The process of inducing evaporation of the moisture contained in wood; a type of hydrothermal treatment of wood is known as wood seasoning.
The purpose of seasoning is to reduce the moisture content in the wood to a level appropriate to the use of the articles made from the wood. This prevents changes in the dimensions and shape of the articles, eliminates wood rot, increases the strength of the wood, reduces the weight of the articles, strengthens glued joints, and improves the quality of finishing. Wood is seasoned in various forms: lumber, peeled or planed veneers, crushed wood particles, and semi-finished articles.
The simplest method of seasoning lumber is open-air seasoning, in which the lumber is stacked in the open air or under canopies for a period of two or three weeks to several months. The principal industrial method, kiln drying, uses kilns fed with hot air, a mixture of air and fuel gases, or superheated steam. Kilns with steam are the most commonly used type. Continuously operating kilns are used primarily for large-scale seasoning of lumber prior to shipping; moisture content is reduced to 18–22 percent. Batch kilns are used to season wood to a moisture content of 7–10 percent.
“The reduction of moisture content causes irregular drying of the wood, and internal stresses arise that may cause cracks to form….”
In order to prevent this, the relative humidity of the drying agent is lowered and the temperature is raised in the course of the seasoning process. Conditions for kiln drying lumber are standardized in the USSR. Depending on the desired intensity of the process and the later use of the wood, the following regimes are used: mild (temperature at the start of seasoning is 40°–50°C), normal (60°–80°C), accelerated (80°–100°C), and high- temperature (over 100°C). The length of the process varies from 15–25 days (mild regimes for hard woods) to 20–30 hr (high-temperature regimes for soft woods). If the wood is to be precision machined, it is steam-treated after seasoning to remove internal stresses.
Seasoning in a high- frequency electric field is also used. Continuously operating roller driers are ordinarily used for peeled and planed veneer. The sheets of veneer are surrounded by hot air (110°–130°C) or fuel gas (150°–250°C) and are moved through the drier on roller conveyors. The process lasts two to 12 minutes. Presses in which the plates are periodically opened and closed may also be used to season veneer.
The purposes of seasoning are:
- To prevent injury by insects and decay before the timber is placed in service.
- To increase the durability of timber in service.
- To prevent shrinking and checking of the wood in service.
- To increase the strength of the wood.
- To decrease the weight of the wood and hence reduce shipping charges.
- To prepare the wood for its injection with preservatives and for other industrial uses.