Reaction of Historical Colours and their Components Irradiated at Different Nd:YAG Laser Wavelengths (ω, 2ω, 3ω, 4ω)

Cleaning of artworks with laser radiation may become a delicate situation in cases where a paint layer is directly underneath the undesired top layer. However, this is the situation in many applications of restoration, e.g. cleaning of paintings. It is known in general that historical colours may react very sensitive on illumination. Within an extensive study first we investigated separately the behaviour of 11 pigments and 3 binding materials under laser irradiation. Then in a next step we investigated the behaviour of 33 historical colours, consisting from a combination of pigments and bindings mentioned above. The experiments have been carried out using four different wavelengths from a Nd:YAG laser with optional frequency multiplier (1064 nm, 532 nm, 355 nm, 266 nm). At various laser fluences and wavelengths the corresponding colour change was analysed by CIE-L*a*b*-System. As a result of our study we have built up a data base containing threshold fluences for various historical pigments, bindings and their combinations.