## 4 Results and Discussion
### Experimental Results
All other series have a slightly higher failure load than the untreated series, except for the SIL-G_94T (Figure 5h) series (similar maximum load) and the SIL-G (Figure 5b) and SIL + 111G_94T (Figure 5l) series (slightly lower maximum load). The untreated series exhibits the highest ultimate displacement, comparable only to that of the EPX series (Figure 5m); all other series (A excluded) have a maximum displacement equal to approximately two thirds of that of the untreated series. The EPX series also has the highest shear stress and strain; the average value of shear strain of the EPX series is comparable only to that of the untreated series (Figure 5a), while the shear strain of all other series with primers is halved (\(0.53\leq\gamma\leq 0.68\)). The highest values of the global stiffness (around 50 kN/mm) are reached by four series, for which at least one primer on the glass side is used (111G--Figure 5c, SIL-G_111T--Figure 5e, SIL + 111G_11T--Figure 5g, and SIL-G_94T--Figure 5h). This results from the improved connection of the adhesive to the glass surface, made possible using adhesion promoters. As a matter of fact, due to the environmental exposure, the bonding surface on the glass side may be subject to moisture penetration at the interface between the glass adherend and the adhesive. This causes a weakening of the bonding and may lead to the detachment of the adhesive from the glass. The use of a Silane primer effectively counteracts this phenomenon. Moreover, the stiffness of the untreated series (Figure 5a) is comparable to that of the painted series, particularly the acrylic types (Figure 5n,o). The series treated with acrylic paint type A (Figure 5n) shows the worst results for all the parameters considered, followed by type B (Figure 5o) series. Once the shear tests were completed, the failure modes of all specimens were analyzed. Only 18 out of the total 65 specimens show a single failure mode, while in all the others, the failure occurs as a mix of two to four different mechanisms. The most recurring one is the stock-break (SB) failure mode, which occurs in three to five out of five cases for all series, except acrylic-treated specimens. The latter all show an adhesive-type failure mode, AF, alone (only for the A type) or in combination with a thin-layer cohesive fracture; both those collapse mechanisms occur at the interface between the adhesive and the glass, which is the most undesirable form, since it indicates low bonding forces between adherends. The TLC mode takes place close to both interfaces, but at the timber interface, it is two times more frequent. The AF never happens at the timber interface, a phenomenon attributable to the fibrous and porous structure of the wood. This is particularly evident in beechwood, which, despite its thin and delicate bark that lends itself well to carving, is still classified as a hardwood due to its high density. In fact, such a structure makes possible the development of greater cohesion forces between the adhesive and the timber adherends. The presence of an interface characterized by a limited roughness (i.e., glass Ra = 0.0012-0.0017 mm outer adherends) determines the presence of a preferential sliding plane for the specimen according to an adhesive failure, AF.