Analysis and surface treatments of the archaeological copper based alloys: application to the detection and comprehension of antic patinas

The analysis of cultural heritage metal artefacts is essential to obtain information on the provenance, the fabrication techniques and the art history, even if it is very critical and submitted to very severe constraints of preservation. This article shows an aspect of this analysis, neglected for a too long time: the analysis of the artefacts surface and of the decorations, in particular a very specific decoration: the black intentional patinas. A specific analytical protocol based on non destructive analysis (ion beam analysis-PIXE-RBS, X-ray diffraction, Raman spectrometry...) is developed and applied to an exceptional corpus of artefacts coming from the French museum collections. This study leads to reveal and assure a better comprehension of the use by antic craftsmen of a decoration technique based on a chemical treatment, providing essential information to the history of techniques and technologies. It led also to study a surface layer on copper based alloys with a very surprising chemistry.