Laser interaction with polychromy: laboratory investigations and on-site observations

Abstract

The impact of laser cleaning of soiled natural stone surfaces possibly with underlying polychromy was studied by means of laboratory samples and during the cleaning of a Gothic tympanum at St Martin’s Church in Brunswick, Germany. Paint layers with pigments (1) frequently encountered in historic polychromies and (2) applied with different binders to cardboard strips and stone tablets (and subsequently coated with an artificial black gypsum crust) were irradiated. Laser light at different wavelengths and fluences was used to (a) determine fluence threshold values for pigment alteration, and (b) to determine if the artificial gypsum crust can be ablated from the paint layers without discolouring them. The wavelength-dependent diffuse reflection and scattering of light were measured for the chosen pigment/binding medium combinations and turned into absorption coefficients according to the Kubelka–Munk theory. These data served as a basis for theoretical considerations about the laser–pigment interaction with respect to less critical wavelength regimes. For the pigments under investigation, the following sequence in order of sensitivity to laser irradiation from highest to lowest was found: vermilion, massicot, lead antimonate (Naples yellow), minium, malachite, red ochre, yellow ochre, azurite, smalt, green earth. In the case of malachite, azurite, minium, and vermilion, the chemical reaction which brings about the colour change was deduced from the X-ray diffraction curves of irradiated and altered pigment powders. Observations made during trial laser cleanings in the laboratory and on-site suggest that historic polychromies are more endangered by the loss of pigment flakes than by discoloration. The role that the binding medium and ageing effects play in the interaction of the laser with polychromies must be the subject of further investigations.