Hasta La Ideas

This is the fifth 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.

(more…)