Multi-analytical investigation of frescoes in Santa Maria della Grotta, Marsala: materials, techniques and conservation study

This study presents the results of a multi-analytical, non-invasive diagnostic campaign on the medieval frescoes and later paintings (up to the 17th century) of the hypogean monumental complex of Santa Maria della Grotta (Marsala, Italy). The investigation aimed to evaluate their conservation state and characterize the materials and techniques employed, with particular attention to pigments and colour palette. In situ analyses using portable instrumentation were integrated with laboratory investigations on selected micro-samples. Multispectral imaging, infrared thermography, X-ray fluorescence, Raman spectroscopy, surface-enhanced Raman spectroscopy, and fiber-optic reflectance spectroscopy enabled pigment identification. The paintings were found to be executed mainly with mineral pigments, including ochres, vermilion, realgar, earth pigments, and carbon black. Conservation issues include detachments, lacunae, saline efflorescence and chromatic alteration. X-ray diffraction identified gypsum-rich sulphate salts as the predominant efflorescence products. Integrated analytical and microclimatic data provide a comprehensive basis for developing conservation strategies for frescoes in high-humidity underground environments.