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What Is “Free Water” and “Bound Water”? When trees are growing there is liquid water moving through the cells of the wood. This water is called “free water” because it exists in water form and can be removed relatively easily from the wood. “Bound water” is water that becomes part of the wood fiber itself, and is more difficult to remove. When wood is dried, the first thing that happens is that the free water evaporates until the lumber drops to what’s called “Fiber Saturation.” Fiber saturation is generally reached when the moisture content gets to about 28%. At that point, all the free water is gone and only bound water remains. Wood does not shrink until it is below fiber saturation and the bound water begins to be removed from the cells of the wood. What’s The Difference Between Drying Hardwoods And Softwoods? This is a commonly asked question, but the actual species of the wood is really more important to consider than just whether it’s a hardwood or softwood. The terms hardwood and softwood generally refer to whether the wood comes from a tree with leaves (hardwood) or a tree with needles (softwood). Some hardwoods are actually softer than many softwoods, so there’s no general drying method that applies as a rule to all hardwoods or all softwoods. Different species require drying at different temperatures and different speeds to produce the best results. Oak has to be dried slowly or it degrades badly. Pine needs to be dried at a fast rate or it stains and molds. That doesn’t mean all hardwoods should be dried as slowly as oak, or that all softwoods can be dried as quickly as pine. There’s a proper method that’s been determined for nearly every species of lumber to produce the best results. Check with your Nyle representative for information on drying a particular species. Once Wood Is Dry Will It Remain Dry? Wood is always trying to come into equilibrium with the air surrounding it, so its moisture content can change somewhat after it’s been dried. In dry air, the wood gives up water to the air until it is dried, as it equalizes with the air. When the air is more humid, wood absorbs water from the air. Wood expands when it absorbs water, and shrinks when it gives up water. In worst-case winter heated conditions in northern climates, wood may dry to a moisture content of 6% as it gives up water to the dry air. In the humidity of summers, it may pick up moisture to about 15-16%. If dried lumber is exposed to outdoor conditions long enough, it will eventually reach about 12% moisture content, which corresponds to the average annual equilibrium point in most geographic areas. 5PDF Image | INTRODUCTION TO KILN DRYING
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