Drying

Lumber and other wood products usually contain considerable moisture after their production, and drying is essential to prepare them for further use. Proper drying reduces the magnitude of dimensional changes due to shrinkage and swelling, protects wood from microorganisms, reduces weight and transportation costs, better prepares wood for most finishing and preservation methods, and increases its strength. Drying is accomplished in yards in the open air or in closed kilns. Other methods of drying also exist.

The object of open-air drying is to reduce the moisture content of wood to the lowest value permitted by weather conditions in the shortest time without producing defects. The level of moisture reduction attainable depends on temperature and relative humidity. Wind reduces the time required, but direct contact with rain and snow hinders the progress of drying.

The air-drying yard is located close to the lumber plant, on a dry site where air movement is not obstructed by tall trees or buildings. The ground surface is kept free of debris and vegetation, and alleys are provided for working areas and air movement. The bottom row of lumber is kept about 40 cm (16 inches) above the ground, with space for air circulation provided as layers are added. When piling is done mechanically, the lumber is first prepiled in packages. A suitable roof, usually made of low-grade lumber or panel material, is placed on top of each pile. The time required to air-dry from green condition to 20 percent moisture content varies from about 20 to 300 days for wood 2.5 cm (1 inch) thick, depending on species, place, and time of year.

Air drying can be accelerated by means of fans, sometimes in combination with low-temperature heating. When this technique is used, the piled lumber is placed in sheds. In the case of beech, walnut, and some other woods, steaming is employed before air drying. This practice reduces drying time by increasing the rate of drying and at the same time darkens the wood, making it more desirable for use in furniture.

Kiln drying is conducted in a closed chamber, under artificially induced and controlled conditions of temperature, relative humidity, and air circulation. This method permits much faster reduction of moisture content to levels that are independent of weather conditions. In wood 2.5 cm (1 inch) thick, moisture is reduced from 20 to 6 percent in 2–15 days and from green condition to 6 percent in 2–50 days. The source of heat is usually steam circulating in pipe coils. Relative humidity is controlled by allowing steam to enter the chamber through a perforated pipe; such control regulates the exit of moisture from the wood and so avoids defects such as splitting and warping. For satisfactory results, air movement is needed to carry the heat from its source to the lumber and to carry away the evaporated moisture. Air circulation is produced by fans located within the kiln and sometimes by blowers placed outside.

Regulation of conditions is usually automatic, and drying is accomplished by the use of drying schedules that have been derived experimentally for various species and thicknesses of wood. Schedules start with high humidity and low temperature and end inversely with high temperature and low humidity. As moisture removal proceeds, samples of wood are removed periodically and weighed. Sometimes moisture content is read from outside the kiln by way of samples wired to moisture meters. Kiln drying normally involves temperatures in the range of 40–75 °C (about 100–170 °F). Such temperatures are high enough to kill insects—another advantage of kiln drying over air drying.

In addition, wood can be dried by special methods that include solar drying (use of greenhouse-type dryers or those equipped with solar collectors), high-temperature drying, dehumidification kiln drying (in which the evaporated wood moisture is condensed and the latent heat recaptured and used for additional evaporation), and boiling in oils (a combination of drying and preservation, usually with creosote). Some other methods, such as drying with applied chemicals (salt seasoning), organic vapours (e.g., xylene), a solvent (specifically acetone), high-frequency electricity, centrifuging, infrared radiation, a vacuum, and microwaves, are inhibitively expensive and therefore are not commercially applicable.

Preservation

Wood can be protected from the action of destructive agents such as fungi, insects, and marine organisms (see the section Degradation) by impregnation with toxic chemicals. Preservatives used against such organisms are of three groups: oils, oil-soluble chemicals, and water-soluble chemicals.

Oils are exemplified by coal tar creosote (i.e., creosote obtained from bituminous coal). Creosote is very effective for treatment of railroad ties, poles, and pilings and can extend their useful life severalfold. Creosote-treated wood, however, resists painting and gluing and can exude the preservative, which is a pollutant. The main representative of oil-soluble preservatives is pentachlorophenol (see chlorophenol). When used with a suitable organic solvent, pentachlorophenol has advantages over creosote in that the preserved wood is kept clean and can be painted or glued. The compound is also polluting, however, and its use is banned in several countries, including the United States. Water-soluble preservatives are salt solutions of various inorganic chemicals such as copper, chromium, arsenic, and mercury. Their main disadvantage is that they leach from the wood under damp conditions, but this can be overcome by the formation of insoluble compounds in the wood—for example, with preservatives prepared from a combination of copper, chromium, and arsenic (CCA).

Wood can be made resistant to fire with chemical retardants. Fire retardants are water-soluble and not toxic. They contain silica and other chemical compounds and act by creating a barrier (charred wood or foam) to the spread of flame or by generating noncombustible gases.

Wood to be treated with preservative is prepared by removing bark (as a rule) and excess moisture (to below the fibre saturation point; see the section Hygroscopicity), machining to final shape, and drilling holes or making incisions to facilitate entrance of the preservative. Preservatives can be applied by brushing, spraying, dipping, steeping, sequential immersion in hot and cold baths, and diffusion (applied to green wood), but impregnation under pressure in closed tanks or cylinders is the most efficient method. (Bark is retained in treating by hydrostatic pressure.) Factors that affect penetration of preservatives include species and structure of wood, density, moisture content, direction of grain, preparation of wood for treatment, type of preservative, and the treatment process used.