The laser cleaning of wall paintings

Abstract

The objective of this work has been the study of the laser cleaning technique when applied to wall paintings. In particular, this study has been devoted to understanding how the characteristics of the laser apparatus and its specific use are linked with the nature of the substances to be removed from the painting. A number of paintings were selected with paint layers that were composed of different pigments such as lapis lazuli, lamp black and ochres. They had been applied using the fresco technique or a secco natural resins, on surfaces that had been treated with organic (natural resins, drying oils, caseinate glues, etc.) and inorganic substances (calcium carbonate or oxalate) applied during previous restorations. The analyses carried out were the following: FTIR spectroscopy with and without microscope and the study of thin sections of the surface of the paintings. The apparatus we used was a Nd:YAG laser operating in Q-switch or in normal mode; in Q-switch operation the laser could emit at two different wavelengths: in the infra-red and in the visible (1 064 and 532 nm). The main objective of the work was to determine the optimum working conditions for the substances to be removed, through the establishment of laser parameters such as the pulse modality (Q-switch or N-mode), the wavelength, the repetition rate and the energy density. The effects of the laser radiation on the surfaces of the paintings were checked by measuring temperature, pH, conductivity and spectro-colorimetric tests.