Study of water content in fresco mock-ups in the microwave and near-IR spectral regions

Water diffusion inside walls and masonry is responsible for the majority of the decay phenomena observed in wall paintings and frescos. Thus the diagnostics of moisture and water content and their monitoring represent a key issue as regards art conservation. In order to preserve the integrity of surfaces of artistic interest and architectural structures, investigations by means of non-destructive techniques (NDT) are preferred over others. The aim of this research is to determine methodologies to quantify the moisture content of fresco by means of the integrated use of two non-invasive techniques, namely Fiber Optic Reflectance Spectroscopy (FORS) in the near-IR region and Evanescent Field Dielectrometry (EFD) in the microwave range. The FORS technique has been employed in order to assess the amount of water adsorbed from the surface by means of the analysis of the reflectance spectra acquired in the Vis-NIR (350-2200nm) range. This technique investigates the electronic and vibrational transitions that are characteristic of each compounds and enables their identification. The water content is evaluated on the basis of the absorption band at 1920 nm (the combination of stretching and deformation vibrations for H2O) and 1450 nm (the first overtone of the OH stretching vibration). The EFD system consists of a resonant probe that is connected to a network analyzer. The resonance frequencies of the cavity under different moisture content conditions of frescos are such that the measurement refers approximately to the 1.0-1.5 GHz range. The Scalar Network Analyzer (SNA) measures the transmission through the probe and sends the measured data (amplitude vs frequency) to a netbook that computes the moisture content in real time. Fresco mock-ups have been prepared in collaboration with the Opificio delle Pietre Dure (OPD) in order to recreate most of the possible chromatic shades obtained by mixing iron oxides and hydroxides based pigments. Measurement were performed by means of both techniques on fresco models after wet-dry cycles by using nebulizer and poultices that had a known water content. The results obtained with these two techniques were compared, and cross relationships between the EFD and FORS data were outlined.