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Kiln dried wood is lumber that has been rough-cut from logs and then dried in a special chamber built for that purpose. There are two reasons wood is kiln dried. The first is to reduce the moisture content to a level where the wood can be used for fuel and won't lose much energy evaporating moisture. The second reason for kiln drying wood is to prepare it for use in construction, either structural or furniture, in which case the drying process will be highly controlled and regulated.
When wood is to be used for fuel, too high a moisture content will make the wood difficult to light and keep lit, and a good amount of the heat produced will be wasted evaporating the remaining moisture. Green wood is generally 60% or more water, but wood used for fuel should be dried to 25% or lower. Letting firewood air dry to that moisture level once it's cut and split will take three to four months, while kiln drying can accomplish the same task in a matter of days or even hours, depending on the temperature used. Firewood merchants, therefore, often kiln dry their firewood prior to distribution. Kiln dried wood is also much lighter than green wood, and is therefore more economical to transport.
Wood intended for construction must be dried before use because wood shrinks as it dries, and if it's used while still green, the structural and aesthetic integrity of the item being built will be compromised. Wood that air dries often has a tendency to dry unevenly, resulting in uneven shrinkage that will twist, cup or otherwise warp the wood. In addition, air drying wooden boards is a process that can take many months. Kiln drying lumber for construction allows the careful regulation of the drying process, generally preventing the more serious warping and twisting that can occur when wood is air dried. Kiln drying is also much faster than air drying, permitting the faster turnaround of batches of lumber. Finally, kiln dried wood usually produces less waste in the milling process, thus reducing overall costs.
The greatest drawback to kiln dried wood is the cost of constructing, maintaining and operating a kiln. While there's no question that a commercial sawmill requires a kiln, many hobbyists who own portable sawmills find the cost of a kiln too expensive, and air drying too unreliable. An alternative is to build and operate a solar kiln, in some respects similar to a greenhouse in concept and construction. While solar kilns aren't as flexible as powered kilns in terms of their ability to control temperature and humidity, or as fast in producing kiln dried wood, they provide a much more stable drying environment than any air drying method. In addition, they're far more economical to operate than a powered kiln, reducing the cost of the kiln dried wood.