Yellowing effect and discoloration of pigments: experimental and theoretical studies
Abbreviated Journal Title
J. Cult. Herit.
Yellowing effect; Discoloration; Light scattering; Encrustation on; marble; Laser cleaning; Structural modification; Laser ablation; Archaeology; Art; Chemistry, Analytical; Geosciences, Multidisciplinary; Materials Science, Multidisciplinary; Spectroscopy
Two issues of great interest in the field of lasers in artwork conservation are the so-called yellowing effect and the discoloration of pigments. We have viewed these issues from a comprehensive point of view, considering all our present experimental results as well as ongoing modeling and theoretical calculations. The first concern to be discussed is the yellowing effect in laser cleaning of marble or stone artifacts. Although, in most cases, a yellowish layer exists underneath the black encrustation, the so-called 'patina', it has become clear that there are situations where yellowing cannot be attributed to an existing layer. In the present study, a light scattering model that may account for the yellowing is presented. This model considers a thin absorbent layer and the surface roughness and/or created voids and accounts for the reflectance spectra measured by (i) hyper-spectral imaging and (ii) integrating sphere. Additional experimental data, such as the absence of yellowing when the third harmonic of a Q-swihed Nd:YAG laser is used, support this model. A thorough understanding of the quantitative characteristics of pigment discoloration, on the other hand, has been attempted by means of X-ray diffraction and theoretical studies. The model developed suggests a nucleation process for cinnabar resulting in a structural modification within the volume of a pigment's crystal or particle close to the 'ablation front', which extends for a few nanometers from its surface. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.
Journal of Cultural Heritage
"Yellowing effect and discoloration of pigments: experimental and theoretical studies" (2003). Faculty Bibliography 2000s. 4152.