Wood is hygroscopic and organic, and as such, it travels in predictable ways. Wood absorbs and releases moisture from its environment by capillary action, with the wood fibers essentially acting as a bundle of straws. The fibers inflate as the relative humidity rises, creating movement across the grain or tangential movement. Understanding how wood moves allows builders to take precautions both before and after installation to prevent warping and checking. They understand that wood movement may be accommodated but that it cannot be prevented. The strongest joints may be made by talented woodworkers through carefully utilizing their understanding of wood movement.
Advantages of Proper Drying
The job of a woodworker or builder is made considerably simpler by properly dried lumber since it is more solid and predictable with its movement. While the drying process cannot be hurried, when it is carefully handled, properly dried lumber is in a position to reach its full potential and offer the user the best possible workability with less customer frustration over the end results.
Benefits of Kiln Drying
Kiln-dried lumber in North America has a typical moisture level of 6% to 8%. Although there may be some variation depending on the region and the season, this norm is generally applicable. The standard in Europe is 12% to 15% due to the region’s more humid environment. The lignin, the fibers between the cell walls of the wood, changes as a result of the kiln-drying process, regardless of the moisture level attained. During kiln drying, the lignin hardens, making the cell walls less flexible and decreasing the capacity for movement. In a nutshell, kiln drying produces lumber that is more stable.
Drawbacks of Kiln Drying
On the other hand, kiln drying has its own set of issues. The most frequent, major negative issue experienced with kiln dried wood is when the lignin is set too rapidly. Wood can sometimes retain moisture because the exterior layers dry out faster than the inner layers. The hard shell that surrounds the stored moisture, known as “case hardening,” causes instability and more pronounced movement following cutting. In addition, the outer layers frequently experience discoloration and breaking.
Principles of Kiln Drying
The fundamental rules of proper kiln drying are very straightforward: proceed slowly and steadily, and follow timetables tailored to each species. When this method of operation is followed, the end result is typically satisfactory. However, some species’ unique supply-and-demand problems can easily result in a hurried drying process that skips crucial processes.
At J. Gibson McIlvain Lumber, we are dedicated to spending the time and effort necessary to produce and deliver only the best lumber. We receive timber from all over the world, and as a result, it arrives at our lumberyard with a wide range of moisture levels, from green lumber with a moisture content of 25–50% to the European standard of 12–15%. We also receive air-dried domestic species, which have a moisture content of 10–25%. No matter how the lumber is delivered to us, we have discovered that the same tried-and-true drying procedure yields lumber with the best stability.
We’ll examine the specifics of our drying procedure in The Proper Drying of Wood, Part 2.