Analysis of the spectacular gold and silver from the Moche tomb 'Señora de Cao'

On the north coast of present-day Peru, between the Andes and the Pacific Ocean, approximately between 100 and 600ad, the Moche civilization prospered. The Moche were very sophisticated artisans and metal smiths, so that they are considered the finest producers of jewels and artifacts of the region. Their metalworking ability was impressively demonstrated by the excavations of the tomb of the 'Lady of Cao' (dated around third-fourth century ad) discovered by Regulo Franco in 2005. Impressive is the beauty of the artifacts, and also the variety of metallurgical solutions, demonstrated by not only the presence of objects composed of gold and silver alloys but also of gilded copper, gilded silver, and tumbaga, a poor gold Cu-Au alloy subject to depletion gilding. About 100 metal artifacts from the tomb of the Lady of Cao, never before analyzed, were studied by using various portable equipments based on following non-destructive and non-invasive methods: energy-dispersive X-ray fluorescence with completely portable equipments; transmission of monenergetic X-rays; radiographic techniques; and optical microscopy. Gold objects and gold areas of nose decorations are characterized by approximately the same composition, that is, Au=(79.5±2.5) %, Ag=(16±3) %, and Cu=(4.5±1.5) %, while silver objects and silver areas of the same nose decorations show completely erratic results, and a systematic high gold concentration. Many gilded copper and tumbaga artifacts were identified and analyzed. Further, soldering gold-silver was specifically studied by radiographs. Additional measurements are needed, particularly because of the suspect that depletion gilding was systematically employed also in the case of some nose decorations.