This investigation is concerned with the effects of UV laser light on inorganic pigments, as extensively used in easel paintings. It has been observed that, under such laser treatment, the two iron oxide pigments raw sienna and yellow ochre darken appreciably. Examination of the X-ray diffraction (XRD) patterns of the pigments has revealed the nature of this discoloration and indicated that alterations to the structure of the crystal lattice of each pigment have taken place. These alterations are probably due to thermal processes which involve the release of molecules of water from the crystal lattice. It has been shown that, although the pigments were initially both composed primarily of the mineral goethite (FeOOH), after treatment the XRD results indicated the presence of haematite (Fe2O3) and possibly magnetite (Fe3O4) as well. Darkening has also been detected, following laser irradiation, of lead chromate, PbCrO4, a pigment that is known not to contain water. After examination by Raman and laser-induced fluorescence spectroscopy, it is proposed in this case that the observed discoloration of the pigment in the binding medium, linseed oil, is due to degradation of the binding medium or impurities therein. (C) 2000 Editions scientifiques et medicales Elsevier SAS
