fester (1962) showed 85.07% copper, 4.36% lead, 5.59% zinc, 3.54% tin, 0.96% antimony, 0.45% iron and 0.16% nickel. These examples confirm the presence of antimony in copper alloys from the region. Antimony, for example, is never found in ancient bronzes from Ecuador above 0.1-0.2% which is why the percentage is so startling here, and why it is understandable in the Argentine context. Ore survey work by Lechtman (1996) indicates that mixed copper-arsenic ores such as enargite, Cu3AsS4 , and tetrahedrite, Cu12(As,Sb)4S13 were available from southern Ecuador through to northern Bolivia, with substantial deposits of these minerals being available to pre-Hispanic miners in northwest Argentina and central Chile. Smelting of such ores could easily produce an antimonial copper alloy. Tin could then be added to the smelted metal to produce a ternary alloy of copper, tin and antimony. Both arsenic and antimony are useful additions to cast bronze, helping to either lower the melting point, increasing fluidity, or mitigating the effects of oxygen absorption on casting, depending on the particular features of the casting process involved.

The imagery of the plaque is of interest, since these anthropomorphic figures are reminiscent of Bolivian rock art imagery, and it would be of interest to study the iconography of these figures to examine in greater detail their possible relationship to other examples of Bolivian or Argentinean art.

The variety of metallurgical studies afforded by pre-Columbian metal objects shows how rewarding the detailed examination of these objects can be from the technological perspective. There mar be very little temporal distance between the areas of La Aguada and Muisca, but there is a startling difference in terms of metallurgy and the smelting of metals. Research on the origins of Colombian copper needs to be undertaken in a systematic manner together with detailed information on Colombian ore sources, although what is really needed is archaeological finds of metalworking in copper, with furnaces and blowpipes and slag.

The unexpected discovery of platinum so far South in a Recuay tax, clearly raises the possibility that there must be more platinum alloys used in ancient Peru than previously thought. Further work is needed on Peruvian silver-looking alloys to examine if they are really made of silver, silver- copper, or gold-platinum alloys.

Some information is available for ancient Peruvian metalwork, especially Moche metalwork, but very little published information can be accessed for Huari or Recuay metalwork. The discovery of traces of lead in the silver used to make the Huari spoon suggests the possible smelting of mixed silver-lead ores followed by cupellation in the Huari cultural area. Further detailed studies are needed to address this issue.

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