Non-Destructive Diagnosis of Detachments in the Covers of Ancient Structures

Detecting the presence, the position and the extension of architectural coverings separations is at the base of the analysis of the state of conservation of ancient structures. Non-destructive investigations used in laboratory to find detachments are hardly executable in situ. For this reason, traditionally, conservators rely on the technique of auscultation, which entails gently knocking with the hand, and listening to the sound produced by the surface. Analysis carried out with such method is accurate, but introduces a strong subjective conservator’s connotation, which makes it not repeatable. For these motivations, we introduced an automatic system to detect and measure the detachments. The proposed technique allows measuring entire surfaces, using a sound correlation method between an acoustic “snap” of the point of interest and an acoustic “reference snap” of a point certainly known. The probe is equipped with a PZT force sensor fabricated in our laboratory, to measure the actual impact force of our “hitting device”. The probe is also equipped with a microphone that allows the capture of the audio signal generated by the impact of the hammer on the surface. The audio signal is acquired using a low-cost microcontroller electronics unit, and the cross-correlation index is calculated. A low-cost infra-red camera is located on the measurement scenario, to acquire the XY position of the probe. All these information (X,Y and cross-correlation value) are stored, and are available to visualization software on a PC. The figure shows the measurement carried out on a real mosaic starting by the acquired points. In addition to confirm the detached area visible to the naked eye, it revealed a further detachment area in the peripheral area on the right. The restorer, using the classic method of "knocking" carried out with the knuckles of the hand, confirmed the presence of the defect but obviously without the precision of the method described so far.