Non destructive evaluation of glaze delaminations in glazed ceramic tiles: laboratory tests

Glazed ceramic tile panels, azulejos, is a very distinctive character of the extraordinarily rich heritage of Portugal. Particular attention is therefore required to study the causes of the decay process, the progress of its effects, and the definition of suitable procedures concerning the conservation practice. Issues related to the presence of glaze delamination are taken into account, attempting to find some possible solutions to the early detection of sub-surface air cavities. In the present paper the basic approach for an extensive laboratory validation of two non contact techniques employing an acoustic excitation source is presented. Using the same acoustic excitation source, an innovative highly directive loudspeaker, the tests provided the velocity of the induced vibrations in a collection of samples by means of a Laser Doppler Vibrometer, correlated to the presence of acoustic energy absorption revealed by means of the Acoustic Absorption diagnostic system. This study compares the acoustic response of a set of historical azulejo samples and of laboratory models with artificial delaminations, and collects preliminary results useful to optimize the experimental settings for a more extensive validation of the acoustical methods.