# The Effect of Pine Wood Thickness on the Freezing and Heating Process in Warm-Air Drying
## 3 Results and Discussion
Both groups of samples, after being placed in an environment with a temperature of \(-25\)\({}^{\circ}\)C, experienced a rapid decrease in temperature to just below 0 \({}^{\circ}\)C. This temperature was reached within 2.5 h. Figure 3 also shows the temperature distribution across the cross-section of the samples, where the temperature decrease was faster at the surface than in the middle. The temperature distribution is consistent with the moisture distribution on the cross-section of the samples. The temperature difference was greater for the 50 mm thick samples. In the second section of the temperature curves, the temperature of the samples hardly changed, with a smaller measured temperature gradient for the thinner samples. At this stage, the free water state changes from liquid to solid. The change of state matter was  
Figure 3: Sample cross-sectional temperatures and ambient temperatures depending on time (thawing process). Figure 2: Moisture gradients of (**A**) 30 mm samples and (**B**) 50 mm samples. faster for the thinner samples, where temperatures in the centre started to decrease after about 7 h, and for the thicker samples (50 mm) after 7.5 h.  
The last section of freezing is also different, where the effect of sample thickness was significant. The 30 mm thick samples had a faster temperature decrease with a gradual equilibration of temperatures between the centre and surfaces of the samples. After 10 h, the temperatures at the centre were identical to those at the surface of the samples. The temperature drop stopped at \(-20\)\({}^{\circ}\)C after 19 h.  
The process was slower for the 50 mm thick samples, and the temperatures at the cross-section of the samples were equal after 12 h. The temperature of \(-20\)\({}^{\circ}\)C was reached only at the end of the measurement, after 24 h. Also, the temperature profile and moisture gradient were larger for the thicker samples. The 30 mm thick samples reached \(-20\)\({}^{\circ}\)C slightly earlier, so it was necessary to wait for the 50 mm thick samples to reach \(-20\)\({}^{\circ}\)C as well. In the freezing process, we would like both samples to reach the freezing temperature of \(-20\)\({}^{\circ}\)C in order to have the same starting point. The study by [PERSON] [4] investigated the effects of combining freezing and heating treatments on tropical wood. According to the findings, the two-stage freeze-heat treatment was more effective than heat treatment alone in enhancing properties such as water absorption, hydrophobicity, and impact strength. This improvement was attributed to the selective degradation of hemicelluloses, which influences the wood's physical and mechanical properties. The combined treatments helped reduce the adverse effects of high temperatures while promoting water loss by increasing the wood's porosity through freezing. After the freezing process, the samples were transferred to a humidity chamber, where they were heated in a humid air environment. Figure 4 shows the temperature profile of the samples and the environmental parameters in the chamber. During the heating of the frozen samples, there was a visible difference in the measured temperatures and their profiles between the 30 and 50 mm thick samples. The defrosting and heating process was faster and with less temperature difference for the thinner samples. The temperature curves can identify three zones. The first zone was an increase in temperature from a negative value (\(-20\)\({}^{\circ}\)C) to \(0\)\({}^{\circ}\)C; the second zone was  
Figure 4: Temperatures at the sample cross-section, ambient temperature, and relative humidity as a function of time (heating process). almost no temperature change, where the supplied heat was consumed to change the state of water in the wood. The third zone was a zone of temperature increase until the required temperature of 65 \({}^{\circ}\)C was reached. The first two zones were minimally affected by the thickness of the samples.