## 3 Results and Discussion
### Degree of Checking
#### 3.1.2 Board Surface: Digital Image Processing  
The relative area of surface checks (%) by exposure time of 'exposed' control and TM boards determined by digital image processing is shown in Figure 5 for the pith side and bark side (boards exposed on their edge were excluded).  
Before exposure, surface checks were mainly present on the path side in the centre of TM boards close to the path (Figure 5a, dotted grey line). These checks were already present before treatment, which is probably related to the fact that pit was enclosed more often in TM boards than control boards as discussed earlier. The relative area of surface checks was largest in the centre of control and TM boards on the bark side (Figure 5b, dotted lines). At this location, growth rings are orientated more or less parallel to the board's surface and the tangential surface is exposed. Here, checks were formed primarily across growth rings (Figure 4a). It was shown earlier for unmodified spruce and pine wood that tangential surfaces have longer and wider checks, and a greater number of checks per unit area than corresponding radial surfaces, because of, inter alia, shrinkage anisotropy [48]. On the bark side, the area with surface checks was larger for control than TM boards. This was attributable to more severe checking (wider and/or longer checks) in the centre of control boards (Figure 5b, dotted orange vs. grey line), which is most likely caused by higher shrinkage and swelling coefficients of control boards compared to TM boards. In detail, the average radial and tangential swelling and shrinkage is 3-4% and 5-6% for Thermo-D spruce, and 3-5% and 6-9% for unmodified spruce, respectively [12; 43; 69]. The relative area of surface checks on the board's pitch side was larger for TM than control boards, both in the centre and at sides. Although differences were quite small, Figure 5a shows that for both control and TM boards the checked area was larger at board sides (dashed lines) than in the board's centre (dotted lines) after 30 months of weathering. At the outer parts of a board on the pit side, growth rings are orientated more or less perpendicular to the board's surface and the radial surface is exposed. Here, checks developed primarily along growth rings (Figure 4c). It was shown previously for unmodified spruce and pine wood that, on radial surfaces, checks are mainly formed at the annual ring border [48]. More checking at the outer parts on the pit side of TM boards compared to control boards is most likely because TMW exposed to weathering is more sensitive to delamination between growth rings [55], as discussed earlier (see again Table 3, checks along rings on pit side). The greatest increase in surface checking was observed after 6 months of weathering. After that, the relative area of surface checks increased at a slower rate (Figure 5). The plateau observed between month 12 and 24, and after exposure, was caused by the fact that boards were wet (i.e., boards were swollen and checks were closed) and/or because of bad light conditions (i.e., lower quality images). After 30 months of weathering, the relative area of surface checks was largest in the centre of boards on the bark side and at the sides of boards on the pit side, both for control and TM boards (Figure 5).  
Results on surface checking from digital image processing were in line with checks measured in the cross-section. However, by using image processing to assess the degree of surface checking, it was shown that the checked area on the bark side is larger for control boards compared to TM boards,  
Figure 5: Mean relative area of surface checks (% of total investigated area) from digital image processing of ‘exposed’ control and TM boards by exposure time: (**a**) pit side (group 1: pit up) and (**b**) bark side (group 2: bark up) [legend in **a**].  
and vice versa on the pith side. Figure 6 shows that the relative checked area after weathering was less on both the pith and bark side of boards exposed on their edge compared to boards exposed flatwise.