LASER INTERACTION WITH COATED COLLAGEN AND CELLULOSE FIBER COMPOSITES- FUNDAMENTALS OF LASER CLEANING OF ANCIENT PARCHMENT MANUSCRIPTS ANDPAPER

Laser cleaning of delicate biological composite materials such as anci ent parchment manuscripts from the 15th and 16th century and printed p aper from the 19th century is demonstrated with an ultraviolet excimer pulsed laser at 308 nm, Laser fluence levels must stay below the abla tion and destruction threshold of the parchment or paper substrate, an d have to surpass the threshold of the contaminant matter. Foreign lay ers to be removed must exhibit a higher optical density than the artif act substrates. Synthetic carbonaceous dirt, modelled by water-soluble black crayons showed a characteristically weak featureless laser-indu ced plasma spectroscopy spectrum near the noise limit. It turned out t hat laser-induced plasma spectroscopy is of limited use in monitoring halting points (or etch-stops) because it relies on the destruction no t only of the laterally inhomogenously distributed contaminant but als o of pigment phases on a microscopically rough parchment substrate. La ser-induced fluorescence spectroscopy, however, promises to be a valua ble non-destructive testing technique for etch-stop monitoring. (C) 19 98 Elsevier Science B.V.