# Effect of Spruce Wood Density on Selected Fire-Technical Parameters during Thermal Loading
###### Abstract  
The paper evaluates the effect of Spruce wood density on the parameters of mass loss and mass loss rate during exposure to thermal load. The intention was to determine whether the effect of density is still evident after the application of flame retardants to the test samples. Groups of samples with different densities under the same retardant treatment were compared. The differences in densities of the compared groups of samples were different for each flame retardant. Water-soluble flame retardants based on inorganic salts were used. For testing, a simple test method was used in which the samples were exposed to direct flame from a Bunsen burner. The results of the study are the findings of how wood density affects the burning process of the samples treated with flame retardants. Statistical evaluation of the experimental results shows a significant effect of wood density on the monitored parameters even when flame retardants are used. For a difference in sample densities of 244 kg\(\cdot\)m\({}^{-3}\), there was a density dependence of the mass loss rate, with the lower density samples having a higher mass loss rate (\(0.158\%\)\(\cdot\)s\({}^{-1}\)) over the whole experimental period compared to the higher density samples (\(0.077\%\)\(\cdot\)s\({}^{-1}\)). The ANOVA test also demonstrated the influence of density on the mass loss of the samples at the above density difference. At lower density differences (51 kg\(\cdot\)m\({}^{-3}\) and below), the effect of sample density on the observed parameters was no longer evident. The fire spread rate parameter was also investigated. Here, a linear correlation between the difference in sample densities and the difference in the values of the above parameter at high and low densities is observed with a reliability coefficient R\({}^{2}\) = 0.99.  
spruce wood; density; mass loss; mass loss rate; flame retardant +
Footnote †: journal: Physics Letters B  
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Footnote 1: email: mik.konarik@uniza. produced under heat load, the determination of heat release and mass loss rate can be performed using the cone calorimeter method [24]. There are other non-normal methods for the evaluation of flame retardants that are sensitive enough to investigate the combustion behaviour of these materials. Several authors [25; 26; 27; 28; 29; 30; 31; 32] have already evaluated some flame retardants using different methods [9; 33]. Among the suitable test methods, one involves exposing test samples to direct flame [34; 35]. In this procedure, actual sample masses are recorded over time, and subsequently, the mass loss rate can be calculated. It is one of the most significant fire engineering parameters used in the past in mathematical models of forest fire spread [36] or in modelling the fire resistance of wooden structures [7; 37]. It is also possible to determine the heat release rate HRR from this parameter under certain conditions [38]. The specific testing conditions were already set by the researchers according to the type and kind of material. To compare the effectiveness of different wood retardant treatments with sufficient accuracy, it is necessary to minimise other factors that affect wood combustion. These are mainly the chemical composition, structure, density, material surface, moisture content and geometric shape of the wood [39; 40; 41; 42]. Chemical composition and structure are mainly determined by the type of wood. Experimental tests are always conducted on the same species of wood when the retardation effects are compared. Also, the shape of the samples (dimensions), the moisture content of the wood and the material treatment must be identical. The above factors (species, moisture content, dimensions and treatment of the samples) shall be minimised in the experiments. However, a significant, frequently neglected, factor is the density of the wood. Wood density indicates the mass per unit volume of wood [43], usually given at a specific moisture content because this directly influences it [44; 45].