Аннотация:Various physicochemical methods are used in the study of historical paper documents. Restoration procedures must be based on comprehensive characterization of samples, which involves the analysis of composition (type of cellulose, fillers, sizing, dyes, etc.).
Annual plants, rags, and wood pulp were used in paper production. Calcite (CaCO3), aluminum trihydrate (Al(OH)3), silica, kaolin, and titanium dioxide are the most wide-spread fillers.
Aging in the presence of atmospheric oxygen and alternative oxidizing agents leads to the oxidative destruction of cellulose, paper discoloration, and mechanical damages. Laser radiation can be used for paper cleaning. It is known that the irradiation at a wavelength of 532 nm leads to a decrease in discoloration in the absence of noticeable damages of old paper materials. Presumably the effect of laser irradiation is related to ablation and/or photochemical reactions that lead to the destruction of chromophores.
In this work, we analyze the bleaching of old paper samples using pulsed laser radiation with a wavelength of 532 nm, demonstrate that the laser ablation takes place, and identify the ablation products. The ablation experiments are performed using the second-harmonic radiation of the Nd:YAG laser with a pulse duration of 10 ns, repetition rate of 10 Hz, and pulse energy of up to 180 mJ. The diameter of the laser spot on the sample is 6 mm, and the corresponding pulse intensity is about 60 MW/cm2. The sample is shifted along the horizontal direction at 0.3 mm/s to avoid thermal damage. The total irradiation time is 3600 s. Comparative FTIR measurements prior to and after irradiation prove the absence of damages of paper samples.
Raman microspectroscopy is used to study the ablation products with a spatial resolution of about 1 µm, which is less than mean size of cellulose fibers (10 µm). Raman spectra are measured with the aid of a Thermo Scientific DXR Raman microscope (50x objective and excitation wavelength of 780 nm). The identification is based on characteristic Raman bands in the spectra of cellulose fibers, fillers, and sizing agents. We study five paper samples of different ages and compositions. Cellulose fragments, calcite, titanium dioxide, minerals, protein structures, and highly fluorescent species are identified as ablation products.
A statistical analysis of the ablation products shows dominant contribution of protein-containing fragments and highly fluorescent species. Apparently, the removal of the latter provides the bleaching effect. The experiment with the paper sample on which a layer with a thickness of about 30% of the total paper thickness was removed using an erasing rubber yields the amounts and relative contents of ablation products that are similar to those in the experiment with the original sample. Therefore, the detected ablation products are hardly related to surface pollutants.
Based on the experimental results, we conclude that the Raman measurements of ablation products can be helpful in the study of components of old paper materials and monitoring of the bleaching effect.
ACKNOWLEDGMENTS
This work was supported in part by Lomonosov Moscow State University Program of Development and Russian Foundation for Basic Research (projects 14-06-00259-a and 14-06-00244-a).