Effect of wavelength on the laser cleaning of polychromes on wood

Abstract

The effect of the wavelength associated with the laser cleaning of polychromes on wood was investigated by using the four harmonics of a Q–switched Nd:YAG laser (1064, 532, 355 and 266 nm). This type of pictorial artwork is very abundant in Spain and substantial effort is directed towards its conservation. Fragments of a XIV century polychrome wooden ceiling of a chapel inside the castle of Mesones de Isuela in Zaragoza were selected for this study. The flat surface of the samples, decorated with red, green, yellow and black paints was covered with dark deposits and polymerised dirt. The modifications induced on the surface of the samples by laser irradiation were studied using optical and vibrational spectroscopies, such as laser-induced fluorescence (LIF), laser-induced breakdown spectroscopy (LIBS) and Fourier-transform Raman (FT-Raman), and infrared (FT-IR). Irradiation in the UV at 266 nm resulted in efficient cleaning of the red (vermilion), green (verdigris) and yellow (orpiment) painted areas. LIF and LIBS spectra showed a relative increase of the pigment features, fluorescence bands and characteristic atomic emissions, respectively, associated with cleaning. Longer wavelengths induced discoloration, although no signs of degradation of pigments or binders were identified. The present work also presents further evidence on how the combined use of the above techniques serves to identify the composition of the paint mixture, including pigments and other inorganic and organic compounds.