The pigments of the frigidarium in the Sarno Baths, Pompeii: Identification, stratigraphy and weathering

In the present research is used a multi-analytical approach to study the wall paintings from the Sarno Baths, located in the southern part of Pompeii. In particular the investigation is focused on the frescos of the frigidarium, though a few samples from other rooms were also analysed. Twenty wall paintings fragments were analysed by laser scanning confocal microscopy (LSCM), optical microscopy (OM), μ-Raman, scanning electron microscope with energy dispersive spectrometry (SEM-EDS), portable X-ray fluorescence spectrometry (p-XRF) and X-ray powder diffraction (XRPD). The XRPD data were used for the mineralogical semi-quantitative phase analyses (SQPA) and the estimation of the hematite crystals size. The obtained data allow the identification of the pigments and the techniques used, and address new question such as whether talc and aragonite were used routinely in the formulation of pigments. The chemical composition of red and yellow pigments is also discussed and compared with data available from the recent literature. The wall paintings are badly preserved and weathering products occur on the pictorial surface. Eight samples of efflorescence salts and patinas were analysed by XRPD: all the samples are composed mainly by alkali sulphates. The systematic difference between the salts present on the northern and the western walls is likely related to the materials inserted during the 19th century restauration.