## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Wood  
In the past, wood was one of the most used materials for buildings, furnishings, and artworks [69]. It is basically an organic, hygroscopic, and anisotropic material. Its porous and fibrous structure is mainly composed of cellulose fibers, a lignin matrix, and hemicellulose; this composition makes wood, and therefore wood artifacts, highly sensitive to moisture, which can promote attacks by microorganisms and fungi. This type of degradation induces a dramatic decrease in the mechanical properties of wood due to the loss in density and cellulose content. To prevent this problem, an urgent consolidation intervention is needed, where the consolidants must have specific properties and characteristics, such as the correct penetration degree, in order to re-establish the cohesion between the wood fibers. Usually, the most common treatments utilize synthetic polymers that are soluble in acrylic, aliphatic, and aromatic resins, which decrease the porosity and improve mechanical properties; otherwise, in the past, natural oils or resins were also used for wood consolidation, therefore introducing a hydrophobic coating that reduces the moisture absorption and stabilizes the wood [60; 70]. However, in addition to the chemical nature of the consolidant, there may be other factors influencing the final performance of wood treatments, such as the method used to apply the consolidant, the use of pre-treatments or post-treatments, and the environmental conditions [69]. Beyond the restoration of mechanical properties, it is also important to recover the appearance of historical wood; different materials are used to achieve this, including epoxy and polyester resins as reported by [PERSON] et al. [71], but also cellulose derivatives as described by [PERSON] and [PERSON] [21], who demonstrated how the nanocellulose-based consolidants can be used to enhance the hydroxypropyl cellulose (Klucel E) properties, usually used for this purpose, without changing the color of the treated wood but increasing the compression strength and adding advantages in terms of increasing penetration within the wood. In the work of [PERSON] et al. [60], a suspension of CNC was used as such or mixed with lignin and/or siloxane derivatives (PDMS) to treat rotten wood samples of Norway spruce belonging to the roof of an ancient 17 th century villa in northern Italy; from the mechanical analysis of these samples, it was demonstrated that the CNC treatment was immediately effective, confirming a considerable improvement in the stiffness even after a few impregnation cycles. [PERSON] et al. [47] proposed the use of nanocellulose-based fillers as renewable reinforcing agents in a polymeric matrix, mainly used as a barrier protection against water absorption, which can cause internal stresses due to changes in pressure or temperature inside wood artifacts. Thanks to the bonds with the polymer chains, these nano-fillers can reduce the diffusivity of liquids, increasing the path tortuosity. Moreover, cellulose nanocrystals showed the highest increment in the stiffness and enhancement of the hydrophobicity of the starting matrix compared to the cellulose microcrystals, which, conversely, provided a less effective coating. The choice of a suitable adhesive is another important aspect in the conservation of wooden artworks, since a wrong adhesive can irreversibly damage the object both aesthetically and mechanically. Therefore, when choosing the right and most adequate adhesive, several parameters should be evaluated, including the stability, reversibility, color, hardness, brittleness, sensitivity to relative humidity, bond strength, cost, and ease of application [72]. In this context, [PERSON] et al. [66] addressed the use of nanocellulose for wood adhesive, claiming that the inclusion of nanocellulose inside the usually used polymeric adhesives could reduce the use of non-renewable petroleum sources and enhance the use of eco-friendly wood adhesives.