Effect of photolysis on 17th/18th century paper

Samples of two historical papers from the late 17th and early 18th centuries were irradiated with a 350-nm lmax light source. Atomic force microscopy examination of untreated samples of these laid paper sheets revealed notable surface inhomogeneity. Diffuse brightness values show that irradiation resulted in an initial decrease followed by a rapid increase in brightness with increasing irradiation time from 0.5 to 28 h. The bleaching effect then slowed, following a logarithmic path that did not plateau within 197 h of irradiation time. Irradiated sheets stored for 3 months exhibited brightness reversion of 7%. Fluorescence also decreased with irradiation time if excitation wavelengths of 320, 350, and 400 nm were applied for samples exposed to narrow-band irradiation at 350 nm. The fold endurance decreased by 34—72% after 197 h of irradiation. Changes in tear and tensile values were less dramatic. The Z-direction ultrasonic velocity decreased after 197 h of irradiation; however, the ultrasonic conductivity partially recovered during 9 months of storage.