Knock-knock, Who’s there? Fresco

Fresco painting, in which an artist applies dry pigments to wet plaster, dates from antiquity. A common mode of failure occurs when the decorative plaster layers separate from the masonry substrate or lower structural plaster. Our research team has for many years used scanning laser Doppler vibrometry to identify regions of delamination of frescos in the US Capitol Building as part of a conservation effort. Historically, such regions have been mapped by experienced conservators who knock on the plaster with their knuckles or small mallets and make subjective assessments of the radiated sound to identify areas needing restoration work. These acoustic signatures carry substantial information about the nature of the structure that produces the sound. Modern signal processing techniques can shed light on the underlying mechanical behavior of the system from those recorded acoustic signatures. This research will enable the use of simple, free smartphone or PC apps, and simple algorithms to put the conservator’s expertise at anyone’s disposal.