In situ non-invasive characterization of pigments and alteration products on the masonry altar of S. Maria ad Undas (Idro, Italy)

A non-invasive characterization study has been performed and here presented for the first time on the masonry altar of S. Maria ad Undas, a parish medieval church on the Idro (Brescia, Italy) lakeshore. The determination of painting materials and of alteration products represent the one of the first steps, together with art history studies, of a wider project aimed to the valorization of the site. Images collected under UV light in fluorescence and reflectance mode provided useful information about the presence of organic residual materials attributable to the application of lost gilding details, while the readability of some particulars was greatly improved with respect to what observable in visible light. Moreover, near infrared (NIR) images led to hypothesize the presence of green earths in green painted areas. Raman and reflectance spectroscopy allowed the identification of the pigments and of several alteration products, such as plattnerite, which derived by the degradation of the lead-based ones, hydromagnesite, gypsum, and niter, as well as of carbon-based depositions.