The use of polysaccharide rigid gels in cleaning treatments : multi analytical approach for the study of their performance, effectiveness and interference with paper artworks

摘要

The cleaning of paper artworks represents a fundamental treatment since external contaminants, metals, spores, etc. can promote degradation of cellulose (the principal constituent of paper). For this reason the removal of such components may partially preserve paper over time. Different cleaning treatments can be applied on paper artworks, for instance dried methods, aqueous cleaning systems. Recently polysaccharide rigid hydrogels (i.e: Agar and Gellan gum) have been used for the cleaning of paper, in order to increase surface wettability of paper and control the diffusion of water molecules in paper. This aspect represents an advantage especially in case of delicate surfaces and in presence of hydrophilic graphic media. The use of water (in a free or gelled form) may promote the extraction of hydrophilic components (either degradation products or original constituents of paper). Therefore the evaluation of the extraction of original constituents of paper induced by cleaning treatments as well as the evaluation of the effects of cleaning on chemical-physical properties of paper represents a relevant aspect in the context of paper conservation. For this reason the aim of the research is the evaluation of the extraction of sizing agents as gelatine contained in paper, products of hydrolysis of hemicelluloses and cellulose, induced by aqueous cleaning treatments and the evaluation of the effects of the cleaning on morphological, mechanical properties of paper. The study is carried out on paper artworks of different periods (from XVI to XX century) subjected to cleaning by immersion in distilled water and to application of Agar and Gellan gum rigid gels. GC-MS analysis show that such cleaning treatments can cause the extraction of products of hydrolysis of hemicelluloses and cellulose as well as the extraction of gelatine. This phenomenon is more evident in the case of the immersion of paper in distilled water and may partially explain the significant variation in the mechanical properties of paper observed by means of tensile testing after cleaning. In fact paper samples subjected to cleaning by immersion show a relevant decrease in the Ultimate Tensile Strength, probably correlated to the evident extraction of water in the free form of hemicelluloses products, and a relevant increase in the breaking strain (probably caused by the swelling of cellulose and the formation of hydrogen bonds between cellulose chains and water molecules). Such effect is reduced and a moderate improvement in mechanical properties of paper is observed when polysaccharide rigid gels are used.The application of rigid gel for the cleaning of ancient paper seems to be respectful of the original surface morphology of paper as well as in the case of the immersion treatment as revealed by 3D profilometer analysis and observation by stereo-microscope and SEM-EDS. On the contrary the application of dried methods (i.e.: wishab sponge) and cellulose ethers by means of cotton swabs, etc. may promote defibration and alteration in the morphology of paper surface because of the mechanical action induced by such applications. An analytical evaluation of the residues left by gels on paper after cleaning treatments is carried out by means of GC-MS analysis. Results show a reduced presence of gels residues left on paper only in the case of the application of 1% gel, because of its soft consistence and its difficult manipulation. The results obtained in this research demonstrate that the effects of cleaning are strongly dependent on the type of paper, its papermaking, its conservation and for this reason the choice of the most suitable method of cleaning should consider materials present in paper and properties of paper. For this reason chemical-physical analysis could be of great help for the knowledge of materials in paper and for the monitoring of cleaning treatments.