Nd: YAG vs Er: YAG: a comparative study of laser varnish removal on easel paintings

摘要

The cleaning of paintings is carried out when an artwork legibility is jeopardized. This can be due to inevitabledeposits on their surface or by a modification of the varnish optical properties as a result of ageing. For the mostdelicate restoration cases, laser processing was widely studied as an alternative to traditional techniques ofpainting cleaning. It encompasses two distinct approaches: the first one involves photo-thermal disaggregationof the unwanted material with an Er:YAG laser (2.94 μm) and eventually its chemical or mechanical removalwhile the second one involves ultraviolet laser ablation, generally with an excimer (193, 248 or 309 nm) or ahigh order harmonic Nd:YAG laser (213 or 266 nm). This study proposes a comparison of these two laserbasedtechnique for removal of an urea-aldehyde resin, Laropal®A81, widely used as a binder or varnish sincethe 90’s, but whose interaction with a pulsed laser was never studied. For this purpose, an Er:YAG laser (El.EnLightBrush2 2.94 μm 500 mJ) and a 4th harmonic Nd:YAG laser (Quantel CFR 266 nm 50 mJ) were used. Thecleaning procedure was monitored in both cases by optical microscopy, spectral domain-optical coherencetomography and UV-induced fluorescence. Results showed that both sources can give satisfying results whenoperated in optimal conditions.