Raman approach to the forensic study of bronze patinas

Abstract

Illicit trade in archaeology is a widespread phenomenon connected to the market of archaeological objects from clandestine excavations or faking manufacture, which includes bronze objects. As a consequence, authentication and provenance studies are required on objects of unknown provenance when they are recovered thanks to the investigation activity of the police, before returning them to the museums. Various strategies and approaches can be used to identify fake bronzes, as for instance patinas studies, since the corrosion can spontaneously occur over time, due to the interaction of the surface with the environment, or induced by artificial treatments, performed to mimic the archaeological aging effects in short time. The artificial patina has outward appearance and colours very similar to the natural patinas, but with different mineralogical composition. In this work, Raman spectroscopy is used to discriminate between artificial and natural patinas by a non-destructive approach, supported by the microanalysis SEM-EDS and optical microscope observations in the case of two ‘Etruscan-Roman’ bronze statuettes of illicit and unknown provenance, and demonstrates their recent and fake manufacture. Indeed, the patinas of both statuettes contain covellite, antlerite, langite, chalcanthite and rouaite, minerals or metastable chemical species traced back to corrosion induced by chemical attack and thermal treatment, as distinct from their stable polymorphs usually formed in burial condition. Moreover, we have not found evidence of the presence of malachite and azurite, typical of natural aged patinas. Finally, the abundance of Zn in the alloy suggest a modern manufacture.