The characterisation of the materials from a Madonna with Child, Saint Catherine of Siena and a Carthusian Prior polychrome earthenware bas-relief attributed to Giovanni Antonio Amadeo: a multi-analytical approach

This research was devoted to the characterisation of the pigments used to paint a polychrome terracotta bas-relief dating from the 15th century. The artwork, which is entitled "Madonna with Child, Saint Catherine of Siena and a Carthusian Prior", was recently and definitively attributed to Giovanni Antonio Amadeo (Pavia, 1447 - Milan, 1522) and it is now stored in the museum of Sforzesco Castle of Milan [1]. Lately, the masterpiece was subjected to restoration work whose main aim was the withdrawal of a thick, dark coating that was hiding its vivid colours. During the intervention, six micro-samples were taken and then investigated through Optical Microscopy (OM), Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDXS), and Fourier-Transform Infrared micro spectroscopy in ATR mode (ATR-FTIR). Moreover, some polished sections were prepared and analysed, leading to the complete stratigraphies of the samples and, therefore, to the distribution of the pigments on the different layers. When the cleaning was completed, some in situ Raman analyses were carried out on some of the points wherein no sample could have been taken previously, especially on the black details. The investigation disclosed the contemporary presence of pigments of natural origin (such as lead white, azurite, yellow ochre, carbon black, calcite, cinnabar, and gypsum) and synthetic ones, especially Prussian blue and barium sulphate, whose presence is to be attributed to an earlier restoration intervention. Furthermore, the presence of gold leaf on the background was confirmed as well as some information about its application technique. Finally, some GC-MS analyses were performed to try to identify both the original binders and the composition of the dark coating that was removed with the restoration [2]. The latter study was especially devoted to the possible elucidation of the environmental factors responsible for that kind of deposit.