## 4 Results of Spruce Thermal Degradation by Thermal Loading  
The largest changes were observed for the sample that was in direct contact with the radiant heat source (sample 1S). The thermal degradation process started at 690 s, at a sample temperature of 354.2 \({}^{\circ}\)C. At 455.8 \({}^{\circ}\)C, the sample began to bend slightly. There was a mass loss of 18.83%. The amount of charring was 12 mm. The surface of the sample exposed to the radiant heat source was charred and cracked (Figure 3a).  
Sample 2S was 12 mm from the radiant heat source. Compared to sample 1S, the thermal degradation process occurred at a later time of 1155 s and at a higher temperature of 410.4 \({}^{\circ}\)C. The charring process was visible over the entire area of the bottom of the sample, especially in the area of the located metal ring (Figure 3b). The layer of charring was 5 mm, and the mass loss was 8.52%.  
At a distance of 25 mm (sample 3S), there was charring of the sample at the sides and at the bottom, where the sample was in contact with the circle (Figure 3c). Charring of the sample was not observed. Towards the end of the experiment (1200 s), a slight bending of the sample was observed. The weight loss was 6.32%, and the charring height was 3.5 mm.  
From the measured values, a temperature curve was constructed for all spruce wood samples (Figure 4). The colored highlighted points represent the monitored processes (smoking, cracks, thermal degradation, and bending). Sample 1S showed a constant temperature rise without fluctuations as a function of time. For samples 2S and 3S, the  
Figure 3: Photo-documentation from spruce experiments. (**a**) The 1 st picture shows the start of the smoking process for 291.2 \({}^{\circ}\)C and at 540 s, and the 2 nd picture shows the charring layer. (**b**) The 1 st picture presents the start of the smoking process for 228.2 \({}^{\circ}\)C and at 630 s; the 2 nd picture presents the start of the smoking process for 351.6 \({}^{\circ}\)C at 1020 s; and the 3 rd picture presents acid cracks, smoking, and thermal degradation (for 410.4 \({}^{\circ}\)C and at 1155 s). (**c**) Pictures for 3S samples without charring layer.  
temperature fluctuations caused by lifting the sample off the plate to observe the thermal degradation processes are visible. Samples 2S and 3S, which were placed on the metal ring further away from the radiant heat source, have the same temperature history up to approximately 400 s. The consequent temperature differences correspond depending on their distance from the source.  
[PERSON] et al. [59] wrote that thermal treatment at temperatures below 200 \({}^{\circ}\)C does not influence the fire safety of an important class of wooden products. The first thermal changes in sample 1S were above the temperature of 227.6 \({}^{\circ}\)C at a time of 405 s.  
The sample of spruce wood even at a distance of 32 mm shows thermal degradation. Previously, [PERSON] et al. [60] reported that spruce wood is more susceptible to heat load independent of the distance from the source, because it has a lower density. In their experiment, they observed the highest mass loss of spruce wood, which was due to the high resin content. The results of the present study are consistent. Higher mass losses in conifers (including spruce wood) were also confirmed by [PERSON] et al. [61]