## 3 Results and Discussion
### Light-Induced Colour Changes in the Coated Wood Surfaces
The process was reversed with about the same speed by continuing the light exposure, the \(\Delta\)E values determined after 48 h and 72 h being considerably lower. The individual patterns for maple and walnut remained very similar to those presented for the samples coated with the other two finishes in Figures 3A(F1) and 4A(F2), being characterised by: (i) changes occurring more rapidly during the first 24 h of exposure, followed then by a slower increase in the case of maple; (ii) changes increasing almost linearly in time in the case of walnut. After 72 h light exposure the measured \(\Delta\)E values for the F3 coated samples, listed in increasing order, were: \(-\)2.25 for ash, 4.74 for walnut and 4.92 for maple. A trend of darkening expressed by negative \(\Delta\)L* values was registered after the first 24 h of light exposure, especially for ash and maple. This trend was reversed by continuing the light exposure in the case of ash but continued at a lower slope in the case of maple. In the case of walnut, only a very slight darkening was registered after 24 h light exposure and the further reversing of the process resulted in a lightening effect after 72 h, contrarily to the other two species. The comparative \(\Delta\)L* values (glass/direct exposure) after 72 h were as follows: \(-\)3.17/\(-\)3.56 for maple, \(-\)0.17/\(-\)0.51 units for ash and +0.63/2.14 units for walnut. From a chromatic point of view, the variation in redness \(\Delta\)a* and yellowness \(\Delta\)b* depended also on the wood species. A progressive increase in redness during light exposure was registered for maple (up to +2.60 units) and walnut (up to +1.61 units), while the values for ash had a slight wavy variation, resulting in only a very low increase (+0.49 units after 72 h). The variation in the chromatic component b* was totally different. A trend of a very slight but continuous decrease in yellowness was registered for maple (up to about +1.61 units). Figure 5: 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 water-based polyurethane lacquer F3 (samples type V2), depending on the exposure situation: (A)—direct exposure to UV light (F3/V2/Z1); (B)—exposure through a 1 mm clear glass lamella (F3/V2/Z3). \(-\)0.5 units). A more advanced decrease in yellowness (up to approximately \(-\)3 units) was registered for ash samples but only during the first 24 h of exposure; afterwards, this trend was reversed so that after 72 h light exposure, almost no variation in yellowness could be observed any longer (\(\Delta\)b* = +0.24 units). An increase in yellowness after 72 h light exposure could be measured only for walnut (\(\Delta\)b* = +3.24 units). Based on the global colour differences \(\Delta\)E after 72 h light exposure, the colour stability of the wood samples coated with the waterborne polyurethane (1 K) lacquer F3 decreased in the following order: F3-ash > F3-walnut \(\cong\) F3-maple. This is different from those observed for the uncoated wood samples (walnut > ash > maple) and those coated with the acrylic-polyurethane finish F1 (F1-ash > F1-maple > F1-walnut) but closer to those resulting for the samples coated with the acrylic finish F2 (F2-ash > F2-maple \(\cong\) F2-walnut). The light-induced colour changes for the same coated wood surfaces, exposed behind 1 mm thick clear glass (F3/V2/Z3 areas), plotted in Figure 5B, were again different but not always smaller than those determined for the directly exposed areas. Looking comparatively at the corresponding evolution patterns of the colour differences in the Figure 5(A1-A4,B1-B4), it can be observed that the most obvious pattern change due to the effect of the covering glass appeared for the yellowness variation (\(\Delta\)b*), especially for maple and ash. When exposed through the 1 mm clear cover glass, the F3 coated maple samples yellowed progressively, whilst when uncovered, there was a very slight trend of decrease.