What is Moisture Content, and how is it Measured?
The moisture content of wood is measured as the ratio of the weight of water in a piece of wood to the weight of the wood when it is completely dry (oven-dry). You will see this referred to as a percentage.
There are two different approaches to determine moisture content: dry basis moisture content and wet basis moisture content. The wet basis is used for pulpwood, chips, and fuelwood. The wet basis is determined as follow: (the weight of the water / the wet weight of the wood) X 100.
Dry basis moisture content is used for solid wood, logs, veneer, and most non-paper and non-fuelwood purposes. This is determined by taking the (the weight of the water / the dry weight of the wood) X 100.
The moisture content of a piece of wood should be as close to equilibrium moisture content (EMC) as possible. Relative humidity and temperature impact EMC. EMC varies on location. The Midwest and Lakes States regions are 7%, and the Southeastern region is 12%.
Moisture meters are also available to help determine the moisture content of the wood. There are various forms and different specifications to use them. Make sure to read your instruction manual on how to use your meter. But moisture meters have limitations and aren’t always accurate. The oven-dried method (the previous method) is the best way to demine moisture content.
Why does Wood Shrink and Swell?
Over 90% of problems in woodworking are related to moisture. Examples of this are drawers sticking and doors warping and cracking. Wood in trees contains a lot of moisture. We must dry the wood down to the equilibrium moisture content (EMC) of the environment.
There are two different types of moisture in wood: free water and bound water. Free water is liquid water found in the pores of the wood (cell lumen). Free water is easy to remove and much of it is released by air drying. Bound water is saturated water that is chemically bound to the cell walls. This water is more difficult to remove.
So why does wood shrink? It’s simple. When the bound water, water in the cell wall, is removed, the cell wall shrinks. Wood doesn’t shrink until approximately 30% MC. This point of the drying process is the Fiber Saturation Point (FSP). FSP is when all the free water is gone, and only the bound water remains.
Wood Movement in Your Projects
For most projects, you don’t need to know precisely how much the wood will move, but understand it will.
Wood doesn’t exchange moisture evenly on all surfaces. Quartersawn lumber has an appearance with long, uniform grain lines. Other names for quartersawn are vertical grain (used in softwoods) and radial grain (used by wood scientists). This surface will move, but not as much as the flatsawn surface. Flatsawn lumber, also known as plainsawn (used in softwoods) or tangential (used by wood scientists), moves the most. We often say that the flatsawn surface exchanges moisture two times faster than the quartersawn surface. Flatsawn lumber has a cathedral grain pattern. But even though lumber shrinks and swells in width and thickness, it does not move longitudinally (end to end). Every species has different rates of moisture exchange. A rule of thumb is the denser the species, the more it will shrink and swell.
The wood movement needs to be accounted for when designing projects. Not accounting for the movement can result in ruining a project. Attach tops to bases with fasteners that allow for movement. You can use tabletop fasteners for small projects or slotted washers for large projects. When running boards perpendicular to each other use breadboard ends. The panel, if solid, on a raised panel door must be free-floating. If glued, it will result in cracking of the panel or breaking of the joints.
Apply the same amount of finish to all sides of the project. This equalizes the exchange of moisture helping to keep your project from warping, cupping, and twisting over the years.