Multi-Analytical Investigation of the Frescoes in the Hypogean Monumental Complex of Santa Maria della Grotta (Marsala, Italy): Materials, Techniques and Conservation state

This work reports the results of a multi-analytical, non-invasive diagnostic campaign carried out on the medieval frescoes and later paintings dating up to the 17th century of the hypogean monumental complex of Santa Maria della Grotta (Marsala, Italy). The study aimed to assess their conservation state and analyse the execution technique, with particular focus on the colour palette and the pigments employed by the artists. The research combined in situ investigations using portable instrumentation with laboratory analyses performed on selected micro-samples. Several techniques, such as multispectral imaging (visible light, UV fluorescence, IR reflectography, blue-induced fluorescence), infrared thermography, X-ray fluorescence (XRF), Raman spectroscopy combined with surface enhanced Raman spectroscopy (SERS) and fiber-optic reflectance spectroscopy (FORS) were used to identify the pigments. The use of a predominantly mineral palette was documented, including yellow and red ochres, cinnabar, realgar, earth pigments, and carbon black. In specific areas, cinnabar also exhibited localized darkening. A microclimatic monitoring was performed for 1 year. The frescoes, executed on a porous calcarenite substrate, exhibit a critical conservation condition. Detachments and lacunae affect both the structural layers and the pictorial film; widespread efflorescence and sub-efflorescence phenomena are present, together with decay caused by moisture and uncontrolled thermo-hygrometric fluctuations; chromatic alteration and staining occur as a result of saline patinas. X-ray diffraction of efflorescence highlights the predominance of sulphate-rich salts, mainly gypsum, closely associated with high relative humidity and capillary rising damp. The integrated results offer a comprehensive understanding of the materials, execution technique and degradation processes, providing a robust analytical basis for the development of future conservation strategies tailored to frescoes in high-humidity underground environments.