## 3 Results and Discussion  
The intensity of one-sided surface charring of the radial and tangential specimens was determined using X-ray density profile analysis and compared to the reference (Figures 1 and 2). The density profile of the reference radial and tangential specimens varied due to the different annual ring orientations in their cross sections and consequently earlywood and latewood proportions in the longitudinal sides. The average density of the reference radial specimens showed uniform distribution, while one-sided charred specimens tended to decrease in their density profile according to the time-temperature regime applied. Application of mild temperature-time regimes (200/6 and 250/4) led to only minor changes in the surface density profile, which decreased by about 4.5 to 8.2%. With increasingly severe charring, the surface density decreased by about 15.5 to 33.5%. According to the results, the mild charring processes produced a surface charred layer of approx. 2 mm, while a higher temperature and longer time affected density up to 4 mm. A significantly greater difference was found when a temperature of 200 \({}^{\circ}\)C was applied for 20 min. As a result of the longer time, the density profile was affected up to 6 mm beneath the surface of the specimen. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature** & **Time I.** & **Coding** & **Time II.** & **Coding** \\ (\({}^{\circ}\)C) & **(min)** & **Coding** & **(min)** & **Coding** \\ \hline Reference & - & Ref & & \\ \hline
200 & 6 & 200/6 & 20 & 200/20 \\
250 & 4 & 250/4 & 6 & 250/6 \\
300 & 2 & 300/2 & 4 & 300/4 \\
350 & 1 & 350/1 & 2 & 350/2 \\
400 & 0.5 & 400/0.5 & 1 & 400/1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Time–temperature regimes of one-sided surface charring. The decrease in density during one-sided surface charring is associated with a number of overlapping thermal degradation reactions as previously presented by several authors [21; 24]. Hemicelluloses are decomposed at temperature up to 250-300 \({}^{\circ}\)C, followed by lignin and cellulose degradation above 280 \({}^{\circ}\)C. The depolymerization of cellulose, production of volatile compounds, formation of oxidation products, and charring result at a temperature of approx. 300 \({}^{\circ}\)C. As the temperature increase, the degree of polymerization of cellulose decrease and the crystallinity increase [25]. This might be due to a preferred degradation of the less ordered molecules during the thermal treatment [26; 27]. Further decomposition of cellulose and lignin takes place at approx. 360 \({}^{\circ}\)C, while at 400 \({}^{\circ}\)C the remaining lignin continues to decompose [23]. The density profiles of the tangential specimens showed greater variability and therefore evaluation is not as straightforward as with the radial specimens. According to the results, most of the charring processes produced a surface charred layer of only approx. Figure 1: Density profiles of charred specimens compared to the reference—radial. Figure 2: Density profiles of charred specimens compared to the reference—tangential. 2 mm, while greater thermal degradation was found in specimens undergoing 250/6 and 300/4 regimes. These differences can be partially explained by the different densities of the radial and tangential specimens and partially due to the different thermal conductivity of wood in the radial and tangential directions (\(\lambda_{\rm R}>\lambda_{\rm T}\)). These differences can be attributed to the significant number of wood rays in hardwoods and their orientation [28; 29]. In addition, in the tangential section reasonably the ratio of earlywood and latewood is different, depending by the position if close to the path or far from the path [3]. The contact angle (CA) evolution with time is given for charred radial and tangential specimens as well as for an untreated sample at different temperatures (Figure 3).