## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Painting Canvases
Figure 5: SEM images of aged cotton canvases coated 3 times with: (**a**) CNF; (**b**) carboxymethylated CNF; and (**c**) CNC, with optical microscopy images as insets (left top). Reproduced with permission. [23]. Copyright © 2018, Elsevier. Moreover, from Figure 5, it can be seen that CNCs tended to form denser structures compared to CNFs and functionalized CNF coatings due to the better packaging ability of nanocrystals than flexible nanofibrils. [PERSON] et al. [61] reported an interesting research work about an alternative method for the strengthening and deacidification of cellulose-based materials with nanocellulose and inorganic nanoparticles, which was already addressed in different previously published works [87, 88]. In detail, calcium carbonate nanoparticles were used to neutralize the acidity, whereas stabilization was approached through a combination of nanocellulose and silica nanoparticles to obtain synergic effects of fiber stiffening and structural reinforcement. In the specific case of iron-tannate inks or dyes, they trigger severe degradation in cellulosic substrates, causing the acid-catalyzed hydrolysis of cellulose; in addition, iron ions not involved in the formation of complexes produce further oxidation. All of the results obtained from this research work demonstrated that the combination of nanocellulose and silica nanoparticles was efficient as a multiscale reinforcement, which is very important for canvases consolidation. The main advantage of this treatment is the lower weight increase: approximately 2.5% compared to a weight increase of 200% for traditional lining. Overall, this study also demonstrated that CNCs were more effective than CNFs because this type of nanocellulose gives the possibility of using higher concentrations in the aqueous dispersion, allowing for a higher strengthening after the equivalent number of coatings and smaller mechanical changes in RH variations. Moreover, CNCs have a higher degree of crystallinity, which makes the treatment much more stable. More recently, [PERSON] et al. [24] conducted a study on the use of CNCs for the consolidation and deacidification of a highly degraded acidic model cotton canvas and canvas from real paintings. Usually, the restoration treatments of a painting canvas are concentrated on the face of the painting, whereas the consolidant is applied to the back of the canvas due to aesthetic reasons; in fact, in this case, the treatment was also applied to the back side of the canvases in order to avoid the formation of cracks and disruption, which endanger the paintwork of the artwork (Figure 6). Another problem related to this is the extreme sensitivity of canvases and paintworks to any type of solvents, whether polar or non-polar; this can be overcome with a short contact time of the solvent and the canvas; otherwise, a very low solvent amount or a solvent with a high volatility can be used. In this context, this research work studied the behavior of polar components such as CNCs in water and non-polar compounds obtained by the silylation of CNCs to allow for its solubility in non-polar solvents as heptane. Therefore, cellulose nanocrystals were mixed in the polymer matrices, obtaining numerous advantages, such as an increase in the tensile strength of the canvas material, a greater chemical compatibility of the consolidant to the substrate, an easier and homogeneous applicability to the canvas, an unchanging  
Figure 6: Application of nanocomposite consolidants on original paintings. Reproduced under terms of the CC-BY license [24]. Copyright © 2020, The Authors, published by Springer Nature. appearance of the canvas and painting, impenetrability through the paint layer, and, finally, a sufficient reinforcement to the canvas. In addition to these advantages, these consolidation nanocomposite systems offer the possibility of carrying out both the consolidation and deacidification of the paintings together thanks to the addition of nanoparticles of calcium carbonate or magnesium oxide.