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Wood Drying
The Wood and Water Relationship
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. Read More...
The EMC of wood is determined by the relative humidity of the surrounding air. The lower the relative humidity, i.e., the dryer the air, the more water that is evaporated from the piece of wood. If the surrounding air is extremely dry as in a hot oven, the wood will eventually lose all of its. water, or become "oven dry". Naturally, as the piece of wood picks up water it will swell and change shape until the FSP is reached. At this point more water may be picked up, but no addition¬al swelling will occur.
Well, that is it-the whole story of when wood, i.e., lumber, shrinks and swells. But, for us to discuss the shrinking and swelling of wood, it is convenient to know the amount of water in a piece of wood at various times. For this purpose, wood technologists use the term "moisture con¬tent" (Me). The MC of wood is measured as the ratio of weight of water in a piece of wood to the weight of wood when it's oven dry. This ratio is usually multiplied by 100 percent and called the "percent moisture content". When a pine or oak tree is cut, the wood normally will have a MC somewhere around 50%. As free water is evaporated the MC will decrease until the FSP is reached,¬somewhere around 28%. Remember that wood will not shrink until the MC is less than approxi¬mately 28%. If the wood continues to lose its bound water below 28%, it will continue to shrink until it is oven dry. Naturally, if the oven dry wood picks up water, it will swell until it reaches a MC of 28%. After 28%, the wood is capable of picking up more water, but no additional swelling occurs.
If fresh cut lumber is exposed to the Mississippi atmosphere, it will lose water until its MC is approximately 11 %. To remove more water requires that the wood be heated to evaporate remaining water. It is critical to understand that the EMC for wood inside of normally heated and air conditioned homes is approximately 7%. This will vary somewhat as the relative humidity of the air changes. Now you know when wood shrinks and swells, and some critical MC(s) of wood. Remember, unless wood is conditioned or dried to the end-use EMC, the wood will shrink or swell as the relative humidity changes. That is one of the natural characteristics of all wood.
Methods of Drying Lumber
There are two common methods of drying lumber, for example air drying and kiln drying. Thus lumber can be purchased either “green” lumber (no significant drying has occurred since the lumber was cut from logs) or as “air-dried” lumber or as “kiln-dried” lumber. All lumber increases in strength, hardness, and stiffness as it dries from “green” moisture content to “kiln-dried” moisture content.
In air drying lumber, green lumber is stacked outdoors in layered bundles with spacers between each layer to provide free circulation of air between layers. After exposure to the circu¬lating air for an adequate length of time, usually between one and three months, the moisture content of the lumber should be between 15 to 25 percent (variables include atmospheric condi¬tions and wood species). It is to be noted that lumber with a moisture content less than 20 per¬cent will not decay.
In kiln drying lumber, either green or air-dried lumber is stacked in layered bundles with spacers between each layer and then placed in a kiln (oven) in which air volume and tempera¬ture is carefully controlled. In kiln drying lumber, the moisture content of the lumber is reduced to a specific amount depending on the particular use for the lumber. Lumber to be used for indoor furniture should be dried to a moisture content between 6 and 8 percent. Naturally, the cost of lumber increases as the moisture content decreases.
