## 3 Results and Analyses
The mass loss rate of these specimens started to increase significantly at a specific time (after approx. the third minute) compared with the other specimens. It subsequently caused a higher mass loss. The reason for the sudden increase in the mass loss rate of TP-R, TP-T, and TF-T specimens is the spontaneous combustion of the specimens that also burn through the sides after a certain time. Either only the lateral specimen side burned, or in some cases, the entire specimen. The specimens all started to combust spontaneously during the experiment. The average time of spontaneous combustion was: TP-R: 5 min 40 s; TP-T: 4 min 43 s; TF-T: 5 min 3 s (Table 6). In each of these groups, there was one specimen that spontaneously combusted at about 3.5 min. This occurrence explains the rapid increase in the average mass loss rate after the above-mentioned time, which increased further. In the other specimens (TF-R and U-R), we did not observe a significant increase in the average mass loss rate as only one specimen in the TF-R group spontaneously combusted and one specimen in the U-R group only at the end of the experiment. A study [30, 36] reported that for thermowood, lower energy is required for spontaneous combustion than for untreated wood. [PERSON] et al. [27], 2013 pointed to findings that heat treatment of spruce wood causes a reduction in ignition time. The above statements explain why the majority of the thermally treated wood specimens ignited earlier than untreated wood. Heat treatment of spruce wood increases its mass loss rate, as observed in the majority of specimens. This claim is consistent with the findings of the authors of ref. [34], who demonstrated that higher temperatures of heat treatment of spruce wood caused a higher burning rate. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline  & **Df** & **Sum Sq** & **Mean Sq** & **F Value** & **Pr (>F)** & **Influence** \\ \hline
**Specimens** & \(1\) & \(1100\) & \(1100.0\) & \(19.91\) & \(0.0111\) & * \\ \hline
**Residuals** & \(4\) & \(221\) & \(55.2\) & & & \\ \hline Signif. codes: ‘influence’ - _p_-value to: ‘*’ - 0.05’ & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: ANOVA of mass loss of TP\(-\)R, U\(-\)R specimens at 780 s.  
Figure 6: Mass loss and relative mass loss rate over time for TP\(-\)R, TF\(-\)R, and U\(-\)R specimens. (**A)** Mass loss; (**B)** mass loss rate. On the other hand, we did not observe a difference in the mass loss rate for TF-R specimens compared to U-R specimens. It suggests that heat treatment of spruce wood will not affect this value unless direct flame burning of the specimens occurs. A similar result was demonstrated by a study [31] claiming that the exposure to a flameless radiant heat source does not affect the maximum burning rate of heat-treated spruce wood compared to untreated wood. We further assume that the higher mass loss rate of TP-R and TP-T specimens (thermowood prism) compared with TF-R and TF-T specimens (thermowood floor) is influenced by the specimen density, which according to several authors [43; 44] influences the mass loss rate. The average densities of the prism thermowood specimens were approximately 50 kg\(\cdot\)m\({}^{-3}\) greater than the other specimens (Table 7). Density differences between specimens were also observed in the density of the annual rings in Figure 1. Specimens TP-R and TP-T feature denser annual rings that affect the wood density [45]. The spruce wood density increases with decreasing annual ring width [46] and decreases as a result of heat treatment [47]. In this case, higher density resulted in a higher mass loss rate and higher mass loss. The causes of this phenomenon have not been clarified yet. One reason could be the changes in the chemical composition of heat-treated wood, containing more lignin than untreated wood [24; 48; 49; 50]. On the one hand, higher lignin content has been confirmed to have an effect on reducing the mass loss rate [37; 51; 52].