Original surface treatment of copper alloy in ancient Roman Empire: chemical patination on a Roman strigil

Metal polychromy has been widely used for decorating metallic artworks from ancient antiquity. In Hellenistic and Roman periods, the famous Corinthian bronzes (Corinthium aes) are a typical example of the skill of metallurgical craftsmen striving to obtain various coloured surfaces on copper based alloys; but the existing investigations are devoted mainly to the 'black bronzes' obtained by oxidation in a wet medium of copper-gold alloys. The present study is part of a general programme devoted to research into different kinds of special surface oxidation treatments on copper alloys and their environmental behaviour. For archaeological objects, such treatments are difficult to observe, as a consequence of corrosion degradation. The examination and analysis of objects kept in the Department of Greek, Etruscan and Roman Antiquities of the Louvre Museum have evidenced the occurrence on one of them, a strigil dated from the first century AD, of a new type of decoration. The strigil is made of copper-zinc alloy, and its decoration exhibits orange-red coloured motifs visibly obtained by chemical oxidation. The present paper describes analyses performed using different techniques: microscopy, particle induced X-ray emission, X-ray diffraction, Rutherford backscattering spectrometry, micro-Raman spectroscopy, spectrophotocolorimetry and optical profilometry. The steps of the decoration process are detailed: the motifs were coloured together with the neighbouring surface. This neighbouring surface was further cleaned by a mechanical process to make the motif coloration appear. The present paper attempts to describe how the Roman craftsman obtained the decorated appearance.