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When boards are first cut from a log, they are considered “green”, which means they contain a high percentage of water weight and must be dried before they are suitable for most uses. Lumber is dried commercially in two ways—by air or by kiln. Air drying is simply that: the stock is stacked in such a way that the air can circulate through and around it. Small pieces of lumber, called stickers, are inserted between the boards at regular intervals. The stack is then left to dry for a long time, sometimes several years, until the moisture evaporates to acceptable levels. Variations on the method involve covering the top layer with plastic or canvas, turning the sides to the prevailing wind, periodically dismantling and rebuilding the stack in reverse, all in an attempt to control the drying process. If lumber is improperly dried, it may begin to mold, which leads to a sometimes desirable defect called spalling (See here).

What is “green” lumber? The American Lumber Standards Committee classifies green wood as having 20% or higher moisture content, and dry lumber as 19% or less. Board moisture is measured in terms of weight, not volume.

Kiln-drying is done in a gas, electric or solar-powered oven. Kilns are expensive to operate, but they offer a precisely controllable drying environment. Some mills may be inclined to speed up the process to save money. However, rapid drying can lead to a multitude of defects, such as case-hardening (See here).

From a woodworker’s point of view, air-dried lumber is a lot cheaper, but it is less common. Most lumber, including everything you’ll find at a home center, is kiln dried, because it is ready for market in a shorter time. The kiln is also a more controllable method than air-drying, especially with large volumes of lumber.