Laser irradiation may alter the colour of certain pigments used in artworks. 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 microscopy were used to study the crystalline structure of
selected irradiated pigments in order to identify the changes that take place upon laser
irradiation. Two representative pigments were studied, cinnabar (HgS) and lead white
(Pb(OH)2 2PbCO3). The results revealed a change in the crystalline phase or chemical
composition, respectively, which can be barely detected owing to its superficial character,
which is of a sub-micrometer scale.