Dielettrometria in campo evanescente, applicazioni ai Beni Culturali - Evanescent-Field Dielectrometry (EFD), application in the Cultural Heritage

Evanescent Field Dielectrometry (EFD) uses microwaves for sensing the dielectric properties of the materials, such as bricks, mortar, plaster , etc., which constitute a wall. The dielectric properties of such materials are very sensitive to the water content, which influences the dielectric constant, and to the presence of soluble salts which affect the effective electrical conductivity. Essentially, EFD is a dielectric spectroscopy limited to the microwave frequency range and, as such, it measures the dielectric contrast in a composite material containing water. An EFD instrument known as SUSI (from the Italian acronym Strumento per la misura di Umidità e Salinità Integrato), developed by the Italian National Research Council, has been patented in USA and in the European Community. The instrument allows a semi-spherical volume of about 2 cm radius to be probed, providing in real time the absolute moisture content (MC, from 0 to about 20%) in such a volume and giving semi-quantitative information about the presence of soluble salts in it, in terms of a Salinity Index (SI). The EFD technique is rather fast (a few seconds per point) and allows to obtain the distribution of moisture inside a wall (in the sub-surface) and also to investigate the dynamics of water diffusion and to provide drying profiles that can be compared to those obtained with other techniques, e.g. the contact-sponge method. Both MC and SI can be processed as a false-colour density-map or a contour-map. The quickness of the measurement and the real-time elaboration allow maps to be obtained in quite a short time. The calibration of the EFD instrument has been performed in the laboratory on material samples (plaster, mortar, bricks, etc.) of varying different moisture content and containing known concentrations of various nitrates, and can be easily performed on any material by measuring the dielectric response in dry and saturated conditions. A theoretical study of how water mass transfer influences the dielectric properties of a wall has also been carried out to make EFD a quantitative technique. EFD has been applied in several monitoring and diagnostics situations, in particular for the study of the moisture conditions and the detection of soluble salts in frescoes. Recent on-site application has concerned the Oratory of San Rocco (Cornaredo, Milan), the Affresco della Fecondità (Massa Marittima, Grosseto), the Church of S.Ignazio (Ponte in Valtellina, Sondrio), and the Cathedral of Monza.