# The Influence of Wood Moisture Content on the Processes of Freezing and Heating
## 3 Results and Discussion
No differences were found in the freezing and heating processes due to the position of the samples in the laboratory equipment. The measured data showed a different course of temperatures in the samples, caused by the influence of wood moisture content. For samples with an average MC of 35.47%, the temperature drop was faster, with a negative temperature being reached in the middle of the sample after only 1 h and 20 min. The difference between the temperature in the middle and surface of the samples was max. 1.5 \({}^{\circ}\)C. Due to the small amount of free water in the samples, no stage of water state change (phase transition) was recorded. After 10 h, the temperatures on the entire cross-section of the samples reached the lowest value of \(-\)13.2 \({}^{\circ}\)C, which did not change further. For samples with an average initial MC of 81.38%, three stages of temperature reduction in the wood were observed. The first stage was from the initial temperature to 0 \({}^{\circ}\)C. In this stage, the temperature decreased more slowly compared to the samples with lower MC. There was a significant difference between the surface and middle layers of the wood. In the middle of the samples, the temperature was higher by a maximum of 2.5 \({}^{\circ}\)C. The time to reach 0 \({}^{\circ}\)C in the center of the samples was approximately 2 h. In the second stage, the temperature in the cross-section of the wood was just below 0 \({}^{\circ}\)C and there was a phase transformation of water in the wood from liquid to solid state. The temperature in the middle of the samples stabilized at \(-\)0.8 \({}^{\circ}\)C and did not change until 7 h. Toward the end of the stage, the difference between the temperature in the middle and below the surface of the samples increased. The duration of this stage was approximately 5 h. In the third stage, the temperatures in the samples decreased and the difference between the temperature in the middle and below the surface of the samples gradually decreased. The temperature dropped and the lowest temperature of \(-\)20.11 \({}^{\circ}\)C was reached after 24 h. Following [1], we discovered that \(-\)20 \({}^{\circ}\)C appears to be the most practicable for reduced shrinkage; the temperature of the samples did not change. Ref. [12] observed phase transitions of water in pine and discovered that the melting temperature of free water depends on moisture content. After the temperature stabilized, the samples were placed in a Memmer humidity chamber where the heating process used in hot air drying was simulated. Figure 3 shows the course of temperatures in the samples and the parameters of the ambient air in the heating process. Again, the temperature difference due to the different MCs of the samples is visible. The defrosting and heating processes of samples with lower MC are significantly easier. The samples had a higher initial temperature (\(-\)13.2 \({}^{\circ}\)C) and the temperature increase was continuous (almost linear). At the end of the heating, the temperature increase in the samples slowed. The desired temperature of 65 \({}^{\circ}\)C in the middle of the samples was reached in 2 h and 20 min. Research [6] established that the optimum temperature in the heating phase when drying frozen spruce is 30 \({}^{\circ}\)C instead of the usual 50 \({}^{\circ}\)C. This value leads to improved drying uniformity without significantly prolonging the drying time. For the second group of samples (MC \(>\) _FSP_), the heating process was divided into three stages similar to those described in the sample freezing process. The initial temperature of the samples was lower (\(-\)20 \({}^{\circ}\)C), and the duration of the first stage, when the temperature rise in the middle of the samples stopped, was 30 min. Temperature differences in wood cross-sections were minimal. The second stage, when the water level in the wood changed from solid to liquid, was 40 min. The difference in temperature at the surface and n the middle of the samples was measured.