## 3 Results and Discussion
### Light-Induced Colour Changes in the Coated Wood Surfaces  
Exposure of the coated wood surfaces to light (V2 samples) resulted generally in smaller colour changes, dependant on the wood species and the type of finishing material. The graphs in (Figure 3A) represent the colour changes in the wood samples coated with the acrylic-polyurethane lacquer, coded F1, in the areas directly exposed to light/UV radiation (F1/V2/Z1) as a function of the exposure duration. The evolution of the global colour changes, expressed by the \(\Delta\)E values, show a quite similar pattern for maple and ash, with maximum changes occurring during the first 24 h of exposure (3.96 units for maple and 2.16 units for ash), followed then by a slower increase up to the 5.71 units for maple and only 3.37 units for ash. The evolution of colour changes for the walnut samples was totally different, almost linear in time, reaching the highest value of 7.70 units after 72 h of light exposure. The changes in lightness varied depending on the wood species: an increase in lightness, expressed by positive \(\Delta\)L* values, reaching (+3.40) units after 72 h exposure was registered for walnut, whilst for the maple and ash coated specimens the lightness decreased, as shown by the negative \(\Delta\)L* values, reaching (\(-\)3.83) units for maple and only (\(-\)0.52) units for ash following a reversed evolution after 24 h exposure. From a chromatic point of view, only slight variations in redness were registered as positive, with \(\Delta\)a* values up to (1.09) units for maple and values up to (1.14) units walnut, while those measured for ash were negative but very close to zero (\(-\)0.06 units). Positive \(\Delta\)b* values indicated for all the coated wood species a continuous increase in yellownness, up to 2.65 units for ash, 4.03 units for maple and 6.69 units for walnut. To conclude, the colour stability of the wood samples coated with the acrylic-polyurethane water-based lacquer decreased in the following order: F1-ash > F1-maple > F1-walnut, which is different from that observed for the uncoated wood samples: walnut > ash > maple. The light-induced colour changes for the same coated wood surfaces covered with 1 mm thick clear glass (V2/Z3) are shown in (Figure 3B). It can be seen that the total colour changes \(\Delta\)E are smaller than those recorded on the surfaces directly exposed (V2/Z1) to light for all the three species, but to a different extent. The effect of covering glass in reducing the global colour changes (\(\Delta\)E) for the coated wood (Figure 3(B1)) was similar to uncoated wood, minimal for walnut and clearly more accentuated in the case of maple (maximum) and ash (medium). However, according to the graphs plotted in (Figure 3(B2-B4)), the effect of glass seemed to be more uniformly distributed on all the colour parameters L*, a*, b*, with some specificity related to the wood species. In the case of maple, for instance, it is clearly visible that the yellownness increase due to light exposure was considerably reduced (from approximately 4 units to approximately 1 unit) for the coated samples covered by glass. It is important to highlight that the order of colour stability among the F1 coated wood samples was not changed by the effect of glass cover. A similar analysis of the light-induced colour changes for wood surfaces coated with the acrylic water-based lacquer, coded F2, is presented in (Figure 4), comparatively for the directly exposed areas (F2/V2/Z1) (Figure 4A) and areas exposed under a glass covering slide (F2/V2/Z3) (Figure 4B). Figure 3: Comparative in-time evolution of light-induced colour changes (\(\Delta\)E, \(\Delta\)L*, \(\Delta\)a*, \(\Delta\)b*) for maple, ash and walnut wood surfaces coated with waterborne acrylic-polyurethane lacquer F1 (samples type V2), depending on the exposure situation: (**A**)—direct exposure to UV light (F1/V2/Z1); (**B**)—exposure through a 1 mm clear glass lamella (F1/V2/Z3).