ToF-SIMS characterization of proteinaceous binders in the wall painting “Madonna and Child enthroned with Saints” by Ambrogio Lorenzetti in the St. Augustine Church (Siena, Italy)

In this paper, we report the results of investigations, by means of ToF-SIMS and HPLC, of samples taken from the wall painting "Madonna and Child Enthroned with Saints" (1335-1338) by Ambrogio Lorenzetti. The aim of this study was to test the capability of ToF-SIMS to identify the nature of the protein-based binders used in the wall painting. The peaks characteristic of egg components were observed in the ToF-SIMS mass spectra of the a secco-applied layers. On the contrary, the ToF-SIMS spectra of the a fresco-applied layers did not show the markers of egg. The PCA results of the protein peaks are consistent with the presence of a rabbit glue-egg mixture. The PCA of protein peaks in the ToF-SIMS spectra was carried out for artificially aged and non-aged reference samples of rabbit glue, casein, egg and their mixtures in order to estimate the effect of ageing on protein-based binding media. Both ToF-SIMS and HPLC data showed that animal glue was present in the a secco- and a fresco-applied layers. On the basis of these results, we can conclude that animal glue was not used as binding medium but was probably applied to the wall painting in prior restorations.