Portable equipment for a non-destructive analysis of pre-Columbian metal artefacts from the Royal Tombs of Sipán by energy-dispersive X-ray fluorescence spectrometry

On the north coast of present-day Peru flourished approximately between 50 and 700 AD the Moche civilisation. They were sophisticated metalworkers and are considered the finest producers of jewels and artefacts of the region. The Moche metalworking ability was impressively demonstrated by the excavations of the 'Tumbas Reales de Sipán', carried out by Walter Alva et al. in 1987. About 50 metal objects from these excavations, now at the Museum 'Tumbas Reales de Sipán', in Lambayeque, North of Peru, were analysed with a portable equipment which uses energy-dispersive X-ray fluorescence (EDXRF). This portable equipment is mainly composed of a small-sized X-ray tube and a thermoelectrically cooled, small-sized X-ray detector. Standard samples of gold and silver alloys were employed for calibration and quantitative determination. The analysed artefacts are mainly gold, silver and copper alloys, gilded copper and tumbaga, the last being a poor gold alloy enriched at the surface by depletion gilding, i.e. by removing copper and silver from the surface. In the case of gold, silver and copper alloys, their composition was determined by the EDXRF analysis in the usual manner, i.e. by employing standard alloys. In the case of gilded copper or tumbaga, the ratios Cu(Kα/Kβ) and (Au-Lα/Cu-Kα) were accurately determined from the X-ray spectra, first to clearly distinguish gilded copper from tumbaga and then to determine the gilding thickness or an 'equivalent gilding thickness' in the case of tumbaga. The combination of the two ratios is a clear indication of the nature of the alloy (gold, gilded Cu or tumbaga) and allows an accurate measurement of the gilding thickness in the case of gilded copper objects or, in the case of tumbaga, the 'equivalent' gold thickness was measured to be ~2.8 μm. From all measurements, the mean approximate composition and thickness of Sipán alloys is the following: 1. Gold: Au = ~70%, Ag = ~20% and Cu = ~10% 2. Gilding of gilded copper: Au = ~97.5% and Ag = ~2.5%; thickness = ~0.6 μm 3. Tumbaga: 'equivalent' Au thickness = ~2.5 μm 4. Silver: Ag = ~92%, Cu = ~5% and Au = ~3% 5. Copper: Cu = ~99% with traces of iron and nickel © 2011 John Wiley & Sons, Ltd.