The main physical properties of wood are：
① Density
　　It refers to the weight of wood per unit volume. The weight and volume of wood are affected by moisture content. The ratio of the dried weight of the wood sample to its saturated water volume, the dried volume and the oven dried volume is called the basic density respectively Absolute dry density and furnace dry density. The ratio of the weight of wood after air drying to the volume after air drying is called the air dry density of wood. Wood density varies with tree species. The air dry density of most wood is about 0.3~0.9 g/cm3. The wood with high density has generally high mechanical strength.
② Wood moisture content
　　It refers to the percentage of water weight in wood to the weight of dried wood. The water in wood can be divided into two parts. One part exists in the cell wall of wood cells, which is called adsorbed water; The other part exists between the cell cavity and the cell gap, which is called free water (free water). When the adsorbed water reaches saturation without free water, it is called fiber saturation point. The fiber saturation point of wood varies from 23% to 33% depending on the tree species. When the moisture content is greater than the fiber saturation point, the effect of moisture on wood properties is very small. When the moisture content decreases from the fiber saturation point, the physical and mechanical properties of wood change accordingly. Wood can absorb or evaporate water in the atmosphere, and achieve a constant moisture content in line with the relative humidity and temperature of the surrounding air, which is called equilibrium moisture content. The equilibrium moisture content of wood varies with the region, season, climate and other factors, and is about 10-18%.
③ Dilatability
　　The volume of wood expands after absorbing water, and shrinks after losing water. The drying shrinkage of wood from the fiber saturation point to the oven dry is about 0.1% in the grain direction, about 3-6% in the radial direction, and about 6-12% in the tangential direction. The difference of radial and tangential shrinkage is the main reason for wood cracking and warping.