Drying Lumber and Defects

To get the wood to dry more evenly, and to eliminate the radial cracking that occurs around the edge of a cut log, the fresh cut trees are sawn into rough lumber before they are seasoned, or brought to their equilibrium moisture content with the environment. Once sawn they can be air dried, kiln dried, or a combination of both (also known as accelerated air drying). Whatever the process, seasoning involves allowing the free moisture trapped inside the inner cavities of the wood fibers to leave the board by evaporation. It is dry when it is around one sixth it’s weight in water. The only true way to tell is by weighing it, which the sawmill does periodically to keep track of its progress. However, as the water leaves the fibers, they shrink.

It is important to note that wood fibers don’t change length, rather they just get fatter or thinner depending on the relative humidity of the surroundings. You will never find a board that gets longer, just wider or narrower, and thicker or thinner. These are things we have to plan for as woodworkers. We also have to consider the fact that as wood is not uniform in structure, it will not expand and contract uniformly. The early wood soaks up more water and tries to straighten out the rest of the growth ring. This process is not restricted to the seasoning of the wood, it will happen throughout the lifetime of the piece of furniture. For this reason you must watch for cup, crook, bow and twist.

Cupping in plainsawn boards causes them to look like a trough or gutter. If the grain does not follow the long axis of the board perfectly, crook can turn it into a hockey stick. Bow occurs when the board bends into the shape of a ski, or a rocker from a rocking chair, and twist makes a propeller out of a board. The majority of the warp will have already happened upon seasoning but after cutting the relief of internal stresses in the board can lead to more changes in shape. Watch the face and end grain and you should be able to anticipate much of the wood movement.

Remember, although you are now ready to pick out that perfect piece of quartersawn, straight, heartwood, cherry, you must also be on the outlook for damage due to fungus, mold, insects, animals, or other things that affect the board. For example, sticker stain, or chemical or fungal discoloration where the stickers contacted the rough lumber when seasoning can affect the utility of a board greatly. Make sure you can saw around imperfections so your cuttings still fit you bill of materials.