Moisture Content of Wood: The weight of water in the specimen expressed as a percentage of the oven-dry weight of the wood.
Oven-Dried Wood: A sample that has been dried in an oven at 100 C to 105 C until the wood attains a constant weight.
Moisture influences weight, shrinkage and strength of the wood. It exists in wood as either bound or free water. Bound water is held within the cell wall by adsorption forces, whereas free water exists as either condensed water or water vapor in the cell cavities.
The level of saturation at which the cell walls are completely saturated, but no free water exists in the cell cavities is called the fiber saturation point (FSP). The FSP ranges from 21% to 32% in most species. The FSP is important because the addition or removal of moisture below the FSP has a large effect on practically all physical and mechanical properties of wood, whereas above the FSP, the properties are largely independent of moisture content.
When moisture content of wood is above the FSP, wood is dimensionally stable. Anything below the FSP will result in dimensional changes. Also, moisture content in wood depends on air temperature and humidity.
The moisture content for the average atmospheric conditions is the equilibrium moisture content (EMC). The EMC ranges from less than 1%, at temperatures greater than 55C and 5% humidity, to over 20% at temperatures less than 27 C and 90% humidity.
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