MOISTURE CONTENT OF WOOD
Wood expands and contracts with changes in the surrounding humidity and to a lesser degree the temperature. More humid air will cause wood to expand; drier air will cause wood to contract. This movement cannot be stopped. You can learn what to expect and techniques to cope with the movement. For most of us it is adequate to know that a table top will move about .5 inches.
Drying lumber, either naturally (air drying) or in a kiln is the process to lower the moisture content to the surrounding environment. For much of the United States, the minimum moisture content of thoroughly air dried lumber is 12% to 15%. Kiln dried hardwood will usually be less than 10%.
Dried lumber has many advantages over green lumber for Woodworkers. Removing excess water reduces weight, thus shipping and handling costs. Proper drying will help confine shrinking and swelling of wood in use to manageable levels. As wood dries, most of its strength properties increase. Properly dried lumber can be cut to precise dimensions and machined more easily and efficiently; wood parts can be joined more securely or fastened together with nails, screws, bolts, and adhesives. Warping, splitting, checking, and other harmful effects of uncontrolled drying are largely eliminated; and paint, varnish, and other finishes are more effectively applied and maintained.
The objective is to bring the moisture content of wood, as close as possible, to the level the finished product will experience in service. Acquire your lumber in advance and give it time to acclimate to the environment in which it will be used or worked. Sometimes it may be necessary to “stack and sticker” lumber to allow it to properly reach equilibrium with the environment.
Finished projects will continue to absorb or give off moisture with changes in humidity and temperature of the surrounding air. The wood will always undergo changes in moisture content and dimension. Consequently doors and drawers may stick during periods of high humidity. Moulding joints may open. Solid wood panels in raised panel doors may shrink exposing unfinished wood along the edges of the frame. Furniture moved from one part of the world to another may change enough in dimension that it must be reworked to function correctly. And, during a dry night a poorly designed furniture panel may explode with a bang!