Colorimetric study of the post-processing effect due to pulsed laser cleaning of paper

The effect of pulsed laser radiation applied for surface cleaning of historical paper samples from the 17th c. and also samples of newly made paper was investigated by means of measurement of the colorimetric parameters: lightness L*, chromaticity C* and yellowness G. For production of the new paper samples the same method and material (linters cellulose) as used in the past were adopted. An artificial sample contamination was simulated by a mixture of charcoal powder with dust. Surface cleaning was performed by means of a Nd:YAG laser (6 ns pulsewidth) at wavelengths from UV, visible, and near IR ranges (355, 532, and 1064 nm). The laser fluence was selected from the range between the ablation and damage thresholds (0.3–0.9 J/cm 2 ) of the substrate materials. The absolute differences ∆L*, ∆C* and ∆G indicate that the smallest changes of the original color parameters and the most effective surface cleaning occurred at 532 nm. Artificial aging equivalent to periods of 25 and 50 years resulted in negligible changes of the L* and G values of laser cleaned, laboratory soiled samples, while for non-soiled samples a marked enhancement of the surface darkening with time was observed. Changes in L*, C*, and G after the laser treatment at 355 nm and 1064 nm are ascribed to the photochemical damage of the cellulose fibers and to enhanced absorption of the laser radiation by soil particles.