How Dry is Your Wood?
Where you'll find the water
Woodworkers work with dry wood. But as you'll find out, it never remains completely so. And it pays to discover why.
Water occurs in wood in two places. First, there's the free water that fills the inside of the wood's cells. That's like water in a bucket.
Second, water also infiltrates the cell walls. That's called bound water. Imagine squeezing a piece of cotton cheesecloth until all the free water has drained away. The cloth, through, remains damp because the material continues to contain moisture -- the bound water.
When wood only contains bound water, it's said to be at its fiber saturation point. And the bound water can be eliminated completely (0% moisture content) only by drying it somewhere with no relative humidity, such as in an air-tight oven.
Wood rates as a hygroscopic substance. That is, it has an affinity for water and readily absorbs it as liquid and vapor. This ability directly depends on the humidity of the surrounding atmosphere. Therefore, the amount of moisture in wood changes as the humidity changes.
The total amount of water in any given piece of wood is called its moisture content (MC). And technically, it's expressed as a percentage of the oven-dry weight of the wood. A piece of green wood is weighed, then dried, then weighed again. Suppose the wood weighed 40 pounds when green and only 30 pounds after drying. The 10 pounds of water lost represents one-third of its oven-dry weight, so the wood would have had a 33-1/3 percent moisture content. If the piece of wood had weighed 80 pounds when green, the 50 pounds of water lost would have reflected a moisture content of 167 percent. The heartwood of black cottonwood, for instance, frequently has a moisture content of 162 percent, and ax blows produce sprays of water!