## 4 Discussion
Moreover, regarding wettability, even though contact angle results (shown in Figure 3d) were different between F and SB paints, they both showed \(\theta^{\circ}\) below 90\({}^{\circ}\), which means that all the surfaces are hydrophilic [61; 62]. Hence, the use of NE as a binder would not influence the wettability properties and would show similar behaviour to water as fresco paintings, thus ensuring the natural breathability of the wall, allowing the exchange of moisture between the wall and the environment. Lastly, regarding the peeling test, the SB layer showed less loss of material compared to the F paints. This may be because the tape removed more grains of sand from the _intonaco_ (i.e., so increasing the weight) or because the paint layer was thicker in frescoes. The presence of fissures observed by SEM on the SB layer should not be considered as a reason to refuse this inpainting procedure. These fissures were reported in other studies [70; 71; 72; 73] in which NE was used as a consolidant product, being caused by polymerisation, after the evaporation of the solvent. Some of these authors claimed that this could affect the durability of the consolidation treatment [72]. Nevertheless, in [68] it was identified that NE as a binder for inpainting did not influence the overall durability of the inpainting after natural ageing tests were carried out. It is also important to highlight that the nano-silica binder did not always form a homogeneous layer including the pigment particles. Some particles were observed protruding from the surface of LA-B-SB and GE-G-SB inpaintings. In [68], where these paintings were exposed for one year to natural environments, LA-B-SB remained stable, while GE-G-SB did not. Lastly, the fact that slight detachment was observed in sulphide- and oxide-based inpaintings could be assigned to sample extraction or sample preparation since in [68] no detachments were reported after natural exposure for 1 year. As for the chemical compatibility between the tested pigments and the NE binder, the blue silicate-based pigments (EG-B, LA-B, and UL-B) showed higher affinity. Observations of the cross-sections revealed fewer fissures, with outstanding adhesion to the _intonaco_, and a more homogeneous dispersion of the pigment particles within the paint layer when compared to the other pigments. Due to their similarity with the binder in terms of chemical composition (both silicate-based), the blue pigment particles bound better with the colloidal aqueous solution of silica nanoparticles, leading to the formation of a solid product of hydrated amorphous silica that behaved in a similar way to a silica gel [74]. On the other hand, despite GE-G being primarily composed of silicates, it also contained many impurities which surely influenced compatibility leading to major fissures and lack of adhesion. As for oxide- (MAR-R and CHR-G) and sulphide-based pigments (CIN-R and VER-R), even though they all showed excellent pigment distribution, fissures and lack of adhesion were observed. This was likely related to their chemical nature as oxides and sulphides. Finally, semi-organic verdigris proved to be unsuitable, regardless of the painting technique (F or SB). When _a fresco_ was applied, the paint layer began to deteriorate 24 h after application. Though the instability of verdigris has been stated [75; 76], it has also been reported to be found in historical frescoes [77; 78]. Therefore, an inaccurate attribution may have been made whereas another painting technique (e.g., tempera) may have been used instead. Regarding the alteration compounds identified by XRD, Ca-Cu acetate paceite forms due to the reaction of acetate salts and calcium carbonate [79]. As for hydrated Cu-silicate apachite, this compound might have been formed by the reaction between the silicic acid from Si-compounds (feldspars from the aggregates) and Cu\({}^{2+}\) ions, under alkaline pH and in the presence of water, as reported by [PERSON] et al. [80]. This possible formation needs further research.