## 4 Discussion  
The chemical, mineralogical, and molecular characterisation of the pigments proved to be useful as impurities were identified. These impurities, whether related to their provenance, manufacturing process or added as inert fillers, could determine the durability of the inpainting in the long term. For instance, high relative humidity environments, in the presence of expandable clays such as kaolinite-Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)- or montmorillonite  
Figure 8: Plane-polarised transmitted-light (PPL-PLM) and SEM micrograph of sulphide-based paints: (**a**) CIN-R-SB and (**b**) VER-R-SB. The black rectangle indicates irregularities in the SB paint layer; the white arrow indicates detachment of the SB paint layer from the substrate. See Table 1 for an explanation of the pigment identification codes (ID). Figure 7: Plane-polarised transmitted-light (PPL-PLM) and SEM micrograph of oxide-based paints: (**a**) MAR-R-SB and (**b**) CHR-G-SB. The black rectangle indicates irregularities in the SB paint layer; white arrows indicate detachment of the SB paint layer from the substrate. See Table 1 for an explanation of the pigment identification codes (ID). (Na,Ca)\({}_{0,3}\)(Al,Mg)\({}_{2}\)Si\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\cdot\)nH\({}_{2}\)O\(\sim\), found in UL-B or GE-G (see Table 1), could lead to swelling processes [67]. Thus, variable moisture conditions could provoke swellable clays to shrink and expand, leading to paint loss. This was observed in a similar study which evaluated the durability of this same inpainting procedure [68]. The authors reported that the presence of expandable clays did indeed lead to paint loss in green earth-based inpaintings [68]. Moreover, the presence of sulphides such as pyrite-FeS\({}_{2}\)-, identified in LA-B, in oxidising environments could favour acidic microenvironments, reacting with the calcium present in the _intonaco_, leading to the formation of calcium sulphates and sulphites [69]. Regarding colour compatibility, the fact that the raw pigment colour varied little once the inpainting layer was dry is of great interest to professional restorers. This would enable them to know approximately what colour will be obtained when mixing the nano-silica with the pigment, as the inpainting did not differ much from the original colour. In fact, considering pigment nature, synthetic pigments generally showed the least colour variation when mixed with the NE binder. The colour difference (\(\Delta\text{E}^{\star}_{\text{ab}}\)) between frescoes and inpaintings when using the same pigment was very high. The colour parameters showed that colour saturation (C\({}^{\star}_{\text{ab}}\)) was always higher in the inpainting. In order to avoid the inpainting work having a strong visual impact, applying the paint _a softotono_, i.e., 'in a lower tone', following traditional inpainting techniques [2], would reduce the C\({}^{\star}_{\text{ab}}\) values and would make sure that the chromatic reintegration would not stand out from the original. The other physical properties evaluated in this study showed a good compatibility between frescoes (F) and this novel inpainting procedure (SB). Inpaintings (except for CHR-G-SB) showed slightly lower gloss values than their fresco counterparts. This meets the compatibility criterion established by [PERSON] in the 20 th century [17] as both the fresco and the chromatic reintegration have a matte appearance (_ergo_ compatible) though slightly different (_ergo_ recognisable). The roughness values were also similar, except for LA-B and GE-G paints where it was higher. This was directly related to their pigment particle size since both showed higher size ranges (0.3-100 \(\upmu\)m and 0.3-125 \(\upmu\)m, respectively). Nevertheless, we should also bear in mind that roughness could also be affected by the brushstrokes and the irregular surface of the _intonaco_ itself.