Cleaning and conservation of parchments from 15th century have been undertaken with excimer laser irradiation with 17 ns pulse duration and a wavelength of 308 nm. Optimized working parameters were identified in the fluence range between 0.2 and 1.4 J cm-2.
The parchments investigated were notarial documents from 15th century Northern Italy. Top stain layers exhibited ablation threshold fluences of the order of 0.1 J cm-2 corresponding to intensities of ~10 MW cm-2. Any foreign layers which have been deposited as viscous or dry phases (e.g. grease, dust etc.) rendered themselves accessible to efficient cleaning while ink scripts could be preserved. A Southern German sheet of music from 15th century used as choir book has also been laser-treated. This parchment artifact was heavily scorched and polluted during a bombing attack at the end of World War II. Soot layers and crystalline dust from tumbling walls adhered to the surface. Particularly soot and pyrolysis product layers could successfully be removed by the laser treatment. Ink notes could be conserved.
Laser cleaning of fine vellum differs from that of normal parchment in that the higher transparency of the vellum collagen fibres and a substantial void interfibre volume allow a deeper light penetration than the comparatively dense collagen fibre composite of ordinary parchments. Thus, even deeply penetrated ink spots could be laser-cleaned on a delicate painted page of a French book of hours from 15th century without deterioration of the microscopic protein fibre matrix.
Generally, laser fluences have to be chosen in such a way that they stay below the ablation and destruction threshold of the parchment substrate, and that they surpass the threshold of the foreign matter to be removed.