Varnish-coated oil paints under laser treatment: Comparative surface- and depth-resolved assessment of 213 nm (15 ns), 266 nm (15 ns), and 2940 nm (160 µs) ablation regimes

This study employs a multi-analytical, non-invasive approach to compare the physico-chemical modifications induced by 213, 266 nm (15 ns pulse duration) and 2940 nm (160 µs) lasers in the removal of varnish films from oil paintings. Artificially aged oil paint films coated with dammar and sandarac varnishes were irradiated at threshold and supra-thresholds fluence levels. Ablation depths, residual varnish thickness ( z var ) and structural changes following laser processing were non-destructively quantified via Multiphoton Excitation Fluorescence (MPEF) depth profiling. Differential Interference Contrast (DIC) microscopy revealed that morphological effects are highly wavelength-dependent. While 266 nm and 2940 nm pulses induced surface texturing via photothermal mechanisms, 213 nm irradiation promoted photolytic bond cleavage, resulting in controlled, micrometric layer-by-layer ablation. Diffuse Reflectance FTIR (DR-FTIR) and Laser-Induced Fluorescence (LIF) confirmed that physicochemical modifications may occur at 266 and 2940 nm in the underlying paint when z var is lower than the optical penetration depth. These results provide important insights into laser-matter interactions, identifying 213 nm as an effective method for “dry” surface cleaning of picture varnishes. Conversely, 266 nm and 2940 nm lasers are better suited for integration into hybrid, solvent-based protocols, where the laser facilitates subsequent removal by mild cleaning agents.