## 4 Applications of Cellulose Nanomaterials in Cultural Heritage
### Painting Canvases
To prevent this problem, it is common to replace the full lining and to use a localized consolidation treatment with the application of an adhesive strip in the degraded zone of the canvas [83]. However, as mentioned before, among the most important properties of any consolidation treatment and, therefore, adhesives for canvases, are its reversibility, compatibility, and stability, which are all factors that the materials used in the past did not always possess. From this perspective, conservators are still searching for novel consolidation strategies based on more eco-friendly materials, but this search has started with the framework of the EU Nanorestart project 2020 [52; 67]. With the introduction of nanocellulose in conservation, these problems have been overcome [67]. In fact, nanocellulose is a material that is chemically similar and thus more compatible to the canvas, unlike the traditional vinyl and acrylic-based resins. Moreover, nanocellulose, with its nano-scale dimension, allows us to obtain a higher physicochemical interaction with the canvas substrates to be treated [19]. In fact, this bio-nanomaterial has the potential to improve and/or modify the properties of various polymers used as a matrix to produce glues or consolidants in preservation when mixed as a filler. [PERSON] et al. [19] were among those who developed and evaluated an alternative approach to structural painting consolidation, which provided the use of natural materials such as nanocellulose. Among the aims of their research was the assessment of the consolidants' response to relative humidity (RH) variations, which is useful for defining the suitability of these materials for canvas consolidation purposes. For this experiment, an aqueous dispersion of cellulose nanocrystals (CNCs) and nanofibrils (CNFs) was applied to the surface of a modern cotton canvas, which was then subjected to aging. The first consideration was that the nanocellulosic treatments did not visibly modify the canvas color; moreover, the mechanical properties of treated samples showed an improvement with an increase in the Young's modulus and, in particular, the best performances were obtained with CNCs, which might result from the greater mass coverage measured for these nanostructures. To evaluate the impact of consolidation treatments on the mechanical properties, RH dynamic mechanical analysis (DMA-RH) was carried out. Overall, this study demonstrated that the CNC treatment is the best in terms of reinforcement and long-term stability compared to the CNF treatment. However, Figure 4 confirms that, with these consolidation treatments, a continuous surface layer was obtained, preserving the topology of the canvas, and, in particular, in the case of CNFs, a network similar to the natural fiber structure was formed across the cotton fibers. In addition, [PERSON] et al. [23] demonstrated for the first time that stabilization due to the use of different types of nanocellulose is more effective than that of the wax-resin formulation Beva 371 commonly used. To achieve this, mechanical tests in the elongation region were carried out to evaluate the response of treated canvases to static and periodic uniaxial stress at different relative humidity values, and the results obtained show that the nanocellulose-based consolidants gave better performances than the traditional consolidant Beva 371. Another important aspect highlighted in this work is the formation of thin films of nanocellulose, with a structure that depends on the type of nanocellulose used--if fibrous or crystalline--which is an aspect that ensures the reversibility of these bio-consolidants since the films are superficial and do not penetrate into the bulk of the canvases as shown in Figure 5.  
Figure 4: SEM images showing the canvas surface topography before and after treatment. Reproduced with permission [19]. Copyright © 2018, Taylor & Francis.