Discoloration of pigments induced by laser irradiation

Laser irradiation may alter the colour of certain pigments used in artwork. Owing to the increasing use of lasers in artwork conservation this problem becomes extremely crucial, especially in the case of a polychromy covered by environmental encrustation. X-ray diffraction analysis and optical micro scopy were used to study the crystalline structure of selected irradiated p igments in order to identify the changes that take place upon laser irradia tion. Two representative pigments were studied, cinnabar (HgS) and lead whi te (Pb(OH)(2).2PbCO(3)). The results revealed a change in the crystalline p hase or chemical composition, respectively, which can be barely detected ow ing to its superficial character, which is of a nanometre scale.