Metallography and microanalysis of precious metal objects from the Middle Sicán elite tombs at Batán Grande, Peru

The Sican Archaeological Project (under the direction of I. Shimada) is centred inthe Batan Grande region of the La Leche Valley in northern Peru. One of the major topics of investigation is Middle Sican (A.D. 900-1100) metallurgy within its broader cultural and environmental contexts. Results of the investigation of copper-arsenic alloy production have been published [1,2]. Two major Middle Siadn elite tombs excavated during the 1991-2 season yielded large quantities of diverse preciousmetal and arsenical copper objects. In particular, the twelve-metre deep shafttomb at thenorth base of the pyramidal temple of Huaca Loro contained some 1.2 tons of grave goods with about 75% of them in weight being metal [3-6]. Samples of precious metal artifacts are being studied using metallographic techniques and electron probe microanalysis (EPMA). At this stage of the investigation, over 40 samples of gold-silver-copper alloy, sheet-metal objects have been analyzed using over 200 EPMA data points on depletiongilded surfaces and multiphase compositions of the centres. The metallographic structuresof the objects are extensively coldworked, but variable in composition including single and multiphase alloys. Surface enrichment is characteristically on the order of 10-20 microns, but several examples are observed with greater thickness. Depletion gilding is documented even in some of the most corroded metal samples. Object types studied include crowns, headdresses, “feathers”, hanging ornaments, wire and foil as well as several exceptional objects. Additional analyses are presented for a gold-silver-copper alloy ingot and standardized copper-arsenic alloy objects called naipes. Due to the archaeological significance of this assemblage of artifacts as well as the quantity, there are several research groups working co-operatively, using different analytical techniques. Compositional results are used to explore issues such as selection of precious metal alloys by surface colour and mechanical properties.