## 4 Conclusions  
In the study, the fire-technical properties of thermally treated spruce wood such as mass loss rate and mass loss were evaluated. Samples with different densities were used. The fire performance was investigated when a direct flame was applied perpendicularly to the radial and tangential sides of the specimen. According to the average mass loss rate and average mass loss achieved when direct flame was applied to the samples using our proposed test method, conclusions here drawn. Thermal treated spruce wood with higher density has a higher mass loss and a higher mass loss rate than thermally treated spruce wood with lower density and untreated wood. The difference in densities was 50 kg\(\cdot\)m\({}^{-3}\). The average mass loss of the higher-density thermowood samples (423 kg\(\cdot\)m\({}^{-3}\)) was 55%, and that of the lower-density samples (370 kg\(\cdot\)m\({}^{-3}\)) was 39%. The radial samples (radial side exposed to flame) of the higher-density spruce thermowood had a significantly higher mass loss and mass loss rate compared with the floor and untreated wood samples (difference in weight loss of 25% and 27%, respectively). The higher-density samples had the highest mass loss rate (radial 0.207%-s\({}^{-1}\), tangential 0.196%-s\({}^{-1}\)) compared with the lower-density samples (radial 0.082%-s\({}^{-1}\), tangential 0.13%-s\({}^{-1}\) and untreated radial 0.086%-s\({}^{-1}\)). According to the results, the fire resistance of spruce thermowood deteriorates with increasing density. Based on the \"fire spread rate\" parameter, we can conclude that a thermowood fire will spread faster on the tangential side. The \(R_{f_{8}}\) values of the tangential floor samples were 29% higher compared with the radial samples. The dependence of the mass loss on the density of the thermowood and the side exposed to the flame was also confirmed by analysis of variance with \(p\) value = 0.00254 resp. 0.09451. The results also showed that the mass loss of radial floor spruce thermowood samples (lower density samples) was not statistically significantly different from the untreated samples (\(p\)-value = 0.31). From the above it can be concluded that the thermal treatment of spruce wood does not have a significant effect on the selected fire characteristics of the wood (mass loss rate and mass loss). However, thermally treated wood is easier to ignite as confirmed by the spontaneous combustion times. While for thermowood, 10 samples out of 12 burned in an average time of 5 min from the start of experiment, on the contrary for untreated wood burned, in which only 1 sample out of 3 burned by the end of the experiment at almost the 10 th min. Figure 7: Average temperatures on the specimen surface during the experiment. (**A**) samples TP\(-\)R and TP\(-\)T; (**B**) Samples TF\(-\)R and TF\(-\)T.  
Our research, aiming to improve the quality of research in this area, confirms the results of a study by several authors who evaluated the fire resistance of thermowood. It provides new findings on the differences in burning of thermowood on the radial and tangential sides and the differences in burning as a function of the specimen density. So far, however, research has only assessed the fire behaviour of thermowood based on laboratory tests. Therefore, research in this area needs to be further focused on conducting large-scale tests to determine the actual behaviour of thermowood under realistic fire conditions. Because thermowood is often used for wall cladding, research also needs to focus on monitoring fire spread in the vertical direction where it is most prominent. Conceptualization, P.M. and M.V. ; methodology, P.M. and M.V. ; software, P.M.; validation, M.K. and K.K. ; formal analysis, K.K. ; resources, P.M. and K.K. ; writing-original draft preparation, P.M.; writing-review and editing, K.K. and M.K. ; visualization, K.K. ; supervision, M.V. ; project administration, P.M.; funding acquisition, P.M. All authors have read and agreed to the published version of the manuscript.