# Effect of Spruce Wood Density on Selected Fire-Technical Parameters during Thermal Loading
## 3 Results and Discussion  
In our case, the fundamental parameters for monitoring the influence of the density of spruce wood samples on the combustion process are the mass loss and the mass loss rate. The tested samples were carefully selected to ensure consistency, free of knots, visible resins and any visible deformations or cracks. They displayed identical wood texture and moisture content before coating and were processed by the same method--smooth planning. However, during the experiment, it was observed that some samples burned through from the sides to the top, resulting in a significant increase in the mass loss and, consequently, the mass loss rate. The flame retardants used dilute the combustible gases released from the wood by heat and thus make it more difficult to ignite. In the above cases, the effect of the retardants was apparently insufficient, which may have caused the spontaneous combustion of the samples. This may be due to the application of less retardant to the samples or an overall lower concentration of active substance in the individual retardants combined with a higher flame intensity on the samples, where after a certain time the retardant failed to produce sufficient non-flammable gases to prevent spontaneous combustion. Why this was the case for only some samples is not entirely clear. It may be due to the different properties of the individual samples (chemical composition, resin, porosity, etc.) or possible variations in the testing. To prevent these outliers from significantly affecting the average values, they were excluded from the overall evaluation. The identification of these extremes was performed statistically using box plots displaying the values of the total mass loss over the 600 s testing time for each group of samples. These box plots were constructed using the statistical software \"R\" (The R Project for Statistical Computing) version 4.1.2 and are presented in Figure 3.  
From the plot in Figure 3, we can see that there were outliers in the total mass loss for each group of samples. The highest number of outliers (three) occurred in the lower-density samples with the application of retardant 3 (R3-LD). It was mainly due to the low variability in the other measured data from that group. The number of outliers for each group of samples was random and did not depend on the treatment applied or the sample density. We then excluded the extreme values (marked by red dots) from further evaluations to minimise their undesired influence on the overall evaluation of the experiment. The mean values of the mass loss of the samples after the experiment, including the standard deviation, and the maximum mass loss rate are given in Table 3.  
The untreated samples exhibited a variability of up to 18% mass loss for the higher-density samples and approximately 55% mass loss for the lower-density samples. This is because the untreated samples experienced spontaneous combustion, with some cases even burning through the side edges to their top. Figure 3: Box plots of the mass loss of each group of samples at 600 s testing time with outliers indicated (red dots). \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multirow{2}{*}{**Total Mass Loss (\(\mathbf{\delta}\))**} & \multirow{2}{*}{**Standard Deviation of Total Mass Loss (\(\mathbf{\sigma_{j}}\))**} & \multirow{2}{*}{**Max Mass Loss Rate (\(\mathbf{v_{r\_max}}\))**} & \multirow{2}{*}{**Standard Deviation of Max Mass Loss Rate (\(\mathbf{\sigma_{v\_max}}\))**} \\  & & **[\%]** & **[\%]** & **[\%-s\({}^{-1}\)]** \\ \hline R1-HD & 19.47 & \(\pm\)1.16 & 0.0816 & \(\pm\)0.0072 \\ R1-LD & 23.88 & \(  
From the beginning of the experiment to the testing time of 300 s, we monitored significant differences in the mass loss rate of the samples as a function of their density. This can be seen in Figure 4. The lower-density samples displayed a higher mass loss rate than the higher-density samples from the beginning of testing.