## 7 Results of Oak Thermal Degradation by Thermal Loading
The samples were positioned 30 mm from the initiator (with the identified temperature at this distance being 130 \({}^{\circ}\)C). The experimental time was 15 min. The obtained results demonstrated an approximate mass loss of 20%. The comparison of the maximum temperatures reached in the samples during the experiment yielded noteworthy results (Figure 13). For spruce, red spruce, and cherry samples, it can be observed that as the distance increases, the maximum temperature decreases linearly. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Distance from** & \multicolumn{4}{c}{**Height of the Charred Layer (mm)**} \\ \cline{2-5}
**Initiator** & **Spruce** & **Red Spruce** & **Cherry** & **Oak** \\ \hline
0 mm & 12 mm & 10 & 12 & 8 \\
12 mm & 5 & 6 & 2 & 1 \\
32 mm & 2 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Visual comparison of the samples and the height of the charred layer in the samples after the experiment (mm). Among the examined wood samples, oak (sample 1D) reached the highest temperature, at 483.5 \({}^{\circ}\)C (Figure 13). [PERSON] et al. [68] also reported the highest temperature of thermal oxidation and the lowest charred-layer thickness in oak samples and lignin isolated from oak. Because the time-temperature curves followed similar patterns (Figure 11), it was necessary to determine an appropriate method to quantify the observed differences. Figure 14 illustrates the comparison of experimental parameters mass loss (Figure 14a), maximal temperatures (Figure 14b), and charring layers (Figure 14c) with sample densities. The results of the experiments are that CS exhibits typical behavior towards other investigated woods. In all of them, the temperature curves of CS were with the lowest values, so the temperature increase was the lowest, so it confirms the thermal stability. The mentioned wood species is not commonly used indoors. Cherry is a softwood. Its results are in agreement with spruce, which is also a softwood. The obtained temperature curves S and C have a higher temperature course than the samples of hardwoods. It is possible to assume that the mentioned woods are more intensively influenced by heat and reach higher temperature values on the surface compared to hardwoods (CS and D). Overall, cherry exhibited the shortest smoldering time (indicating the onset of thermal degradation) (Figure 7a). Spruce showed temperatures approximately 100 \({}^{\circ}\)C higher than cherry. Smoldering in oak began at 525 s (Figure 9c). Figure 13: Mutual comparison of maximal temperature in wood samples from the various distances of radiant heat source. Legend: dark blue is spruce, light blue is red spruce, red is cherry, and green is oak. Figure 14: Mutual comparison of experimental parameters: mass loss (**a**), maximal temperatures (**b**), and charring layers (**c**), with sample densities of wood samples from the various distance of radiant heat source. Samples with lower density exhibit a significantly higher rate of weight loss. [PERSON] et al. [73] conducted research to investigate the effect of spruce wood density on weight loss and the rate of weight loss. Their study revealed that sample density influenced the rate of fire spread. They attributed the variability in mass loss primarily to the density of the spruce wood samples. The findings here indicate that, in addition to sample density, the distance from the initiating source also affects mass loss and the layer of charring.