A detailed micro-Raman spectroscopic study of wall paintings of the period AD 100-200: effect of atmospheric conditions on the alteration of samples
Résumé
In this study, optical microscopy and micro-Raman spectroscopy were used to determine the chemical compositions of the materials used in the preparatory and pictorial layers of wall paintings from four different sites of the Roman Empire of first to second century AD. The use of UV excitation for the Raman work for the study of the preparatory layers made it possible to overcome the problems of fluorescence. The fresco technique, revealed by the presence of calcite in the Greek and Roman samples, was not used for the Gallo-Roman works. The studies indicated that the same compounds were used for red and green pigments throughout the Roman Empire: red ocher (hematite) and green earths (celadonite, in the case of the Gallo-Roman fragments). It is shown that the impact of the atmosphere on the sample surface is very important, because the only fragments to have suffered identifiable deterioration are those from the 'open-air humid room' in Kenchreai. The presence of gypsum in these samples in the absence of pigment indicates the implication of SO2 in the deterioration and makes it possible to suggest that the pigments in the pictorial layer can serve as a protection for the calcite preparatory layers.