A diagnosis of the yellowing of the marble high reliefs and the black decorations in the chapel of the tomb of Saint Anthony (Padua, Italy)

This paper focuses on the chemical characterisation of samples of black decorations and inscriptions, and of yellow-brown patinas collected from various panels of the chapel of Saint Anthony (Padua, Italy) ark, by using three analytical procedures based on gas chromatography/mass spectrometry (GC/MS). In particular, analytical pyrolysis in the presence of hexamethyldisilazane followed by gas chromatographic-mass spectrometric analysis (Py-GC/MS) and two procedures based on gas chromatography/mass spectrometry after wet-chemical treatment of the samples (GC/MS) have been chosen for the recognition of the organic substances (used originally or in the course of restoration) and of their degradation products. As far as the two GC/MS procedures are concerned, one is suitable for the characterisation of proteins, lipids, resins, waxes, bituminous materials and their degradation products, and the other was used to characterise the saccharide materials. The three analytical procedures used allow us to obtain information on the presence of egg proteins, lipid materials (beeswax, siccative oil and animal fatty material), saccharide materials and Pinaceae resin. Beeswax, animal fat, egg and saccharide materials have been used as past restoration materials, and pine resin and siccative oil are the main ingredients of the black decorations and inscriptions.