Lumber Moisture Content

The moisture content (MC) of wood is usually determined by the weight of the water in the wood. Since the water that is present can weigh more than the wood itself, the MC of green hardwood lumber can be up to 150% of a completely dried product. It is not desirable to dry the wood out completely except for samples used to determine the MC. Instead, it should be dried to an MC similar to the estimated median MC of the environment where it will be used. This will depend on the product, location and application (especially indoor or outdoor use). Seasoning is not something that is done once and for all; wood will not remain completely dry and can still shrink, swell and become vulnerable to decay from uneven moisture absorption in unfavourable circumstances.

Benefits of Wood Seasoning

Drying can increase the strength of the wood by most measures (the main exception is impact-bending strength) when dried to moisture contents of below 30%. Resistance against decay, fungal infestation and insect attack increases. Dry wood becomes a better electrical and thermal insulator and can be more effectively treated with chemicals for further protection, as the chemicals are better able to penetrate it. Finishes and paints adhere better, and screws, nails and glue hold better when wood is properly seasoned. Finally, the wood is lighter and cheaper to transport. The wood’s improved properties and cheaper transport costs contribute to higher profits.

The Drying Process

Wood that is to be seasoned is usually sawed first and air-dried or kiln-dried. The goal for wood that must be worked is to ensure that external vapour pressure in the environment where the product will be used is the same as vapour pressure within the wood. Drying defects and high energy use are the two main problems that occur during the process. Up to 80% of sawmill energy use may be for drying, and badly managed drying can lead to warping, splitting, checks, case hardening or honeycomb.

Wood initially dries fairly easily. Water evaporates from the surface, so that the moisture content on the surface needs to be lower than at the core to remove water. Likewise, the ambient air should be dryer than the wood. Eventually, the moisture content throughout the wood will be equal. The drying rate depends on the temperature, air velocity and RH of the air. Higher temperatures and air velocities, and a lower ambient RH, contribute to fast drying.

During the process, a milestone called the fibre saturation point (FSP) is reached, after which it becomes harder to remove moisture. It is after this point that the wood begins to shrink and gain in strength. Improper drying in a kiln leads to much surface shrinkage that is much greater than shrinkage at the core of the wood, resulting in stresses that can lead to damage. This phenomenon is known as case hardening. Hardwood species are more difficult to dry.

Seasoning Methods

Wood is sometimes dried simply by using natural ventilation, or in sheds fitted with fans. This method is relatively slow and does not reduce the moisture content as much as kilns can. Dry kilns function either with dehumidification or by heated air circulation. The latter involves blowing air over the wood in an enclosed chamber and may use air of various temperatures. In heated-air driers, high air velocities can dry out the wood comparatively quickly. Reversible impellers are also an advantage, as the direction of airflow is often reversed for the sake of even drying when kiln charges are large.

Various elevated temperature kilns are more common. Steam drying kilns, the most common solution for a long time, operate at temperatures of up to 82 °C and use boilers that burn various fuels, ideally available wood waste to save on fuel costs. Elevated temperature kilns can be expensive to operate, but save money on transportation and increase the value of the product. The heated air is periodically vented to get rid of the water vapour in the kiln. In this process, a great deal of heat is lost, so that reheating is required. The species, size and condition of the wood determines the timing of this process and the temperatures and humidity levels applied (the drying schedule).

High-temperature kilns (operating at about 93-115 °C) are used for large-scale softwood drying, but is suitable for only a few kinds of hardwood.

Some kilns use refrigerative dehumidification as their operating principle. While dehumidification kilns are considered low-temperature systems, temperatures can still rise to 71 °C. Dry kilns can reduce the moisture content to 5-6%, and the hotter ones can dry most kinds of wood at the fastest possible rates. They do not need vents, but vents can be used for closer temperature control. A pre-dryer can be used to reduce the moisture content to approximately 25% before a dehumidification kiln is used. Pre-dryers can produce better quality lumber and faster drying times than air drying, but they may not be cost-effective unless very large amounts of wood are dried.

Concerns during the Drying Process

Drying defects such as case hardening can occur with elevated- and high-temperature kilns. “Seasoning checks”, lengthwise cracks that appear with uneven or excessively fast drying, are one problem. The appearance of excessive checks is one reason why high-temperature kilns have a limited scope of use.

Because of the danger of drying, stress relief is often applied. This usually entails moistening the wood’s surface with steam for hardwoods, while liquid water is used with the majority of softwoods and fast-drying hardwoods, causing the surface to swell somewhat and reducing the effect of faster surface shrinkage.

Proper air circulation is critical to kiln drying and strongly affects the final result. High-efficiency fans are needed so that an evenly distributed moisture content can be attained without driving up electricity costs unduly. A product with consistently high quality can thus be produced. Due to the importance of even air distribution, most larger kilns use reversible fans.