# Effect of Spruce Wood Density on Selected Fire-Technical Parameters during Thermal Loading
## 2 Materials and Methods  
### Materials  
The samples of spruce wood (_Pica abies_) with dimensions 200 \(\times\) 100 \(\times\) 10 mm were used for the experiment. They were selected to be free of lumps, resins and other defects that could affect the combustion process. Before the experiment and the application of flame retardants, all samples were placed in the laboratory for two weeks at a temperature of 21 \({}^{\circ}\)C and a humidity of approximately 40% to ensure that the samples' moisture content matched the end-use conditions. The final moisture content of the samples before applying the flame retardants was approximately 12%. For the first experiment, untreated samples were first used. Due to the high variability in the measured data caused by the samples being burnt through, samples treated with flame retardants were used for further experiments to see if the wood density would affect the experimental results with such samples. Three different wood flame retardants were applied to three groups of samples. The applied retardants were transparent, water-soluble, colourless coatings based on inorganic salts. Their specific composition is not specified here as the aim of this paper is not to evaluate the retardants themselves. They were applied to all sides of the samples, including their edges, in one coat using a brush. The average amount of coating varied between 60 and 80 g\(\cdot\)m\({}^{-2}\) depending on the specific retardant (according to the instructions of the manufacturers of the individual flame retardants). The retardants on the samples were dried for two months in a room with a temperature of 21 \({}^{\circ}\)C and a humidity of about 40%. The samples were supplied by an external contractor. When measuring the densities of the individual samples, we found a very large variability. The aim was to take advantage of this variability and create groups of samples with various density differences. To investigate the effect of density on the mass loss rate and mass loss, each studied sample group included ten higher- and ten lower-density samples when one type of flame retardant was applied. The designations of each sample group are specified in Table 1. The average sample densities used in the experiment and the range of variation of the densities of the individual samples within a test set are depicted in Table 2.  
As demonstrated in Tables 1 and 2, the samples were intentionally selected to exhibit varying mean densities within each group. For each retardant coating, the samples comprised both higher (>450 kg\(\cdot\)m\({}^{-3}\)) and lower (<450 kg\(\cdot\)m\({}^{-3}\)) densities, with the magnitude of density differences differing for each coating. The boundary between high and low densities was determined approximately as the median of the densities of all samples used for the research. This design enabled us to establish the relationship between the difference in sample densities and the resulting testing parameters. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Samples** & **High-Density Wood Samples** & **Low-Density Wood Samples** \\  & **(\(\rho\) > 450 kg\(\cdot\)m\({}^{-3}\))** & **(\(\rho\) < 450 kg\(\cdot\)m\({}^{-3}\))** \\ \hline Samples treated with Retardant 1 & R1-HD & R1-LD \\ Samples treated with Retardant 2 & R2-HD & R2-LD \\ Samples treated with Retardant 3 & R3-HD & R3-LD \\ Untreated samples & U-HD & U-LD \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Designation of individual sample groups.