## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Painting Canvases  
Traditional painting canvases present a multi-layered structure made of the canvas, the preparation made by adhesive and inert charge, the paint layer consisting of the pictorial medium and pigments, and the varnish, consisting of a mixture of natural or synthetic resins, which is not necessarily present on all canvases. All of these layers that constitute a painting artwork have different compositions, roles, and chemico-physical characteristics. The painting canvases can be made by different materials, but the natural fibers linen and cotton have always been the favorite surfaces for artists. These canvases have a woven structure that comprises two thread directions, which are defined as warp and werf. This characteristic structure of the canvases is responsible for their anisotropic behavior and directional-dependent properties. Moreover, in a painting, the canvas plays a fundamental role of mechanical support for the tensile stresses to which it is subjected to over time and, for this reason, it is the focus of the work on painting structural consolidation [83, 84]. The first canvas consolidants were applied in the XVII century, when the support of the first oil paintings started to show signs of degradation. Between the XVII and XX centuries, lining treatments gained the attention of the artistic community and were carried out increasingly frequently. Currently, minimal possible intervention is required in the field of cultural heritage conservation in order to preserve the original aspect of the painting canvases. For this reason, even if it is rarely possible, attempts are made to obtain stabilizing products with a double effect of both deacidifying and consolidating in order to prevent the chemical and mechanical degradation of the canvases. The most important  
Figure 3: SEM images of freeze-dried control wood (FH) and blocks treated with bacterial nanocellulose (BCH) and cellulose nanocrystals (CNCH). **External and internal sections:** for FH cell, lumens are empty and secondary cell walls appear almost completely detached; for BCH sample, some of cell lumens are filled with consolidant, which creates sporadic superficial deposits; finally, for CNCH-treated wood, the cell lumens are empty and there is a thin layer of consolidant but it is not everywhere. **Radial section:** the fiber walls of sample FH appear smooth and the channels that they form are clearly visible; in BCH sample, there is a homogeneous and compact layer of consolidant, visible on the cell walls; in the CNCH sample, cellulose nanocrystals form a compact and homogeneous coat completely covering the pits. Reproduced under terms of the CC-BY license [18]. Copyright © 2020, The Authors, published by Front. Chem. degradation mechanism of these artworks is acid-catalyzed hydrolysis, which leads to a decrease in the polymerization degree (DP) and, as a result, in a decrease in the mechanical resistance of the materials. Another characteristic result of canvases degradation is aging, which is caused by several reactions taking place in the material and phenomena such as temperature and humidity variations that produce the cracking of the paint layer, resulting in irreversible damages of the painting [25; 61; 85]. Even the biodeterioration of canvas paintings is a frequent problem that involves the inorganic materials, and it is produced by microorganisms and bacteria that are present in the textile supports due to different environmental conditions such as pH, the presence or absence of oxygen, temperature variations, light, and pollution, which, in turn, can affect their growth. Among these, the percentage of cellulose also influences the resistance to microbiological attacks; in fact, the higher this is, the more resistant the canvas [86]. Further, the mechanical stress itself can also trigger the chemical degradation of the canvas, and both mechanical and chemical processes make the painting canvas increasingly brittle.