How Dry is Your Wood?
Why dry wood and finish it?
In a tree, a liquid called sap (primarily water) carries dissolved minerals and food that's manufactured by the leaves throughout the tree. Green wood refers to boards sawn from logs that had much of the original sap still in them. But it can describe rewetted wood as well. Dry or seasoned wood has had most of the sap removed.
Generally, there's a higher moisture content in the sapwood than in the heartwood of softwoods. That's because the sapwood is "living" wood that carries nutrients. In hardwoods, the difference between sapwood and heartwood varies with the species. And due to growing conditions -- a north slope, river bottom, etc. -- moisture content can vary from tree to tree. Moisture in the cell structure of wood makes it weaker than wood without water. Of course, it's impossible to keep moisture completely out of wood?finishes only serve to slow its movement and the reaction of the wood.
Wood exposed to an atmosphere containing constant humidity will, in time, reach a steady moisture-content condition -- it doesn't gain or lose moisture. The numerical value of this stabile moisture content (MC) is called the equilibrium moisture content (EMC) of the wood, and is dependent on the relative humidity and air temperature. Over much of the United States, the outside conditions average 65 percent relative humidity, which is 12 percent EMC. Therefore, wood under cover, yet exposed to outside conditions winter and summer, will reach a 12 percent MC.
On the other hand, wood stored inside heated buildings in mid-winter can reach an MC that ranges from 4-8 percent. If you know the atmospheric conditions, you can estimate the wood's moisture content with a chart like the one below. Remember, though, that changes in wood's MC are a function of time, too, with rapid fluctuations on the wood's surface at first and very slow changes inside.
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