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Spectrocolorimetric control of ancient documents postablation with excimer lasers.

The application of excimer lasers in ablation, cleaning, and restoration for the recovery of paper and parchment manuscripts is a recently implemented technique. A report of the use of excimer lasers in a cleaning process by which mud was removed from Islamic papers and parchments is presented. It was found that, because of the close proximity of the binding energies of paper to paper and of paper to mud, it was difficult to maintain control of the ablation process. However, the substrate was not affected. Spectrocolorimetry was used as a technique to detect the effects of ablation on cleaned areas of the manuscripts in terms of change in color appearance and severity of aging postablation. The analysis was performed by comparison of treated and untreated areas. Mathematical modeling was developed to define a representative original color and a color-distribution parameter. Improvements in the measuring method were made to yield the required precision for evaluating differences in color produced by laser ablation and to follow the color evolution after ablation. Results show that the effects of restoration, aging, and the environmental conditions can be individually identified under certain conditions. The method has applications in other domains.