Gold coated copper artifacts from the Royal Tombs of Sipan (Huaca Rajada, Peru): manufacturing techniques and corrosion phenomena

Twenty five years ago, close to the northern Peruvian town of Lambayeque (Huaca Rajada) beneath two large and eroded pyramids, built of adobe mud bricks, Professor Alva discovered the world-famous unlooted pre-Columbian burial chambers of the Royal Tombs of Sipan. The tombs contained a large amount of objects of exceptional artistic and historical value including the greatest intact number of gold and silver artefacts in the Americas to be considered one of the most important archaeological discoveries of the last century. Some copper based objects coated with thin layers of gold have been studied by means of the combined use of analytical techniques such as optical microscopy (OM), scanning electron microscopy coupled with energy dispersive X-ray micro-analysis (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) in order to identify the chemical composition and the manufacturing processes of the gold layer as well as the corrosion products formed during the long-term burial. The micro-chemical and structural results give useful information about the manufacturing techniques used by the Moche metalsmiths to modify the surface chemical composition of the coated artefacts likely based on the depletion gilding process carried out by oxidising the surface copper containing the noble metal and etching away the copper oxides. Furthermore, the results reveal that the main degradation agent is the ubiquitous chlorine and that copper has been almost completely transformed during the burial into mineral species giving rise to the formation of stratified structures constituted by different mineral phases such as cuprous oxide (Cu2O) and copper carbonates [azurite (Cu3(CO3)2(OH)2 and malachite (CuCO3Cu(OH)2)] as well as dangerous chlorine-based compounds such as nantokite (CuCl) and atacamite (Cu2(OH)3Cl) polymorphs. These information evidence the strict interaction of the alloying elements with the soil components as well as the occurrence of the copper cyclic corrosion as post-burial degradation phenomenon. The present study confirms that the combined use of micro-chemical and micro-structural investigation techniques such as SEM-EDS, XPS, XRD, and OM can be successfully used to investigate the technological manufacturing processes of the ancient coated artefacts and to achieve information about degradation agents and mechanisms useful to define tailored conservation strategies possibly including new, more reliable, and safer materials.