Analytical spectroscopic investigation of wavelength and pulse duration effects on laser-induced changes of egg-yolk-based tempera paints.

The application of laser cleaning methodologies to light-sensitive substrates such as those encountered in artistic paintings is an extremely delicate issue. The cleaning of paintings and polychromes is an irreversibly invasive intervention; therefore, prior to the implementation of laser cleaning methodologies, a thorough characterization of the interaction between laser pulses and painting components is required. In this work, the modifications induced by irradiation with pulses of 150 picoseconds (at 1064 and 213 nm) and 15 nanoseconds (at 213 nm) on unvarnished aged model egg-yolk-based paints were examined following a spectroanalytical approach. Laser-induced chemical changes on samples of unpigmented and widely used artist's pigment temperas were investigated by spectrofluorimetry and Fourier transform Raman spectroscopy, while color changes were quantified by colorimetry. Noticeable modifications of the Raman and fluorescence bands attributed to pigments are absent except for vermillion, for which the pigment bands tend to disappear upon irradiation at 1064 nm. Interestingly, no discoloration was observed on most of the pigments upon irradiation at 213 nm (150 ps), including the light-sensitive vermillion, while no indications of carbonization or charring of the paint layers, which could give rise to amorphous carbon bands, were obtained at any of the irradiation conditions explored. Comparison of the results using the two different pulse durations and wavelengths illustrates the participation of mechanisms of diverse origin according to the chemical composition of the pigment and highlights the importance of the optimization of the laser parameters, specifically fluence, pulse duration, and wavelength, in conservation treatments of paintings.