Expansion and Shrinkage in Wood
The success of a woodworking venture partly depends upon whether or not allowance has been made for expansion and shrinkage of the wood. Moisture being released from or absorbed by the wood is what creates the movement. It makes no sense, for instance, to use fresh-cut wood for a project because the wood will undergo a significant amount of shrinkage as it dries out.
A living tree is full of moisture because every day many gallons of water and sap move through the tree from root to branches and leaves. A percentage of this moisture is released back into the atmosphere. The relationship between wood and moisture in its environment continues after a tree is cut down; in fact, it’s ceaseless. Wood adapts to the moisture level in its environment by either releasing moisture, which causes shrinkage, or absorbing moisture, which causes expansion.
Dry Wood to Establish Equilibrium
Most lumber manufacturers use kilns to dry lumber to proper moisture levels for woodworking purposes. Kiln drying uses heat sources to dry out stacks of lumber. Planks can also be air-dried, but the process is much slower. Hand-held wood moisture meters are convenient to use in determining the level of moisture in a piece of lumber.
The ultimate goal is to dry wood to the same moisture levels as the environment in which the final product will be used or installed. Establishing equilibrium that matches the wood’s ultimate environment will minimize the amount of movement in the wood.
Changes within an environment sometimes create problems with wood. In the upper Midwest of the USA, for example, the summers are considerably humid but the winters are quite dry. Most homeowners use humidifiers during the winter so that some of the moisture in the air is replaced and hardwood floor and furniture doesn’t lose so much moisture that the wood becomes damaged. This type of seasonal change in the amount of humidity in the air is why dresser drawers and doors tend to stick during the summertime but are problem-free in winter.
Another consideration as regards equilibrium in wood is moving a piece of furniture to a significantly different climate. A finely crafted hardwood chest of drawers that has achieved balance in a humid environment such as in Louisiana but is then moved to a dry climate such as Arizona could actually crack as the wood releases a substantial amount of moisture.
Understand How Wood Expands
A skilled woodworker knows how the wood being used will expand as it equalizes with the moisture in its environment. The movement in lumber created by gaining or losing moisture occurs across the grain and not along the grain. In other words, an 8-foot-long 2 x 4 will usually always remain 8 feet long. The width and thickness of the same 2 x 4, on the other hand, could vary significantly.
Build with Expansion in Mind
If you’re building a cabinet, for example, it could help to use wood from the same source of original stock. In any case, the grain of the lumber should be oriented in the same direction. Expansion can have an effect on drawers, however, making them difficult to open. For this reason, plywood is used to build most cabinet carcasses. The advantage offered by plywood is that it’s virtually unaffected by humidity as compared with dimensional lumber.
The following is a tip for dealing with potential cupping of boards when gluing boards together to make a table top: As you place the boards side-by-side, alternate end grains facing up and end grains facing down. This should achieve a balance which eliminates cupping.
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