# Beyond the surface: Raman micro-SORS for in depth non-destructive analysis of fresco layers.

Chiriu D., Ricci P.C., Fiorino D.R., Grillo S., Carbonaro C.M.

Comunicazione
VI - Fisica applicata, acceleratori e beni culturali
GSSI Ex ISEF - Aula A - Mercoledì 25 h 10:00 - 13:00
We applied microscale spatially offset Raman spectroscopy (micro-SORS) to perform the analysis of the layer composition in a fresco of the San Giuseppe church in Cagliari. Standard Raman analysis of the surface evidenced the presence of expected pigments in the final plaster layer, such as hematite and gypsum, beside the contribution of calcite. The latter is observed in its anhydrous form on the surface (main Raman peak at 1024 cm${}^{-1}$) and in its hydrate form (1006 cm${}^{-1}$) from the substrate layers. With micro-SORS analysis we evaluated the thickness of the different layers of the plaster by monitoring the intensity ratio of the calcite and the anhydrous gypsum. Four different layers with separation lines at about 80 $\mu$m, 600 $\mu$m and 2400 $\mu$m in depth were evidenced, corresponding to the painting layer (down to 80 $\mu$m), the plaster (down to 600 $\mu$m), the arriccio layer (down to 2400 $\mu$), and the rinzaffo layer (below 2400 $\mu$m). The cross-sectional analysis by SEM imaging of a fragment from the fresco confirmed the micro-SORS findings, promoting the technique of a new non-invasive imaging tool for cultural heritage.