Everyone has experienced the results when wood absorbs water and swells (desk and kitchen drawers stick), or when wood loses water and shrinks (tool handles loosen). To become familiar with this relationship between wood and water, consider a piece of wood, or a board, as consisting of many individual cells cemented together. Visualize the piece of wood as a handful of drinking straws. Right after the tree is cut down and processed into lumber, the individual wood cells have their walls saturated with water and their centers are filled with water. The water in the wall is called “bound water,” and the water in the center of the straw is called “free water.” As the wood cell is exposed to relatively dryer air, the free water leaves the cell, but the cell wall stays saturated.
As long as the cell walls remain saturated the piece of wood will not change in shape, i.e., it will not shrink or swell. But, when the wood loses all of its free water, the bound water in the cell wall starts to evaporate. This is a critical point and is given the name "fiber saturation point" (FSP), i.e., all of the free water has evaporated, but all of the bound water is still present. Now as water continues to evaporate from the cell wall, the piece of wood begins to change dimensions; it shrinks. Wood will continue to lose water from the cell walls until it reaches equilibrium with the surrounding air. Wood technologists call this point the "equilibrium moisture content" (EMC) for the wood. Remember, that the piece of wood continued to shrink as bound water was removed below the FSP.