Characterisation of organic patinas on indoor bronze sculptures

The surface of bronze sculptures was often treated by applying organic translucent substances (patinas), in order to modify their appearance and to cover melting defects. The aesthetical impact and the chemical composition of patinas on bronze sculptures have been studied in the literature, mainly focussing on inorganic materials and corrosion phenomena. On the other hand, the identification of original and restoration organic materials constituting the patina on a bronze object can yield precious historical and artistic information, and can contribute to plan efficient cleaning and restoring procedures. This paper reports the characterisation of the patinas on two bronze heads belonging to the Cantoria di Donatello in Santa Maria del Fiore currently under restoration at the Opificio delle Pietre Dure (Florence). The samples were collected at different cleaning stages. The samples were subjected to GC/MS analysis to characterise organic materials by identifying the specific biomarkers: a Pinaceae resin, a siccative oil, beeswax and paraffin wax were recognised. The study highlighted meaningful differences between the two samples, and allowed for understanding the technology employed in the application of patinas on the two bronze heads