## 2 Materials and Methods
### Methods
#### 2.2.2 Heat Treatments  
Heat treatment modification was investigated on 75 samples of sapwood and 40 samples of the heartwood of stone pine (_P.pine_) with a width of 25 mm, a thickness of 15 mm (in crosscut direction), and a length of 50 mm, which were cut according to the EN 113:2006 [28]. As a control, 75 samples of beech (_F. sylvatica_) and sapwood Scots pine (_P. sylvestris_) were used with the same size of stone pine. Fifteen samples of sapwood of stone pine and eight samples of the heartwood of stone pine were used with the relevant controls for each different treatment temperature. All samples were oven-dried (103 \(\pm\) 2 \({}^{\circ}\)C; 24 h) before and after heat treatment and weighed to calculate the mass loss due to thermal modification. The thermal modification processes were performed as prescribed by the commercial Silvapro treatment [29]. Five maximum modification temperatures were applied: 180 \({}^{\circ}\)C, 195 \({}^{\circ}\)C, 210 \({}^{\circ}\)C, 220 \({}^{\circ}\)C, and 240 \({}^{\circ}\)C. The samples were modified at the target temperature for 3 h. Each different heat treatment, regardless of the maximum temperature, was characterized by seven different heating stages that gradually increased the temperature and kept it constant for the required time. The total heating time was eight and a half hours. The treatments were carried out in a laboratory-scale modification chamber (Kambic, Slovenia) in an atmosphere with reduced oxygen concentration achieved by applying vacuum in the first phase. In the following phases, the atmosphere consisted of superheated steam to avoid the formation of cracks and changes on the wood in the early phases of the treatment. At the end of the treatment, a cooling phase was carried out in the chamber at atmospheric pressure. The thermally modified wood was characterized by determining the mass loss and hygroscopic properties. The hygroscopicity of the heat-treated wood, i.e., the ability of the material to absorb water from the ambient air, was tested by placing the samples in a glass chamber over distilled water (RH = 98 \(\pm\) 2%). Seven of fifteen thermally modified samples were used for this test. The mass of the samples was determined after 24, 72, 168, 240, 408, 600, and 720 h until the constant mass was determined. After thermal modification, the color of the samples was determined. Color changes are one of the first and easiest indicators of changes in the wood. Two radial sides of each sample were scanned. The color in the images was evaluated using the CIE L*a*b* system (Commission Internationale de l'Eclairage), where L* represents lightness, varying from 100 (white) to 0 (black); a* (along the \(x\)-axis) red (+) to green (\(-\)), and b* (along the \(y\)-axis) yellow (+) to blue (\(-\)) being the chromaticity coordinates [30; 31] using CoreIDRAW 8 software (Ottawa, ON, Canada). Scanning was performed with the HP Scanjet G4050. The scanning parameters were set separately for dark, semi-dark, and light samples to obtain the most representative color of the surface. The average color coordinates of the respective plane were calculated directly with the software. It should be noted that the entire sample surface was analyzed. The color change (\(\Delta\)E) between the control samples and the treated samples were calculated using Equation (1):  
\[\Delta\mathrm{E}^{*}=\sqrt{\left(\Delta\mathrm{L}^{*}\right)^{2}+\left(\Delta \mathrm{a}^{*}\right)^{2}+\left(\Delta\mathrm{b}^{*}\right)^{2}} \tag{1}\]  
where \(\Delta\)L*, \(\Delta\)a*, and \(\Delta\)b* are the differences between the initial and final values of L*, a*, and b*, respectively. Statistical Analysis  
Statistical descriptive parameters (i.e., mean value and standard deviation) are reported for each measurement. Moreover, the significance of the mean differences within the different preservative products and between the different impregnation methods was tested using [PERSON]'s HSD post hoc test (\(p\)-value \(<\) 0.05).