## 3 Results and Discussion
### Electrokinetic Analysis of Treated Lithotypes
In fact, according to the generally accepted criteria of conservation science, a protective coating for natural stones must prevent penetration of condensed water while preserving the open porosity required for water vapour permeation through the treated layer of the stone surface. Thus, while a non-uniform surface distribution of the hydrophobic coating material (e.g., with cracks or lacunas in the coating layer leaving the outer mineral grain exposed) may facilitate water infiltration and absorption by capillarity into the stone, minor differences between the IEP value measured on the coated stone and that of the coating material itself (as measured on a nonporous substrate) may indicate that accessibility to the mineral inner pore surface by the probing electrolyte solution during the electrokinetic measurement is not prevented. The relative variation \(\Delta\)IEP (Equation (2)) for the six lithotype-treatment combinations reported in Figure 4 does not appear to be correlated in a simple manner with the amount of product applied. In particular, in the case of ANC, a large difference in dry matter from Apuan marble to Balegem stone only corresponded to a negligible increase in \(\Delta\)IEP, while a comparable increase in the amount of product from Balegem and Obernkirchen gave rise to a comparatively large increase of \(\Delta\)IEP; similarly, in the case of WNC, a minor increase of dry matter from Balegem to Obernkirchen resulted in a large  
Figure 3: Isoelectric points (IEP, in pH units) of the three lithotypes before and after treatment with WNC/ANC. Values for untreated lithotypes were obtained by averaging the two sets of values reported in Table 6.  
increase in \(\Delta\)IEP, actually much larger than that recorded from Apuan marble to Balegem in spite of a higher increase in dry matter. The two products indeed showed markedly different penetration ability, as it is possible to infer from the amount of absorbed dry matter upon treatment (Table 3). WNC has a limited penetration and tends to accumulate on the stone surface, leading to a significant increase in IEP at a comparatively low amount of product applied (Figure 4). By contrast, the higher penetration of ANC results in a tendency to saturate stone porosity, so that a larger amount of product is needed to increase the isoelectric point of the stone surface, as it is especially clear when considering the trend from Apuan marble to Balegem stone (Figure 4). This evidence highlights the complexity of the picture resulting from the contribution of a multiplicity of factors in determining the extent of surface modification, in terms of both the amount of product actually deposited on the surface and the homogeneity of deposition. In particular, among the factors that are most likely to contribute are: (i) the different chemical composition of the alkylalkoxysilane sols, resulting in possibly different reactivity and surface charge of organosilica nanogel particles that are likely to be present along with monomeric and oligomeric silanes; (ii) the different size of nanoparticles dispersed in the two sols (Table 2); (iii) the different porosity, average pore diameter and surface roughness of lithotypes (Table 1); and (iv) their different chemical composition, resulting in different surface charge and specific chemical interaction (covalent or hydrogen bonding) with the sols. The lower penetration of WNC may be tentatively explained as the result of incipient deposition of nanoparticles onto the outer surface, due to the presence of cationic (IEP = 10.3; Table 5) and reactive organosilica particles generated by sol-gel partial condensation of the aminated alkylalkoxysilane precursor.