Trees are in general are 50% water while they are living and when they are freshly felled. As anyone who has tried to burn fresh cut wood can attest freshly cut wood is too wet to burn effectively. The primary reason for this is that the tree has capillaries that allow sap, which is mostly water, to flow from the roots to the top of the tree. Some species of tree have such a high moisture content that they drip water from where they have been cut.
Fresh cut lumber is inherently unstable and unsuitable for fine woodworking as it still has a high moisture count. As the lumber is dried, either air dried or kiln dried, the wood reaches a point of equilibrium. Equilibrium is the point where the wood will no longer gain or shed moisture. This equilibrium will depend on the species of wood in question and the location in which the wood is located. Geography can play a huge role in the amount of moisture in which lumber would be considered to be in equilibrium, in Arizona wood will have a lower moisture content once dried than it will in the Pacific Northwest.
Humidity's Effect on Woodworking
As stated above any wood that is to be utilized in fine woodworking should first reach its equilibrium. However, there are a few considerations to keep in mind:
Any wood that has not reached its equilibrium is out of balance with the moisture in the surrounding air can still give and take moisture from the air. As the wood takes the moisture from the air the wood will swell, and when the wood gives its moisture up to the air the wood will shrink. While some areas have a relatively consistent level of humidity, some locations such as the Mid-West have very humid summers and dry winters. This change in humidity can cause changes in wood that is even in equilibrium. However, any wood that has not been properly dried will have larger shifts in expansion and contraction.
Any piece of furniture that was made from wood that reached equilibrium in a humid place, such as Honduras, will undergo a significant change in the form of contraction if it is moved to a dry place, such as Denver, Colorado. All wood will change depending on the environment and if the woodworker does not take this into consideration while building the piece failure of the piece can likely occur. For example, if a woodworker is building a piece of furniture in a non-climate controlled shop and then moves the finished piece into a climate controlled environment the wood will respond to the changes.
Now that we have established that wood will naturally change to reach equilibrium with its environment we must now discuss how the wood itself will expand. Any wood that takes on moisture will naturally expand across the grain and not along the grain direction. A 2 x4 that is 8 feet long will continue to be 8 feet long even after expansion, but the width of the wood will vary depending on the wood species and the amount of moisture the wood has absorbed to reach equilibrium. The thickness of the piece can also vary but to a lesser extent than the width.
Methods for Dealing with Equilibrium Changes
When building a cabinet carcass, the grain direction of all four sides should be oriented in the same direction. With all four sides oriented in the same direction any expansion and contraction should be equal all of the way around. This is especially true if the wood utilized in the carcass construction have originated from the same board. However, as the outer carcass expands and contracts with seasonal changes it can cause difficulties in opening and shutting any drawers or creating gaps around the drawer faces. This is the primary reason that plywood has become the choice for building a cabinet carcass. Plywood is created by using multiple thin veneers of wood with the grain directions changing with each subsequent layer and creating a stable product that is effected very little by moisture changes in the air.
When gluing up boards to create a larger panel or tabletop all of the board should have their grain orientation going the same direction. It is also important to ensure that the end grains of each board are running in opposite directions. Essentially, when looking at the end of the first board the end grain could be curved down, like a rainbow, and the end of the second board would be the opposite where the curve looks more like a bowel. By changing the end grain direction on each board it will help prevent any cupping that can occur when the moisture levels change.
The orientation of the boards during the creation of a large panel or tabletop should place the end grain of the boards on the two short sides of the panel. When connecting a tabletop to its frame there are multiple methods that will work. However, it is important to only secure one board to the frame where it will not move. All of the other boards should be secured with hardware in a slot that allows for the expansion and contraction of the tabletop. If all of the boards are secured to prevent movement it will likely lead to joint cracking during contraction or cupping of the tabletop during expansion