An integrated approach to mapping moisture and salt content in two frescoes in the Basilica of San Clemente

Two physical non-invasive techniques bave been used to measure the moisture content in two frescoes in the Basilica of San Clemente in Rome: evanescent-field dielectrometry (EFD) and unilateral nuclear magnetic resonance (uNMR). Both techniques are portable and allow mapping the moisture content in frescoed walls and murai paintings. The integration of the two techniques results in a true synergistic effect, by exploiting their peculiar characteristics: uNMR is inherently independent on the materia!characteristics, as it is directly sensitive to the water content, and allows the obtainment of a 2D map of the water content relative to a depth of a few millimeters; EFD measures the sub-superficial moisture content (up to about 2 cm) and allows the detection of soluble salts in the wall, but its response slightly depends on the material density, as moisture and salt content are obtained from the dielectric properties of the wall. Moreover, uNMR operates without needing to contact the surface of the fresco, and thus allows the obtainment of information also about criticai regions where, for example, there is a risk of det chment of the painted layer. EFD, that needs a direct contact with the wall, is very fast (a few seconds per measureod point, with in-line elaboration), also allowing the study of the dynamics of water absorption after a consolidation treatment. The integration between the two techniques allows the full characterization of a fresco with regard to the water content and a screening of the locations where an invasive chemical analysis of the salts is in order.