Electron Microprobe and Neutron Activation Analysis of Gold Artifacts from a 1000 A.D. Peruvian Gravesite

Abstract

The gold-silver-copper ternary alloy contents of gold objects from an unlooted Middle Sicin (ca, 1000 AD) burial site on the north coast of Peru were determined by instrumental neutron activation analysis (INAA) using 10 μg metal rubbings from the interior of the objects. Electron microprobe analysis (EMPA) was also performed on a few of the objects allowed to be exported from Peru in order to measure thc interior and surface alloy contents and to determine if the INAA samples are representative of the interior alloy. The near-perfect agreement between the probe data for the interior metal and INAA sample data confirms that the INAA sampling method results in metal characteristic of the interior alloy. The thin outer edges, which are only about five μm thick, are depleted markedly in copper (18 wt% interior copper, 7% edge copper, for example) and suggest that, for the high-quality gold objects (typical contents: 45 wt% Au, 37% Ag, 18% Cu), surface depletion of copper (and enrichment of gold and silver) may not have been deliberately induced but could have been the result of the repeated hammering-annealing required to produce the thin gold sheets used for the construction of the objects. Lower quality tumbaga gold sheets (typical interior contents: 25 wt% Au, 45% Ag, 30% Cu) also show marked surface depletion of copper.