Some useful information from the theory of drying wood

Wood consists of different plant cells, mostly half long. The structure made of cells creates a net in wood that is well permeable to liquids and gases in the direction along the fibers, but much less across the wood, in a right angle to the cells. Water in wood may also be held, that is in the tubes of the cells as wells as in the space between them and lastly the cell walls.

Water contained in the wood limited. Maximum relative humidity of the cell walls (saturation) ranges from 25 to 35%, the maximum content of free water depends on size of cell cavities and varies for different types of wood between 60-250%. Wood growing or freshly chopped always has higher humidity, which is dependent on the type of tree, location and the season of growth.

Wood is a hygroscopic material, meaning that it has the characteristic to change the moisture in response to the changes in his environment. If the wood is stored in peace, its humidity will fall to a level, which is called stable humidity. Wood can not only pick up moisture from the water, but even steam from moisture in the air. That is why it depends so much on how much it was dried before use, and what is the humidity of the place where the product made from it is used. In general, the higher the temperature, the lower the constant moisture content.

By changing the humidity of wood, his size changes as well; this might cause a major problem. By reducing humidity the dimensions and size varies, this phenomenon is called contraction. This happens only in reducing the amount of bound water. The shrinking stops when the wood is completely dried. Even in the forest wood shrinks depending on the humidity of the surrounding air. Both acts, the acquisition and contracting, are reversible.

Shrinking depends on the wood structure, the scale of changes in humidity and in the tree species. In general, it is very important that in the tangential direction the wood shrinks 1.5-2x more than in the radial direction. Shrinking in the longitudinal direction is negligible and therefore not included in the calculation.

Shrinking is approximately dependent on humidity changes. By reducing the moisture from the limit of saturation of the cell walls to zero, the maximum shrinkage will be reached, which is called shrinkage complete. On average, the overall decrease in the tangential direction is 6-10%, 3-5% in the radial and in the total volume 12-15%.

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