## 3 Results and Discussion
It has been demonstrated using 13C nuclear magnetic resonance and X-ray analyses that heat-treated wood has a higher crystallinity, leading to a more hydrophobic material [26; 31; 33]. The fiber alignment and degree of crystallinity can also affect the hydrophobicity of the samples, the larger the alignment of the fiber, the higher the degree of crystallinity [34]. In addition, higher amounts of non-polar components on the surface of samples exposed to higher temperatures cause an increase in hydrophobicity, which is attributed to extractive migrations and deposition of volatile organic compound substances [33]. Moreover, the hydrophobic character is correlated with low values of electron-donating components. After heat treatment, the lignin content increased, while carboxylic acid functions that are mainly present in hemicelluloses decreased significantly; an important decrease in wood acidity was noted [35]. The dehydration reactions of hemicelluloses that are ongoing during heat-treatment are able to degrade OH-groups, which cause a decrease in water penetration into the wood surface [36]. The heat treatment causes a greater gas permeability and an increase in porosity and decrease in gravimetric density [37; 38; 39]. Finally, extensive charring (more than 400 \({}^{\circ}\)C) leads to intensive cracking and an increase in the capillary absorption of water, which is related to the high standard deviation values measured for wood charred at 400 \({}^{\circ}\)C.  
The results of wettability of the one-sided surface charred specimens were determined by surface water uptake and are presented in Figures 4 and 5. Similar to the contact-angle measurements, varying results of water uptake were found for the radial and tangential specimens. The average water uptake of the radial specimens after 72 h was statistically (\(p\) = 0.000207) lower than the reference. The most significant decrease of 56% in water uptake was found in the 200/20 group after 72 h of floating in water. The reduced water absorption is likely to be due to a reduction in the number of hydroxyl groups (-OH) in the celluloses and hemicelluloses as a result of high temperature treatment. Furthermore, the decreased accessibility of water molecules to cellulose hydroxyl groups due to the increase in cellulose crystallinity and cross-linking in lignin can also play an important role [40; 41; 42]. The water uptake of the 250/4 and 250/6 specimens after 72 h decreased by about 38% and 36%, respectively. With increasingly severe charring, specimens exhibited only minor differences, though the standard deviation increased more than with the others. There was no statistically significant difference between the water uptake of the tangential specimens (Figure 5). The average water uptake in 200/20, 350/2, and 400/1 samples increased when compared to the reference. The variability of measured data was also significantly greater than those of the radial specimens. The main reason can be found in the highly anisotropic character of the tangential specimens, which undergo more severe dimensional changes (swelling) during water soaking than the radial specimens [43]. The charred surface layer is hydrophobic, cross-linked and aromatic, but also porous and brittle, especially at temperatures above 300 \({}^{\circ}\)C.  
When capillary absorption takes place and water molecules penetrate into the cell wall, wood starts to swell and change its transverse dimensions. As a result of the surface micro cracks in the charred layers, water uptake is increased and causes additional surface damage due to the different internal stresses of the charred surface and untreated inner wood. The results indicate that the most important factors influencing the performance of one-sided surface charred beech wood are, not only the process parameters, i.e., temperature and time, but also the orientation of annual rings towards the heat flow, which may play a major role during a product's life [13; 44; 45; 46].