APPLICATIONS OF A FREQUENCY RESOLVED ACOUSTIC IMAGING TECHNIQUE IN THE AUDIO FREQUENCY INTERVAL: FRESCOES, CERAMICS, PANEL PAINTINGS

The development of innovative and cost-effective non destructive techniques for paintings diagnostics may highly support their preventive conservation. Some applications of an acoustic imaging technique, whose current equipment is named ACoustic Energy Absorption Diagnostic Device (ACEADD), are here reviewed: after almost two decades of laboratory tests and on site investigations since the invention, today a reasoned overview of this activity helps a thorough understanding of the applicability of this technique for different typologies of paintings. The acoustic method reveals detachments of the painted film and other kinds of structural damage in paintings. The technique evaluates the absorption of the acoustic energy occurring in a damaged artefact when this is irradiated with an acoustic wave, while the non-contact instrumentation scans its surface. The validation on laboratory models of frescoes, containing artificial detachments, and successive tests on real frescoes evidenced the agreement of the resulting acoustic images with the traditional restorers' inspection, and with IR thermographs under particular conditions. On glazed ceramic tiles panels, the potential of the method was explored for detecting the tile/mortar adhesion failure and the delamination of the glazed layer from the clay body. Laboratory validation has recently been undertaken using suitable models with artificial delaminations. The acoustic response of panel paintings was recently investigated: a first collection of frequency resolved acoustic images of panel paintings shows interesting features, potentially related to critical conditions of both the wood substrate and the painted layer. A composite experimental activity has evidenced that the ACEADD method reveals some forms of structural damage in multilayer structures, although for each class of paintings a peculiar acoustic response emerges and the advancements are at different phases. In any case a similar approach based on metrological principles has been adopted, encompassing the laboratory validation on artificial models, the field experimentation, a specific data analysis, and the comparison with other methodologies.