# The Effect of Pine Wood Thickness on the Freezing and Heating Process in Warm-Air Drying
## 3 Results and Discussion  
The average moisture contents of the samples at each stage of the research and the average dry density values are given in Table 1.  
The average moisture contents of the samples were similar. The difference of 2.34% between the 30 and 50 mm samples was insignificant, and similar results were obtained for the average wood density as well. Therefore, the groups of samples could be compared. The average moisture loss of the samples due to the freezing process was greater for the thicker samples (50 mm), but the difference between the thicknesses compared was only 2.4%. The freezing time was the same for both sample thicknesses, which was 24 h. Moisture loss after defrosting and heating was greater by an average of 7.3% for the thinner samples (30 mm) compared to the 50 mm thick samples. This occurred because the thinner specimens overheated more quickly throughout their cross-section than the thicker specimens, leading to faster moisture evaporation. These findings are consistent with the research conducted by [PERSON] [8] and [PERSON] [15], who studied the effects of prolonged exposure to repeated cycles of freezing and thawing due to natural, gradual temperature changes. Their results demonstrated that under slow freezing conditions, the rate of freezing is the key factor that influences the extent of damage caused by the freezing process. Based on the Missio studies [4; 5], the freezing rate plays a critical role in altering the physical properties of wood, impacting its durability and structural integrity. Other researchers, [PERSON] and [PERSON] [16], explored the use of pre-freezing as a method to reduce shrinkage-related degradation in collapse-prone wood. Their findings revealed that the formation of ice from free water in the lumen creates significant pressure on the cell walls when moisture levels are high. This pressure drives moisture from the cell walls toward the frozen water in the lumen, effectively mitigating the risk of shrinkage and collapse during the drying process. This process helps maintain the structural integrity of the wood by managing moisture distribution more effectively. The values of moisture gradients and moisture distribution over the thickness of the samples were measured before and after the freezing process and after the heating of the samples, as shown in Figure 2.  
Higher moisture gradient values were observed for the 50 mm thick samples. The freezing process did not affect the magnitude of the moisture gradient, and the heating process reduced its value by approximately 3%. The 30 mm thick samples had moisture gradient values that were, on average, 12% smaller than those of the 50 mm thick samples. For the thickness of 30 mm, the freezing process increased the moisture gradient, and the heating process again reduced the differences between the moisture content at the centre and at the surface. The moisture gradient confirmed a remarkable (directly proportional) effect of sample thickness on the magnitude of the moisture gradient. The time profile of the temperatures measured on the cross-section of the samples during the freezing process is shown in Figure 3. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **30 mm** & **50 mm** & **30 mm** & **50 mm** & **30 mm** & **50 mm** \\ \cline{2-6}  & **Average Moisture Content (\%)** & \multicolumn{2}{c}{**Moisture Loss**} & \multicolumn{2}{c}{**Density in Dry State (kg\(\cdot\)m\({}^{-3}\))**} \\ \hline Before freezing & 81.38 & 79.04 & 0 & 0 & \\ \hline After freezing & 77.27 & 72.53 & 4.11 & 6.51 & 429.5 & 427.1 \\ \hline After heating & 59.76 & 62.37 & 17.51 & 10.16 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average moisture contents and moisture losses of the samples before and after freezing and after heating and average densities in the dry state of the samples. The temperature profiles measured in the samples can indicate the impact of the thickness of the samples on the freezing process of the water in the samples.