This is the sixth post of an ongoing series about wood. Understanding its nature, the way it moves and changes, and the implications for designers. Check back every Wednesday for the next installment.
The previous posts are here:
» How Logs Are Turned Into Boards, Part 1: Plainsawn
» How Logs Are Turned Into Boards, Part 2: Quartersawn
» How Logs Are Turned Into Boards, Part 3: Riftsawn
» Why Does Wood Move?
The number one enemy facing designers working with wood is water. Long after it's cut, a board will attempt to reach its equilibrium moisture content (or EMC), which will cause it to change shape, sometimes dramatically. The EMC is the amount of water that the wood will hold in order to match the relative humidity of the surrounding environment. If the wood moves from Seattle to Phoenix there will be some shrinkage and change as the water is pulled from the wood cells and into the surrounding hot and dry air.
There's no way to tell how dry your cut lumber is by just touching it. You will need a moisture meter, a tool that measures the percentage moisture content of the wood, based on its volume or weight. (You can learn more about moisture meters here.) An acceptable measure is about 6 to 7 percent. But remember that each species of wood has its appropriate EMC.
The only way you can properly work with lumber is to have it dried. So how wood is dried becomes an important variable for anyone working with wood.
One of the first things you need to know about drying wood is that drying begins in the outer layers of the log and then moves inward, the inner zones will only start to dry once the outer layers have reached appropriate dryness. This process can create its own issues. If the outer layers dry too much (below what is called the fiber saturation point) and the center layers remain wet then so-called "drying stresses" cause rupturing in the wood, which leads to checking. Checks are splits in the wood that often appear during the drying process.
There are two ways to dry wood: Air dry or kiln dry. The important difference between the two is time.
Kiln drying is perhaps the safest way to dry wood, because the process is controlled. Basically the wood is stacked in a kiln, where humidity and temperature is controlled using steam and drying is controlled with fans. It's fast, only taking about six to eight weeks. Because it is dried at high temperatures, drying wood in a kiln also takes care of any pockets of pitch (also known as resin) that can sometimes liquefy and seep out from the board later on if the wood is placed in a very hot area after being dried.
Air drying takes much longer, typically about a year for every inch of board thickness. Of course, this timeframe can vary greatly depending on the environmental conditions and species of wood. Here is a general tip: Low-density wood tends to be easier to dry than high-density wood. Low-density logs have thinner cell-walls so the moisture movement is faster and this of course leads to faster drying. Also, the weaker cell structure of lower density wood reacts more favorably to the stress of drying rather than resisting and checking.
To air dry wood, lumber is stacked between "stickers," two by two pieces of wood, and then either strapped or weighted down by a plank of particle board and cinder blocks. (Note that the stickers are placed directly above and below each other, to avoid the uneven weight transfer that would lead to sagging.) Then, as long as there is sufficient air flow between and around the individual boards, it is simply left to dry.
Air drying is trickier since it's up to you to consistently check the rate of drying and to monitor the wood for moisture, fungus and any developing checks. But overcoming the challenges involved in air-drying wood often rewards you with more stable wood, as the wood has more time to work through the process and develop any inevitable twists and bows it might have. An important thing to remember: Boards that are dried outside must be brought indoors to the environment where they will ultimately stay, because they will continue to absorb or release moisture depending on the surrounding humidity. And this should happen a few weeks or months before you plan to work on the wood.
Here's a video from the Woodworkers Guild of America explaining the air-drying process:
And here's a video on wood drying within the context of an actual furnituremaking operation. George Martin of Pennsylvania-based George's Furniture shows you his set-up and process:
Ultimately neither method of drying has any advantage over the other as far as moisture content is concerned. Dried wood will always re-absorb moisture in the same ways.
Stay tuned next week for how design itself can safeguard against any further wood movement that can happen even after the drying process.
Material Matters: Wood
» Why Does Wood Move?
» Controlling Wood Movement: The Drying Process
» Dealing with Wood Movement: Design and Understanding
An Introduction to Wood Species:
» Part 1: Properties & Terminology
» Part 2: Pine
» Part 3: Oak
» Part 4: Maple
» Part 5: Walnut
» Part 6: Cherry
» Part 7: Mahogany
» Part 8: Rosewood
» Part 9: Ebony
» Part 10: Teak