Quantitative spectroscopic characterization of laser-induced effects on oil paint films using 213, 266, or 2940 nm

Removal of aged varnish layers from easel paintings is a challenging task requiring methodologies capable of preventing any alteration of the underlying painted surface. Laser-ablation approaches using the fourth and fifth harmonics of a Q-Switched Nd:YAG (266 and 213 nm, 15 ns) and Free-Running Er:YAG laser (2940 nm, 160 µs pulses) can allow for very thin removal rates but their discrimination of the underlying paint film and self-termination potential are very limited. Thus, there is a need to develop suitable analytical methods for promptly identifying the possible occurrence of any undesired effect on the paint layer beneath the varnish, which is intentionally or unintentionally irradiated during the laser thinning treatment. Such side effects depend on the irradiation parameters and the specific material features of the pictorial film. Linseed oil paint models were prepared using lead white, vermilion, chrome yellow, ultramarine, Prussian blue and zinc white. Two distinct pigment volume concentrations (PVC) were used to investigate the dependence of the laser-induced alterations on oil binder volume ratio: low PVC (gloss paint) and high PVC (matte paint). After determining threshold exposure limits for each laser wavelength, laser-induced effects slightly above the threshold fluence were qualitatively and quantitatively assessed using FTIR and µ-Raman spectroscopies. Results showed that IR-reflectance bands of glossy and matte films exhibited appreciable shifts and distortion upon laser irradiation while reductions of the signal-to-background ratio were observed in µRaman spectra. The developed method, based on the processing of a number of spectral acquisitions, enabled for the first time the quantitative assessment and comparison of the corresponding undesired laser-induced effects in the outermost painted layer.