One of the many aspects of lumber that affects both its pricing and its performance is drying. Why is this important? As a natural building product, wood moves. The good news is that the way wood moves is fairly predictable, and by utilizing proper drying processes appropriate to the species and application, we can greatly reduce the chances of warping, cupping, checking, and other generally destructive results.
Predicting and Allowing for Movement
As a hygroscopic material, the fibers of wood mimic a bundle of straws, taking in moisture and expelling moisture according to the humidity surrounding it; as the humidity levels change, the moisture content changes, as well. As humidity increases, capillary action causes fibers in the wood to swell, which results in the wood moving across the grain.
For that reason, it’s essential that installation occurs with room for movement in mind; wood movement cannot be stopped, so it must be accommodated. Many seasoned woodworkers actually find ways to use this characteristic of wood to create stronger joints that grow tighter as the fibers swell.
Understanding the Importance of Proper Drying
No matter what kind of wood project you create, it helps to start with properly dried lumber. While the wood will still move, it will do so stably and predictably.
High-quality lumber is not just about what it’s like when it’s initially sawn into lumber — it’s also about how the wood is dried. Proper drying cannot be rushed: it’s a carefully controlled multi-step process performed with specific species and applications in view.
Promoting the Ideal Moisture Content
When it comes to determining ideal moisture content, a lot depends on the climate where the wood will be installed. The North American standard for kiln-dried lumber is 6-8%, while the wetter European climate makes 12-15% moisture content preferable. There is some fluctuation within those large geographic areas, as well.
In addition to helping the wood to achieve an equilibrium with its intended environment, kiln drying also helps by hardening the lignin, setting it in place. While the wood will still move some, the cell walls become less flexible, allowing for less dramatic movement. So while all drying helps wood acclimate to a local environment, kiln drying has the added benefit of creating lumber that’s generally more stable.
Avoiding Potential Problems
As you might guess, setting the lignin can cause problems, if it isn’t done correctly. When kiln drying occurs too quickly, case hardening can result. Basically, this means that the outer layers dry before the inner layers, creating a hard shell that traps the moisture inside the inner layers. The result is increased instability, causing drastic movement when the wood is cut. This kind of problem shows up in the outer layers even before cutting, with cracking and discoloration. When done improperly, kiln drying can be more of a detriment than a benefit.
Part 2: The Importance of Slow & Steady Air Drying for Lumber.
Part 3: Why Lumber Is Slowly Dried in a Kiln.