ToF-SIMS study of gilding technique in the fresco Vela della Castità by Giotto's school

Time of flight secondary ion mass spectrometry (ToF-SIMS) has become a fundamental analytical technique in cultural heritage studies to obtain the identification and distribution of organic components present in artworks. A sample taken from the soldier's golden shield decoration in the fresco Vela della Castità (San Francesco Lower Basilica, Assisi) was studied. The metal leaves for gilding decoration on wall paintings are usually applied using an oil-resin mordant as adhesive. In this work, organic and inorganic compounds used in this peculiar painting technique were identified by ToF-SIMS technique. High mass resolution spectrometry was applied for the chemical characterization of organic materials present in oil-resin binding media and red lake paint layer. Presence of azelaic, palmitic and stearic acids proved the nature of the oil binder between plaster and tin leaf and between tin and gold leaf. The analyses revealed also the presence of colophony, a natural resin used as an adhesive in this painting technique. Furthermore, the fresco cross section was studied using ToF-SIMS imaging. ToF-SIMS results agree with and are complementary to those obtained by other analytical techniques: scanning electron microscopy with X-ray microanalysis and Fourier transform infrared spectroscopy.