# The Influence of Wood Moisture Content on the Processes of Freezing and Heating
## 1 Introduction  
The process of water freezing in wood has not been fully elucidated theoretically. It is known from molecular physics that the smaller the radius of the capillary, the greater the cooling required to initiate the crystallization process. The walls of macrocapillaries in wood are thin (\(\sim\)0.005 mm) and highly elastic. The water push by the ice can move into the secondary capillaries, and therefore the pressure in the macrocapillaries of the wood cannot increase if it is not at the fiber saturation point. This causes an increase in the freezing temperature of the water in the wood's capillaries. Lignin reduces the elasticity of cell walls and causes a very low freezing temperature of free water. The content of nutrients in the water of living trees also has an influence, whose freezing temperature is lower than felled ones, even if the lignin content is the same. The freezing temperature of water in the surface layers of wood is lower than inside because its cooling begins from the surface, where the first places of crystallization of water also form. Researchers [1] described that water to ice transformation leads to an expansion of liquid water in the lumen, thus imparting a compressive stress to the cell wall, which, together with the moisture loss, increase the rigidity of the cell. The freezing process of bound water in wood is complicated. Due to the thin dispersion construction of bound water, there is no constant and determined freezing temperature. With decreases in water volume, the likelihood of the formation of crystallization centersdecreases remarkably, and the duration and the degree of cooling of the water increase. The adsorption water in the monomolecular cannot crystallize because water molecules must be arranged not in one plane, but must form a spatial lattice. To form new H bonds with secondary water molecules (create ice), they must be released from H bonds with capillary walls. Because the H bonds between water molecules are energetically equal to the hydroxyl groups of the wood, under these conditions ice can form in the submicrocapillaries of wood, even at low temperatures [2]. Previous research [3] focused on the latent heat of bound water in wood during freezing and defrosting by mathematic equations. As also shown [3], the specific latent heat of fusion and crystallization of the bound water in wood decreases with a decrease in the temperature. This result explains that the absorption of the bound water in capillary-porous materials, including wood, is accompanied with heat release and with an increase in the alignment of the water molecules in the materials. Heating of frozen wood is fundamentally different from the heating of unfrozen wood. The complexity of the problem lies in the variability in the temperature field over time and in the conditions of the phase transformation of water. Ref. [4] observed phase transitions of water in pine and cell walls. Samples were prepared on the freezing and melting of water at 10-20 different moisture contents. The results showed that the melting temperature of the free water depends on moisture content. Complete thawing of lumber by moist air occurs during kiln drying in winter conditions. Ice is first completely dissolved during heating. Then, it is heated from 0 \({}^{\circ}\)C to the desired temperature in the middle of the dried wood, which is lower than the temperature of the environment. The humidity of the environment in this method is 80% to 90% [2]. Research [5] also showed that a significant amount of water is removed from wood during the beginning of the heating phase (thawing). Next, results [6] clearly showed that the freezing rate influences the defrosting time and speed. Similarly, others [7] clearly indicated that the freezing rate significantly influences the thawing time and speed, which increase by 13-17% in the case of slowly frozen wood (at \(-1\)\({}^{\circ}\)C\(\cdot\)min\({}^{-1}\)) compared to rapidly frozen wood (at \(-10\)\({}^{\circ}\)C\(\cdot\)min\({}^{-1}\)).