## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Historical Papers
In particular, in this research work, both bacterial (BNC) and fibrilated nanocellulose (CNF) were studied since they have different characteristics regarding their morphology (Figure 8), crystalline and polymerization degrees, and purity level. In the function of the type of nanocellulose used and the modification performed to decrease the content of the water of the two suspensions, the results obtained were different: for example, the substitution of water with ethanol gave a faster absorption and greater aggregation for the CNF suspension, making the application of the consolidant more complicated. On the other hand, the freeze-drying modification of the BNC suspension caused a loss of viscosity and, in this case, agglomerations were formed. Otherwise, in both cases, the modification performed with the addition of gelatin increased the stability of nanocellulose film but left the treated surface very sensitive to the moisture and stickier compared to the nanocellulose suspensions without gelatin. However,  
Figure 8: **Left:** paper treated with BNC; a clear borderline between the treated (**left**) and the untreated (**right**) areas is visible. **Right:** paper treated with CNF; the borderline between the treated (**left**) and untreated (**right**) areas is not as clear as with BNC. Reproduced under terms of the CC-BY license [89]. Copyright © 2017, The Authors, published by Springer Nature. with these treatments, the damaged areas of paper were consolidated, as visible in Figure 9, and the results of the application of BNC and CNF suspensions were comparable with the adhesive consolidation; they did not alter the visual appearance of ancient papers, even in the long term, and permitted an innovative approach for paper stabilization. Other research works, such as those reported by [PERSON] et al. [96] and [PERSON] et al. [97; 98], showed the use of bacterial nanocellulose (BNC) for the paper reinforcement as a replacement of the usual Japanese paper coating method, obtaining excellent results in terms of the minimum thickness of the coating and better legibility of the characters after processing, although with a slight yellowing of the surface due to the aging process. Moreover, the treatment with BNC decreased the wettability due to the reduction in air permeance, an important aspect that prevents the formation of mold. Hence, bacterial nanocellulose is a promising material for the restoration of paper because it preserves the cellulose fibers of paper against degradation agents such as atmospheric pollutants and humidity, with results similar to those obtained for Japanese paper consolidation treatment. A similar study conducted by [PERSON] [99] compared nanocellulose coatings with different kinds of Japanese paper and confirmed that nanocellulose provided a better consolidating film that was able to withstand aging, light, and temperature. [PERSON] et al. [16] proposed a new treatment for the conservation and consolidation of ancient papers based on the use of cellulose nanocrystals (CNCs) with the addition of silver (Ag) nanoparticles in order to obtain both a consolidating action due to nanocellulose and biocidal activity by Ag nanoparticles (Figure 10). Figure 10: Growth inhibition of Aspergillus niger on Whatman paper samples, not treated (NT) and treated with CNCs obtained from different sources (CNC\({}_{\text{CL}}\) from cotton linter and CNC\({}_{\text{WP}}\) from waste paper), CNCs/Ag and Ag nanoparticles. (**a**) CNC\({}_{\text{CL}}\) (**b**) CNC\({}_{\text{WP}}\). Reproduced with permission [16]. Copyright © 2019, Elsevier. Figure 9: Cracks and leakage of damaged paper before treatment (**left**). After a direct application of nanocellulose, the cracks are closed (**right**) and the damaged paper is consolidated and stabilized. Reproduced under terms of the CC-BY license [89]. Copyright © 2017, The Authors, published by Springer Nature.