What is the origin of the black film in the sculpture "Death of Cleopatra"?

Abstract

This work arises from the need to identify the origin of a black film which is currently covering a large area of the sculpture Death of Cleopatra, promoting its disfiguration. This piece, sculpted in Carrara marble, belongs to Tropical Botanic Garden, in Lisbon. In situ works involved colorimetric measurements, adhesion tests and mapping of decay patterns. Samples of the black film were taken from the sculpture and also from black areas present in botanic species surrounding the sculpture, which were observed macro and microscopically. Individual samples of the black film were analysed by Fourier-transform infrared spectroscopy. The results obtained show the collected samples possess saccharides such as sucrose, found in honeydew exemplars. Therefore, it is proposed that black film formation is due to sooty mould. This black film is responsible for aesthetic damage, as well as stone corrosion processes, which was observed by scanning electron microscopy.