Wood cannot be dried.
To put that another way, the wood that we use to build things is really only mostly dried.
It always retains some moisture in its cells, no matter how long it has been aged, or how many coats of finish are applied to it.
Intially, wood is full of sap and moisture. The tree from which it has been cut has been using it to conduct minerals and nutrients to its leaves and roots, and the long hollow shapes of the wood cells retain most of this watery liquid.
Since water has to migrate from deep inside the wood to get out, it takes time. If the outermost layer of wood dries out faster than the rest, it will shrink more than areas deeper inside, creating stresses that will split and warp the lumber. These stresses also remain in the wood long after it is dry.
If the process proceeds gradually, however, tensions in the wood can be minimized. The slower the drying, the better the behavior of the dried wood as a working material, and the more stable and long-lasting the resulting furniture.
Most commercial lumber mills want to dry wood as quickly as possibly. They apply gentle heat and dehumidify the air inside their drying chambers to speed up the drying process. Usually this does no harm, but is never as good at avoiding problems as a slow, patient approach.
For the intial drying--the loss of the "free water" trapped inside wood-cell walls--an outdoor environment is ideal, because it will generally be more humid outdoors than indoors, lessening the difference in humidity between wood and air. Thin strips of wood called stickers are placed at invervals along each board before the next board is stacked. This allows air to circulate fully. Raising the stack well off the ground, and covering the top to shed rain and snow, protects the wood from the elements as it slowly dries.
To dry to a point of equilibrium with the moisture content of the air will take about one year for every inch of the lumber's thickness. Thicker wood takes proportionally longer to air-dry, and because of its larger size is even more prone to drying stress.
Eventually, the wood neither gains nor loses water to the atmosphere. At that point about twenty percent of its weight is in the form of water.
Even when the wood has done that, though, it is still not ready for use in indoor furniture. Because the indoor air is usually so much drier than outdoor, the wood must be dried further before being used inour heated and cooled homes. Commercial hardwood lumber, meant for building furniture, is usually dried to a moisture content of about eight to ten percent. At that level it will be in balance with the moisture level of the typical indoor.
By the way, when buying lumber it is worth asking how long it has been stored out of doors after being kiln dried. Wood stored in outdoor or even indoor lumberyards can easily gain moisture again, especially in wet seasons.
The simplest way of drying wood sufficiently for indoor use is to actually store it indoors, in the same location or conditions where it will be used. Even kiln-dried wood benefits from this patient conditioning before being made into cabinets and furniture.
Once the wood has been acclimated to the environment, it will continue to shrink and expand throughout its usable life. The traditions of cabinet design address the problem of dimensional stability in many ways, some of which are described in the About Building section.
Wood that has been properly dried is not only a pleasure to work with but can be part of furniture and utensils which will last for many lifetimes.
© Copyright 2002-2011