Analysis of the laser-induced reduction mechanisms of medieval pigments

This study is concerned with the effect of infrared Q-switched laser radiation on four of the most commonly used medieval pigments. Vermilion, red lead (minium), white lead and yellow lead (massicot) have been irradiated with Nd:YAG laser radiation at 1064 nm. Visual observation, reflectance spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy analyses have been employed to study the colour alteration, identity the chemical composition and observe the surface of the irradiated pigments. It was shown that, contrary to what was generally accepted, the mechanism responsible for the discoloration is reduction. It was suggested that under the intense laser ablation conditions and the plasma formation, oxidation is not favourable and instead the pigments decompose to their constituent elements. This scenario can also explain the temporary blackening on red and white lead as it is a well established tendency for metallic lead to become oxidised as soon as favourable conditions are obtained. Recovery mechanisms are currently being studied to prove the model.