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Nd:YAG Q-Switched laser cleaning at 1064 nm can sometimes lead to a more yellow appearance of the stone surface in comparison with other cleaning techniques. The yellow hue can originate from different contributions among which the presence of nano-sized residues generated by the laser interaction with the surface materials to be eliminated. In this study, the nature of such residues has been investigated. The analyzed materials are (i) particles collected from a pure gypsum reference plate; (ii) a synthetic crust, composed of 80 wt% natural black crust and 20 wt% synthetic gypsum; (iii) particles ejected from the synthetic crust during laser irradiation. Optical, scanning electron and transmission electron microscopies were used to link color changes at the macro-scale to morphology changes at the submicronand nano-scales. Chemical composition was also obtained at the nano-scale using TEM coupled with energy dispersive X-ray (EDX) spectroscopy. This multi-scale approach was combined with electron paramagnetic resonance spectroscopy (EPR) analysis at low and room temperatures to examine the possible presence of iron-containing species presenting particular magnetic properties in the sample before and after irradiation. Under laser irradiation, both the crust sample and the ejected gypsum particles take a yellow color. This color shift can be linked with morphology changes occurring at the nano-scale: gypsum crystals from the reference plate show a smooth surface, while those coming from the synthetic black crust are, after irradiation, covered by many spherical nanoparticles and a rough nano-layer ranging from less than 20 nm to more than 100

Laser yellowing effect: study of the nanophases created by laser irradiation of synthetic black crusts using transmission electron microscopy (TEM) and electron paramagnetic resonance (EPR) spectroscopy