Evaluating the effectiveness of consolidation through laboratory and in situ tests for the conservation of frescoes in hypogean environments of Marsala (Italy)

The conservation and restoration of frescos located in hypogeal environments require specifically tailored strategies to address critical thermo-hygrometric conditions, such as high humidity, poor ventilation, con- densation, and capillary rise. These factors often lead to the accumulation of sulphate salts both on the surface and within their inner layers. Under such microclimatic conditions, the choice of a suitable con- solidant is crucial to ensure material compatibility and long-term breathability. This study reports the results obtained by using barium hydroxide nanoparticles in hydroalcoholic solution for the consolidation of frescos in hypogeal environments, through a comparative evaluation with nanolime and nanosilica. The research involved both laboratory tests on artificially aged mock-up samples simulating frescoes and in situ applications on frescoes in the hypogea of Santa Maria della Grotta and of Crispia Salvia (Marsala, Italy). To evaluate the performance of the nanostructured consolidants, a series of standardized tests were carried out, including capillary water absorption, water vapor permeability, contact angle measurements, colorimetric analysis, surface cohesion (scotch tape test), and SEM-EDS analysis on the mock-ups. In situ, the consolidation effectiveness was evaluated using colorimetry, contact sponge test, and Scotch tape test, followed by a 12-months monitoring campaign. The results provide valuable insights into the relative per- formance, compatibility, and limitations of each consolidant under hypogeal conditions, contributing to the development of more effective conservation strategies for frescoes affected by moisture and sulphation.