## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Historical Papers
The main advantage of this new approach is the absence of alterations in the appearance of original papers, even after adding the Ag nanoparticles to the CNCs suspension. Moreover, the CNCs coating was tested by mechanical stresses, and it was demonstrated that it increased the interaction between fibers, allowing for the stress to transfer into the material, thus obtaining a greater resistance. [PERSON] et al. [17] suggested the use of nanocellulose-based composites containing metal nanoparticles, which are commonly used for the conservation of paper collections. In particular, in this work, ZnO nanoparticles were used as an antimicrobial agent in order to increase the resistance to the biological degradation of paper due to some fungal strains present in archives or museums and to improve its chemical stability, including under UV radiation. In fact, once exposed to UV light, cellulose, which paper is made of, undergoes oxidative degradation due to the formation of hydroxyl radicals and carbonyl groups, which promotes the yellowing of the paper. However, these nanoparticles need stabilizers that avoid their agglomeration and, to achieve this, cellulose nanocrystals were used, thus increasing the tensile strength and folding strength of degraded paper. In this way, [PERSON] et al. demonstrated how the ZnO/CNCs nanocomposite system can simultaneously act as a consolidating agent and bacteria, fungi, and UV light protection. Another innovative treatment of damaged ancient paper that makes use of nanocellulose was shown by [PERSON] et al. [59]. First, they demonstrated the reversibility of their studied treatment thanks to the use of a hydrogel suitable for removing nanocellulose, such as Gellan gel. This is a very important aspect of consolidation treatment because, in the field of cultural heritage, the reversibility is a fundamental goal for all restorations. Moreover, in this research work, the importance of the surface functionalization of cellulose nanocrystals was underlined to evaluate the influence of surface functional groups on their behavior as paper consolidants. In particular, it was observed that the presence of sulfate groups (S_CNC) affected aggregation properties and the aging response of treated materials, worsening the conservation in the long-term. This behavior is related to the pH of paper treated with S_CNC, which is lower compared to that of paper treated with uncharged CNC. For this reason, S_CNC suspension requires the addition of deacidifying agents to eliminate the acidity excess and to prevent paper depolymerization. On the contrary, a neutral CNC suspension did not compromise the pH and mechanical properties of the treated paper and therefore seemed to be the best solution between the two treatments. Finally, from the visual analysis of the Breviarium pages treated with the two types of CNCs (Figure 11), it is possible to observe how transparent CNC films do not alter the characteristics of the written parts since they remain clearly legible in all samples. [PERSON] et al. [100], in their work, proposed an interesting solution for providing both consolidation and deacidification for the degraded papers with a single-step treatment,  
Figure 11: Images of pages and details from Breviarium romanum ad usum fratrum minorum treated with (**A**) water (pristine); (**B**) S_CNC; and (**C**) CNC. Reproduced under terms of the CC-BY license [59]. Copyright © 2021, [PERSON], published by American Chemical Society. thus reducing the risks and stresses usually associated with a conservation intervention. To achieve this, they grafted cellulose nanocrystals with oleic acid to improve their dispersibility in organic solvents, such as ethanol, and then added alkaline nanoparticles of Ca(OH)\({}_{2}\) or CaCO\({}_{3}\), thus obtaining an ethanol-based \"hybrid\" system able to improve the mechanical properties of the paper and increase its pH, hindering further cellulose depolymerization. Figure 12 shows the graphical schematization of the conservation proposal made by [PERSON] et al. [PERSON] et al.