Chemical characterization of protein based binders in painting samples by means of ToF-SIMS: Tests on ancient and model samples

• Proteinaceous binders used in tempera painting are investigated by ToF-SIMS and PCA.
• Analysis of samples from a 15th century painted crucifix and of model samples.
• Role of matrix effects in the identification of proteinaceous binders.

In the present work we investigated the possibility of identifying proteinaceous binders, typically used in “tempera” painting, by means of ToF-SIMS and PCA method. ToF-SIMS measurements were performed on cross-sections taken from a painted wooden crucifix of the 15th century. The results of the ToF-SIMS analysis are compared with those obtained by GC–MS. ToF-SIMS measurements were carried out also on model samples simulating the layered structure of real painting cross sections to elucidate how matrix effects can influence the identification of protein based binding media. A discrimination between rabbit glue and casein on the basis of the comparison of the ToF-SIMS results for the ancient samples with those obtained from reference compounds appears to be difficult. This is mainly due to matrix effects that are relevant for these complex, multicomponent samples, as indicated by the PCA results for model painting samples of known composition. On the other hand, PCA method and the observation of characteristic fragment ions in the spectra should allow to determine the presence of egg in painting layers.

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