## 2 Materials and Methods
### Determination of the Ignitability of Wood Samples by the Action of a Small Ignition Initiator  
Solid wood panels are rated as building elements according to EN 13353 as D-s2, d0 [62]. The terms of the classification are \"reaction to fire D-s2, d0 by [63]\". Reactions to fire testing methods are designed to simulate the fire incipient and growth phases. The purpose is to evaluate how products and materials contribute to the early stages of a fire in terms of ignitability, flame spread, heat release, smoke production, and occurrence of flaming droplets or particles [64]. D-s2, d0 means \"combustible materials--medium contribution to fire, with the speed of emission absent or weak during combustion\" according to [63]. The priority of the experiment was to monitor the critical parameters of the ignition based on the change in board thickness. Most wood products fall into classes D-s2, and d0 [62]. The 2 tests were conducted for the classification of building products (i.e., wood-based materials) according to EN 13501-1. The first test followed the test method of EN ISO 11925-2 Inflammatory of building products with direct exposure to flames and the second test followed the test method of EN 13823 Thermal load from a single burning object [65]. Our research applied the first test.  
The experiments were conducted in the Fire Chemistry Laboratory of the Department of Fire Engineering FBI UNIZA. The test method was determined according to STN EN ISO 11925-2 [66] to test the ignitability of building materials due to the influence of a small flame combustion source. The test was performed in a test chamber (Fire Engineering and Expertise Institute, Bratislava, Slovakia) (Figure 2).  
Figure 1: Oak sample in the process of applying protective coatings.  
The sample was fixed in the sample holder, and the holder was placed in the test chamber in a horizontal position. The combustion source was a small propane gas burner (Figure 3c). The height of the test flame was 20 mm. The burner was adjusted to an inclined position at an angle of 45\({}^{\circ}\). The test started the moment the burner was pressed against the sample surface. The time of the experiment was set at 30 s. After 30 s, the burner was moved away and the processes of ignition, spontaneous combustion, falling off, or dripping of the sample were observed.  
The experimental data to monitor the behaviour of the samples exposed to flame for 30 s included the mass loss and the flame spread over the surface during the experiment.  
The experiments were carried out in two steps:  
1 st step: monitoring the effectiveness of the HRP-retardation coating applied to the wood samples.  
2 nd step: monitoring the effectiveness of the HRP-retardation coating on wood samples previously treated with fungicidal coatings (FUN, BOCHFUN, BOCH, N  
Figure 3: (a) Prepared experiment; (b) experiment at 10 s; (c) close-up of flame–sample contact at 20 s.  
Figure 2: (a) Test chamber according to STN EN ISO 11925-2 [66]; (b) scheme of small flame test, EN ISO 11925-2 (sample size 0.09 \(\times\) 0.25 m\({}^{2}\)) [62; 66]; (c) demonstration of the experiment on a sample of SMR + HRP (retardant) in 30 s.  
The evaluation of the experiment focuses on the following:
* Evaluation of the effectiveness of the fire-retardant coating on the selected wood samples. A significant dependence of the mass loss during the experiment and flame spread within 30 s on the wood species were determined using the QtiPlot software program (QtiPlot 0.8.5 in the GNU/Linux OS environment of the UBUNTU 6.06 distribution, Bucuresti, Romania) [67].
* Evaluation of the influence of the fungicidal coating on the retarder effectiveness and search for the optimal fungicidal coating as an undercoat for the selected retardant, again, based on the mass loss during the experiment and flame spread within 30 s.