Pre-Columbian alloys from the royal tombs of Sipán and from the Museum of Sicán

Abstract

On the north coast of present-day Peru, approximately between 50 and 700 AD, flourished the Moche civilization. It was an advanced culture, and the Moche were sophisticated metalsmiths. The Moche metal working ability was impressively shown by the excavations of the “Tumbas Reales de Sipán”, discovered by W. Alva and co-workers in 1987. The Sicán culture is a successive civilization (750-1375 AD) which extended as far as present day Piura in the north and Trujillo in the south. The Sicán culture was strongly influenced by the Moche culture, particularly in its metallurgical development (Shimada and Griffin, 1994). The metal objects from the Museums of Sipán and Sicán were analyzed with a portable equipment which uses energy-dispersive X-ray fluorescence (XRF). This portable equipment is composed of a small size X-ray tube and a Si-PIN thermoelectrically cooled X-ray detector. It was determined that the analyzed artefacts are composed of gold, silver and copper alloys, of gilded copper, of silvered gold, and of tumbaga, the last being a poor gold alloy enriched at the surface by depletion gilding, i.e. removing copper from the surface. In the case of gold, silver and copper alloys, their composition was determined by EDXRF analysis employing standard alloys. In the case of gilded copper, silvered copper and of tumbaga, the ratio Cu(Ka/Kb) was accurately determined from the X-ray spectra, first in order to clearly distinguish them, and subsequently in order to determine the thickness of the gilding. Mean values of 0.4 µm and 2.7 µm were determined for gilded copper and tumbaga, respectively. For gilded silver, the ratio Ag(Ka /Kb) was measured. The ratios Au-Ka/Cu-Ka, Ag-Ka/Cu-Ka and Au-Ka/Ag-Ka (gilded copper, silvered copper, and gilded silver, respectively) also depend on the thickness of gilding or silvering, and were employed to this aim.