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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.

Varnishes are employed in painted artworks for protection from atmospheric pollution and oxidation and for improving the esthetic appearance by providing an even and brilliant surface finish. Also, in some cases, colored varnishes are applied intentionally to modify the chromatic appearance of paintings. Varnishes undergo complex and differentiated structural and chemical changes over time depending on their composition and conservation conditions. The present work investigates the extent of degradation of the outermost layers of varnish coatings due to aging as a function of depth by using nonlinear optical microscopy (NLOM) in the modality of multiphoton excitation fluorescence (MPEF). This non-invasive technique has been employed for the determination of the thickness, with a microscopic axial and lateral resolution, of the affected regions of pictorial terpenoid varnish layers, such as dammar, mastic, shellac and sandarac, subjected to various types and degrees of aging, artificial, natural and a combination of both. A homemade nonlinear optical microscope, based on a tightly focused pulsed femtosecond laser emitting at 800 nm, was used for the investigation. Single-photon laser-induced fluorescence (LIF) measurements served to determine the degree of surface aging and the optimum NLOM-MPEF operating conditions and helped to interpret the results obtained by applying the latter. These results signpost the correlations between the extent of degradation, non-invasively determined by NLOM-MPEF, the type and length of aging, the nature of the varnish and the initial thickness of the varnish layer.