EXCIMER-LASER RESTORATION OF PAINTED ARTWORKS - PROCEDURES, MECHANISMS AND EFFECTS

Excimer laser ablation is demonstrated to afford a novel, highly effec tive 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 t o be appropriate for achieving sufficient etching rates in combination with good surface morphology. With proper optimization of the irradia tion parameters, excimer laser cleaning can be a highly selective proc ess. surpassing by far the degree of selectivity afforded by tradition al restoration methods. Furthermore, a number of different laser analy tical techniques can be used for on-line monitoring and control, there by safeguarding against damage. Broadband reflectography, providing st ructural information about the uncovered layers, and laser-induced bre akdown spectroscopy, providing information on the elemental compositio n of the ablated material, appear to be particularly effective in this respect. Furthermore, the plausible consequences of the laser irradia tion on the state of the paintings are investigated in experiments inv olving model and realistic systems. In particular, the importance of p hotochemical effects is addressed by chromatographic analysis of irrad iated realistic samples for the detection of photoproducts, and by exa mining the degree of photolysis of photosensitive dopants incorporated in model samples. Potential consequences of laser-induced photomechan ical 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 insi gnificant. (C) 1998 Elsevier Science B.V.