Excimer laser restoration of painted artworks: procedures, mechanisms and effects

Excimer laser ablation is demonstrated to afford a novel, highly effective method of restoration of painted artworks. The application relies on the strong UV-absorptivity of these substrates ensuring efficient material removal, with minimal light penetration to the sublayers. On the basis of structural and analytical examinations, 248 nm is shown to be appropriate for achieving sufficient etching rates in combination with good surface morphology. With proper optimization of the irradiation parameters, excimer laser cleaning can be a highly selective process. surpassing by far the degree of selectivity afforded by traditional restoration methods. Furthermore, a number of different laser analytical techniques can be used for on-line monitoring and control, thereby safeguarding against damage. Broadband reflectography, providing structural information about the uncovered layers, and laser-induced breakdown spectroscopy, providing information on the elemental composition of the ablated material, appear to be particularly effective in this respect. Furthermore, the plausible consequences of the laser irradiation on the state of the paintings are investigated in experiments involving model and realistic systems. In particular, the importance of photochemical effects is addressed by chromatographic analysis of irradiated realistic samples for the detection of photoproducts, and by examining the degree of photolysis of photosensitive dopants incorporated in model samples. Potential consequences of laser-induced photomechanical effects are addressed via the use of holographic interferometry. In all, these experiments indicate that optimal fluence ranges can be defined in which damaging effects to the substrate are minimal or insignificant. (C) 1998 Elsevier Science B.V.