## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Historical Papers  
Paper is the most used material for transmitting knowledge, and it represents most of the world artistic heritage. The degradation of historical papers in museums, libraries, and archives is due to different factors such as humidity, light, air, pollution, and bacteria attacks; moreover, this type of artworks is often subject to mechanical damage in terms of cracks, cuts, and deformations, especially in the borders, and is more exposed to atmospheric agents that induce the lowering of mechanical properties of paper. However, regarding the other cellulose-based artworks discussed before, in this case, the main degradation process is also acidic hydrolysis, which produces a considerable decrease in the mechanical strength of cellulose fibers [16; 89; 90; 91]. To provide a structural reinforcement of paper, many strategies have recently been developed using both synthetic and natural materials [92]. For example, among the synthetic materials, we find 3-aminopropyl-methyl-diethoxysilane (AMDES) and 3-aminopropyl-triethoxysilane (APTES); however, they give stability problems [93]. On the contrary, natural materials such as nanocellulose are better solutions due to their compatibility with paper and eco-friendliness. Figure 7: Real linen canvas with high-density weaving before (**a**) and after (**b**) treatment with polar nanocomposite and before (**c**) and after (**d**) treatment with non-polar nanocomposite; real linen canvas with low-density weaving before (**e**) and after (**f**) treatment with polar nanocomposite and before (**g**) and after (**h**) treatment with non-polar nanocomposite. Reproduced under terms of the CC-BY license [24]. Copyright © 2020, The Authors, published by Springer Nature. In this context, many studies were conducted on the potential of nanocellulose for the stabilization and reinforcement of ancient papers. Among these, [PERSON] et al. [94] recently described how the mechanical properties of treated paper change with the nature of nanocellulose: if crystalline or fibrous. In fact, the mechanical behavior depends on the bonding of fibers inside the paper and on the forces that provide these interactions between the fibers, which are especially hydrogen bonds, van der Waals bonds, and frictional forces. Therefore, the introduction of nanocellulose with many hydroxyl groups on the surface and a very small size promotes the formation of hydrogen bonds between fibers and cellulose nanoparticles, thus increasing the strength of paper. In this specific case, CNCs and CNFs were suspended in two different solvents, water and ethanol, to assess the possible consequences on tested properties, including the tensile index, elongation coefficient, and bending strength. Additionally, the CNF was also functionalized with carboxyl groups in order to value the penetration of nanocellulose inside the paper structure. From this study, [PERSON] et al. were able to conclude that nanocellulose did not always improve the mechanical properties of paper; in addition, the medium in which it is suspended can be crucial because its polarity influences the strength of hydrogen bonds and thus the paper strength. [PERSON] et al. [89] discussed the possibility of using nanocellulose-based suspensions for the treatment of damaged historical papers. Usually, Japanese paper additions are used to rehabilitate damaged areas of these artworks, with a minimum acceptable alteration in the surface aspect and visual changes, since they are especially applied on very fragile paper [95]. However, the use of these very thin Japanese tissue papers and glues to attach these little fragments of papers to the damaged artwork involves several problems, such as the difficult control of humidity and influence of moisture. For this reason, [PERSON] et al. focused their attention on nanocellulose because it is the same material as the matrix to be treated; in fact, cellulose nanofibers bond with fibers of the papers and stabilize the fibers of the artwork without the addition of anchoring materials such as adhesives.