Organic and hygroscopic, wood moves in predictable ways and requires careful drying. Basically performing as a bundle of straws, wood absorbs and releases moisture in its environment through capillary action. As the surrounding moisture level increases, the fibers swell, causing movement across the grain, or tangential movement.
Builders who understand how wood moves can take steps prior to and during installation to avoid warping and checking. They realize that wood movement cannot be stopped; however, it can be accommodated. The most gifted woodworkers can use their knowledge of wood movement in order to create the strongest possible joints.
Benefits of Proper Drying
When lumber is properly dried, the woodworker or builder’s job becomes much easier, because the lumber is more stable and predictable. The drying process cannot be rushed; however, when lumber undergoes a carefully controlled drying process, it is in a place to achieve its fullest potential and provide the user with optimum workability.
Pros of Kiln Drying
The North American standard moisture content for kiln-dried lumber is 6-8%. While there is some fluctuation based on the local climate and time of year, this standard generally applies. Because of Europe’s wetter climate, the European standard is 12-15%.
Regardless of the moisture level achieved, the kiln-drying process effects a change in the wood’s lignin, the fibers between the cell walls. Because the lignin hardens during kiln drying, the cell walls become less flexible, translating into less potential for movement. In short, the kiln drying results in more stable lumber.
Cons of Kiln Drying
On the flip side, kiln drying can cause unique problems. The most common and significant potential issue occurs with setting the lignin too quickly. Sometimes the outer layers of wood dry more quickly than the inner layers, trapping moisture inside. Referred to as “case hardening,” the hard shell surrounding the trapped moisture leads to instability and more dramatic movement after cutting. In addition, discoloration and cracking of the outer layers often results.
Basics of Kiln Drying
The basic tenets of proper kiln drying are pretty simple: slow and steady and according to species-specific schedules. Simple doesn’t always mean easy, though. The supply-and-demand issues accompanying some species can easily lead to a rushed drying process in which important steps are eliminated.
Here at J. Gibson McIlvain Lumber, we are committed to taking the time and effort to provide only the best lumber possible, regardless of the timetable. Because we receive lumber from across the globe, we receive it in various moisture contents, ranging from green lumber with a moisture content of 25-50% to the European standard of 12-15%; we also receive air-dried domestic species with a moisture content ranging from 10-25%. Regardless of the state in which we receive the lumber, we have found that the same time-tested process leads to lumber with optimal stability.
In Lumber Drying, 102, we’ll look at the details of the process we use.