Spectroscopic assessment of the UV laser removal of varnishes from painted surfaces

In the present work, we focus on the assessment of the chemical and physical side effects induced by UV laser ablation of varnish samples (i.e. mastic, oil-mastic, dammar, and bleached shellac) after natural curing and artificial ageing using micro-Raman spectroscopy, laser-induced fluorescence and environmental scanning electron microscopy. The varnish films were irradiated with the fifth (213 nm) and fourth (266 nm) harmonics of a Q-switched Nd: YAG laser. Besides the systematic microscopy inspection of the irradiated areas, a significant effort was devoted to the study of the optical properties of the varnish films by means of UV-Vis absorption spectroscopy, which allowed the determination of linear absorption coefficients at the present laser irradiation wavelengths. Single-pulse laser ablation thresholds were measured by applying the spot regression method, and processing of the films was carried out using three different scanning speeds that resulted in 1, 5, and 10 laser pulses on each irradiated area. The results achieved using laser fluences well above the single-pulse ablation threshold of the treated varnishes, indicated that the quality of the material removal process crucially depends on the irradiation wavelength, type of varnish, and on its degree of polymerization. In contrast with the undesired modifications observed at 266 nm, that led to the formation of microbubbles, which produced a whitish appearance of the irradiated area, the absence of relevant side effect at 213 nm emphasizes the importance of using a highly absorbed wavelength to finely remove the altered uppermost layer of different types of aged varnishes.