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Types and Techniques for Drying Wood

Views: 262     Author: Wendy     Publish Time: 2023-07-11      Origin: Site


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Types and Techniques for Drying Wood

Prior to usage, wood is dried to minimize its moisture content (also known as seasoning lumber or seasoning wood). The product is known as kiln-dried timber or lumber when the drying is carried out in a kiln; air drying is the more conventional technique.

Wood should be dried for two main reasons:


Wood will absorb or release moisture until it is in equilibrium with its environment when utilized as a construction material, whether as a structural support in a building or in woodworking objects. Equilibration (often drying) causes the wood to shrink unevenly, and if it happens too quickly, it may destroy the wood. To protect the wood, the equilibration needs to be kept under control.

Burning wood:

Typically, it is best to dry the wood (firewood) before burning it. In contrast to drying for woodworking, damage from shrinking is not an issue in this situation. Unburned hydrocarbons escape from the chimney as a result of moisture's impact on the burning process. About 5% of the energy from a 50% wet wood is lost through the evaporation and heating of the water vapour when it is burned at a high temperature and with good heat extraction from the exhaust gas, resulting in an exhaust temperature of 100 °C. Although the efficiency can be further boosted with condensers, the key to burning wet wood in a regular stove is to burn it very hot, possibly by beginning the fire with dry wood.

Various Woods:

According to its botanical source, wood can be split into two categories: softwoods, which come from coniferous trees, and hardwoods, which come from broad-leaved trees. Hardwoods are tougher and more complicated in structure, whereas softwoods are lighter and typically simpler. However, in Australia, Sclerophyll species (Eucalyptus spp.) are classified as Hardwood, while Softwood refers to trees found in Rain Forests.

Water and Wood Relationship:

Fresh logs and wood from living trees both include a significant amount of water, which frequently makes up more than 50% of the weight of the wood. Wood is significantly influenced by water. The pace at which wood exchanges moisture or water with its surroundings is significantly influenced by how well the wood is sealed.

Wood includes three types of water:

No cost water

Only capillary forces are able to keep the majority of the water inside the Lumina cell in place. It is known as free water since it is not chemically bonded. Since energy is needed to overcome capillary forces, free water is not in the same thermodynamic state as liquid water. Additionally, free water may contain pollutants that change the way wood dries.

Hygroscopic or bound water

Hydrogen bonds hold the bound water to the wood. The presence of free hydroxyl (OH) groups in the cellulose, hemicelluloses, and lignin molecules in the cell wall is what causes wood to be attracted to water. Negatively charged hydroxyl groups are present. The free hydroxyl groups in cellulose attract and hold water by hydrogen bonding because water is a polar liquid.


At normal temperatures and humidity levels, the amount of water in cell Lumina in the form of water vapour is typically insignificant.

Drying flaws

The most prevalent type of degradation in lumber, after natural issues like knots, is caused by drying faults. Despite the fact that some faults have both sources, there are two categories of drying defects:

Defects such as cupping, bowing, twisting, crooking, springing, and diamonding that come from shrinkage anisotropy and cause warping.

Defects such as checks (surface, end, and internal), end splits, honeycombing, and case hardening that occur as a result of uneven drying and cause the wood tissue to burst. Collapse, which is frequently visible as corrugation or "wash boarding" of the wood surface, can also take place (Innes, 1996). Collapse is not a type of shrinkage anisotropy; rather, it is a flaw that arises from the physical flattening of fibers over the fibre saturation limit.

The five drying quality indicators are as follows:

Gradient in moisture content and the existence of residual drying stress (case-hardening); Surface, internal, and end checks; Collapse; Distortions; and drying-related discoloration.

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