New perspectives for the inhibition of salt damages in mural paintings in hypogeal contexts: the case of Saint Augustine in the rupestrian church of San Pietro Barisano (Matera, Italy)

This paper focuses on research aimed to reduce salt damage on architectural decorated surfaces by using salt crystallization inhibitor products. A phosphonate product, namely aminotris(methylenephosphonic acid) (ATMP) at 0.1 percentage concentration, was selected for application in real conditions. A pilot area of the fresco painting within the church of San Pietro Barisano (Matera, Italy) was chosen for on-site testing. A comprehensive methodology was adopted, combining laboratory and in situ investigations. Prior to the application, the materials of the wall painting were identified by XRD, SEM-EDS, FTIR. Salts, as well, were analysed with these techniques and by ion chromatography, and calcium sulphate was found prevalent. Attention was also devoted to verify the compatibility of the product solutions with some pigments and determining the application method. Before and after the application on-site, a thermo-hygrometric monitoring within the chapel was performed and the painted surface was periodically checked by visual observations. Results indicate a reduced salt damage in the treated areas, compared to the untreated ones. These findings support that salt inhibitor treatments could be an innovative approach to the preventive conservation of mural paintings, especially in hypogeal contexts, where the salt source is difficult to eliminate, and reducing salt damage could be an effective strategy. However, longer-term checks of the treatment performance would be appropriate to confirm these findings and fully to assess the effectiveness.