Tip #39
Drying Your Own Lumber (continued)
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Kiln-drying - Pros & Cons

You may have found a sawmill that will supply you with green lumber, and you’re considering having it kiln-dried. (The sawyer should be able to put you in touch with kiln operators in the vicinity.) If that’s the case, you’ll want to know a few things about kiln-drying.

The main advantage of kiln-drying is that it’s fast. Air-drying wood takes at least a year. In a kiln, however, green wood can be dried in as little as two weeks. Although most kilns will dry only their own wood, you may find one that will dry your lumber. This could enable you to get started working in a matter of weeks.

The other advantage to kiln-drying is that it takes the moisture content a bit lower than air-drying because it drives out the ‘bound water’ -- water held inside the cell walls. This process, however, makes wood ‘thirsty’. Kiln-dried lumber will have a tendency to take on moisture in your shop unless the ends ofthe boards are painted and you store your lumber in a very dry area, such as a loft or attic.

Be careful that the storage area isn’t too hot, though, ends of the boards may check. There is still some moisture present in kiln-dried lumber (about 7%), and excessive heat will drive this out too quickly. However, if you have to choose between a hot storage area or a wet one, take the heat. Changes in humidity are far more destructive to wood than changes in temperature. If the only space you have available for wood storage is damp, install a dehumidifier to keep it dry.

Figure 1. If the moisture is removed from a board too quickly, it will form 'case hardening'.

There are three distinct disadvantages to kiln-drying. First, kiln-dried lumber tends to be more brittle. Some woodworkers use air-dried wood exclusively because it has a lively resilience that is lost in kiln-drying. Secondly, drying wood too quickly can cause surface checking and/or case hardening. This is especially common with oak. (See Figure 1.) And finally, the color may be changed slightly. Walnut is a prime example. When walnut is kiln-dried, it’s usually steamed. The steam disperses the tannins responsible for the dark colors evenly throughout the rest of the wood. The dark and light areas (heartwood and sapwood) blend together. This process makes more of the wood into high-grade lumber, since sapwood is worth less than heartwood, but it washes out the depth and intensity of the color. A piece of air-dried walnut often has a rich purple cast to it which is lost in the steaming process.

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