Evolution of Pre-Columbian Metallurgy from the North of Peru’ Studied with a Portable Non-Invasive Equipment Using Energy-Dispersive X-Ray Fluorescence

On the north coast of present-day Peru, between the Andes and the Pacific Ocean, prospered approximately between 1000 BC and 1375 AD, several relevant cultures: Chavín (1000-200 BC), Vicús and Frías (200 BC - 300 AD), Moche (400 BC-700 AD), Sicán (700-1375 AD). These cultures are interconnected and characterized by a high metallurgical ability, demonstrated by the presence of beautiful artifacts on gold, silver and copper alloys. More than hundred metal artefacts from these cultures were analyzed with a portable equipment which uses the non-destructive and non-invasive technique of energy-dispersive X-ray fluorescence (EDXRF). Following objects were analyzed: (1) gold, silver and copper objects from the Chavín culture (Museo Municipal of Piura and and Museo Enrico Poli in Lima); (2) gold and silver objects from the Vicús and Frías cultures (Museo Municipal of Piura); (3) gold, silver and copper objects from the Moche culture (“Museo Tumbas Reales de Sipán”, “Museo de Sitio de Huaca Rajada” in Lambayeque and Museo Enrico Poli in Lima); (4) gold, silver and copper objects from the Sicán culture (Museum of Sicán, Ferrañafe). Portable equipments were employed, mainly composed of a small size X-ray tube and a thermoelectrically cooled, small size, Si-PIN or Si-drift X-ray detectors. The main characteristics of the EDXRF-technique is of being non destructive and multi-elemental. Standard samples of gold and silver alloys were employed for calibration and quantitative analysis. The aims of this campaign of analysis were: (1) to analyze a large number of objects; (2) to differentiate gold, gilded copper and tumbaga (this last being a poor gold alloy enriched at the surface by depletion gilding); (3) to correlate, when possible, composition of analyzed alloys to the specific culture; (4) to determine a possible evolution of metallurgy; (5) to better determine characteristics and beginning time of tumbaga production. It was determined that the analyzed artefacts are composed of gold, silver and copper alloys, of gilded copper or silver and tumbaga, the last being a poor gold-alloy rich on copper and enriched at the surface by depletion gilding, i.e. by removing copper from the surface. About 120 alloys were analyzed. In the case of gold, silver and copper alloys, their composition was determined by EDXRF-analysis by employing standard alloys. In the case of gilded copper (or gilded silver) and of copper based tumbaga, the ratios Cu(Kα/Kβ) andAu(Lα/Lβ) were determined from the X-ray spectra, first to clearly differentiate gold, gilded metal and tumbaga, and then to determine the gilding thickness. Concerning the correlation composition-culture, it seems that the Chavín did not use to mix copper in the gold alloy. This element, with a mean concentration of 1.5%, is only present in Au-alloys as in nature. Further, silver was measured in all cultures of the north of Peru at relatively high levels of purity, but by the Chavín was associated to Cu and Pb, by the Moche to Cu and Au, by the Sicán to Cu, Au, Pb and Br. Also copper was measured in all cultures of the north of Peru at high levels of purity. However, this element seems to contain small quantities of Zn by the Chavín, is almost pure by the Moche, and contains as by the Sicán. With reference to copper based tumbaga, it is not clear when it was “produced” for the first time in the north of Peru, may be by the Vicús; in any the largest number of tumbaga was produced by the Moche, and are characterized by a mean “equivalent” Au-thickness of ~2.5 μm.