Airborne acoustic characterisation of cultural heritage paintings: A preliminary study

In order to assess the health of cultural heritage paintings, for example prior to their restoration, visual inspections as well as techniques such as X-ray imaging are commonly used. Non-contact acoustic methods could become another exploration modality thanks to their non-destructive character and the fact that they are expected to be more sensitive to certain defects, such as a delamination, than other techniques. A setup has been designed for such applications, based on airborne ultrasonic transducers operating in the several hundred kHz to over 1 MHz range, specific transmit/receive electronics and automated scanning stages allowing 2D through-transmission scans to be performed on areas up to 80 × 80 square centimeters. Measurements on well-known thin plates made of plastic, wood and gesso on wood were first performed to evaluate the possibilities of the technique, then simple paintings on canvas were tested and finally a real ancient painting was scanned. A method where the transmission amplitude is plotted versus the angle between the propagation axis and the normal to the painting surface is developed for local characterization of the elastic constants of the sample under test, thanks to comparison with simulated Lamb wave dispersion curves using a fitting procedure. These promising results open the way towards comparison between acoustic, radiological and other imaging results on paintings with severe defects that require restoration.