Tips from J. Gibson McIlvain: A Lumber Leader Since 1798
As an organic material, wood is hygroscopic, by nature. Imagine a bundle of straws, and you’ll have an idea of the structure of lumber. The capillary action allows the wood to take in moisture, causing fibers to swell. The result is wood movement across the grain. The absorption and expulsion of moisture due to changing humidity levels can make woodworking both exciting and frustrating.
Understanding wood movement can do more than lessen your frustrations; it can help you avoid warping and cupping in the finished product. Without this understanding, projects can literally fall apart when they lack the room to expand, as needed. Even though you can’t stop wood from moving, you can anticipate and accommodate the movement, avoiding catastrophic problems. In fact, if you embrace the movement, you can even use it to create stronger joints, entirely.
When you start with properly dried lumber, your job will be much easier. While dried lumber will still move, it will be more stable and more predictable. Proper drying takes place in a controlled environment, over time. The multi-step process yields stable lumber that has the lowest possible risk of extreme movement.
Depending on the climate, stable lumber can have various amounts of moisture content. For most of North America, a 6-8% standard moisture content for kiln-dried lumber makes sense. For some areas, the content might need to be higher, but properly kiln-dried lumber will be close to this standard. The European standard is higher, due to a generally wetter climate, at 12-15% moisture content. In addition to arriving at these standard moisture levels, properly kiln-dried lumber will harden the lignin and make sure it remains in place. This process ensures that the cell walls won’t be as flexible, therefore reducing the amount of movement possible.
The stability that can be achieved by setting the lignin can be compromised when kiln drying is done too quickly, though. Since the outer layers dry faster than the inner layers, the outer layer can set up too fast, creating a hard shell around inner layers and trapping moisture inside them. Referred to as “hardening,” the result of this situation is unstable wood that moves significantly when it’s cut. Cracking and discoloration are telltale signs of hardening.
Drying schedules for specific species are commonly published, so there should be no reason for improperly dried lumber. However, when there’s a gap between a species’ supply and demand, the desire to rush the process can eclipse a supplier’s good sense. Skipping steps may get the product out the door earlier, but it won’t provide the best long-term results. J. Gibson McIlvain realizes the kinds of problems that can come from doing a rush job, and we’d rather stick to our tradition of providing only high quality materials than fill an order with less-than-the-best lumber.