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Moisture and soluble salts are the main causes of degradation of mural paintings, in particular, frescoes. Water is the ‘driving force’ of damage such as the detachment of the painted layer and the whitening of the painting due to the crystallization of salts (efflorescence). Indeed, the appearance of efflorescence itself is related to the alterations caused by moisture in the process of the evaporation of water through the surface of the wall. Early detection of the presence of moisture under the wall surface is therefore essential for avoiding such kind of damage. In this paper a non-invasive microwave system is described which allows the measurement of the moisture content and the detection of salts in frescoes and mural paintings. The system performs a sub-surface measurement with an investigated depth up to about 2 cm. The measurement system consists of an evanescent-field resonant sensor, a network analyser and a numerical code. The method has been validated by measurements performed on some reference materials with known dielectric characteristics. Several tests on moistened plaster samples, some of them containing calcium nitrate at different concentrations, have been performed in order to verify the effectiveness in quantifying the moisture and salt content. In situ investigations have been carried out by measuring both the moisture content and salt content on frescoes in several museums and churches. The preliminary results prove the usefulness of the method as a diagnostic tool for investigating the health status of frescoes.

An open-coaxial probe suitable for measuring the permittivity of solid materials has been designed and tested. The peculiar structure of the probe allows an easy and firm contact with the surface of the material under measurement, resulting in highly reliable and reproducible dielectric measurements. The permittivity measurement is absolute, as the calibration procedure - based on a genetic algorithm - does not require the use of dielectric standards. The measurement system - including the probe, a network analyser, and the numerical code to determine the permittivity from reflection measurements - have been tested on solid materials of known properties, and used for determining the dielectric characteristics of several types of wood.

Many applications of microwave energy to wooden materials have been developed in the last few decades, both for treatment and for diagnostic purposes. All these applications require a reliable estimation of the permittivity of the wood species of interest, which is the physical parameter of crucial importance in the absorption of electromagnetic energy. This paper presents results obtained in the dielectric characterization of five wood species in the frequency range from 2 to 3 GHz, including the ISM frequency of 2.45 GHz. Permittivity was measured by an open-ended coaxial-line probe of new design on wood samples conditioned at several moisture levels. The influence of the natural variability of wood characteristics on the measured permittivity was also investigated by a suitable experimental setup consisting of a poplar table including both sapwood and heartwood regions. Finally, a theoretical discussion on the meaning of a scalar measurement on anisotropic dielectrics is conducted in terms of an isotropic-equivalent permittivity, which is related to the permittivity tensor of the dielectric material.