You probably know that wood has to be dried in order to be useful as lumber. Removing the water makes the material dimensionally stable, predictably machinable, and accepting of glues and finishes.
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But have you stopped to consider how water is stored in wood? Do you know how that water is removed, or what happens to wood fibers as they dry?

Knowing the answers to those questions, and understanding what can go wrong if wood gets dried improperly, will make you a smarter buyer and user of this natural resource. Here's the short course on wood moisture and its effects.

What happens when wood releases water

Whether dried naturally (air dried), or in a kiln, wood dries to a certain percentage, and then stops. The relative humidity of the air around the wood determines this point, called equilibrium moisture content (EMC). This is why you always should let your wood acclimate to the conditions in your shop before beginning a project.

As bound water (see the wood cell cutaway for an explanation, above) is removed, the cell walls shrink, causing the wood to change dimension. Boards shrink most across their tangential plane (face grain), so flatsawn boards shrink more than those that are quartersawn, as shown below.

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Because wood cells vary in size, density, and orientation, they may shrink unevenly, causing boards to warp in various ways. To minimize these problems, shy away from boards that show wild figure, inconsistent growth-ring spacing, or an off-center pith.

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Shrinkage also stresses the wood. If not properly relieved, this tears the fibers apart, as illustrated below. You can stop shrinkage by replacing bound water with a stable substance that will remain in the cell walls. Turners do this by soaking blanks in polyethylene glycol (PEG). But this technique remains impractical for use with whole logs or large pieces.

Careful drying results in high-quality lumber

Wood dries from the outside in, as the water it contains moves naturally from areas of highest to lowest concentration. This creates uneven pressures in the outer portion (the shell) and the inside of the board (the core).

Lumber producers constantly manipulate the temperature, humidity and airflow inside the kiln, and cut thin, tuning-fork shaped cross sections regularly to look for signs of problems before they become too severe to correct.

Diagram of good board drying
Drawing of problems wood not dried properly

Calculating water content

Moisture content (MD) tells us the ration of the weight of water in a piece of wood compared to its weight completely dry. We express it as a percentage. To determine MC, first weigh a piece of wood. Then, dry it until it contains no water (determined with the weight loss stops). Next, compare the weights as follows:

Formula to determine MC

For example, if a piece weighs 25 lbs. wet and 20 lbs. dry, its MC equals 25 percent:

(25-20)/20 = 5/20 = .25 or 25%

Thankfully, you don't have to weigh and dry lumber to determine its MC. A moisture meter does the job for you.

For indoor projects, MC should lie between 6 parent and 11 percent to cope with dehumidified conditions. See the map, below. Construction lumber and outdoor woods, subjected to high humidity levels, should range from 15 to 20 percent MC to maximize expansion and contraction.

Map displaying typical moisture content in parts of North America
Drawing showing how milled wood warps

Preventing problems with moisture and movement

Wood becomes most stable when it reaches EMC, and that's controlled by relative humidity. So what happens when humidity levels fluctuate? Wood is described as hygroscopic, meaning it will continue to take on and shed moisture and, because of this, expand and contract.

You can see how this happens in glued-up panels. At some points during the year, they may be dead flat. At other times, though, panels warp, as shown below. This movement won't cause problems if you build your projects to withstand it as discussed here.

To slow down the movement of water vapor into and out of the wood, always apply finish to your completed projects. Apply the same number of coats to all surfaces to equalize the rate of moisture exchange. This helps prevent cupping.

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