Photoacoustic real-time monitoring of UV laser ablation of aged varnish coatings on heritage objects

A novel, simple and non-invasive diagnostic methodology is presented herein, as an effective approach to monitor in real-time the UV laser-assisted thinning of aged varnished coatings from heritage objects, using the acoustic signals generated upon light interaction with matter. Detection of the photoacoustic (PA) signals is realized by means of a non-contact air-coupled ultrasonic transducer with a frequency response in the MHz regime. This proposed statistical approach utilizes the information the PA signals provide during the laser ablation process, embedded in their temporal delay and amplitude reduction during varnish removal. Based on the cross-correlation of the PA signals, this monitoring technique determines the critical laser pulse, responsible for complete varnish removal. The efficiency of our methodology has been examined on a series of aged varnish mock-ups on different substrates. The laser treated surfaces were further assessed using various techniques, in order to confirm the results of our PA monitoring strategy. Application of this technique is aimed at validating its potential and suitability in addressing different varnish cleaning challenges. Based on previous encouraging results on encrusted stonework, primary objective of this study is to establish the PA monitoring methodology as an essential, real-time monitoring tool for controlled laser ablation of aged varnish coatings from heritage surfaces (paintings, metallic objects, wall paintings etc.), ensuring their efficient removal while preserving the integrity of the objects. Implementation of the PA technique may pave the way for numerous applications as regards the reliable monitoring of removing unwanted layers from various surfaces of interest.