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WHAT KILN OPERATORS NEED TO KNOW ABOUT WOOD STRUCTURE AND PROPERTIES Wood is a natural composite. It’s made from different chemicals (with different degrees of water absorbency) that combine to make fibers. Unlike man-made composites which can be manufactured with the properties we want, we have to take the trees as they are given to us by Mother Nature. Just like the kids we went to school with, some trees grow faster than others, some are fatter or skinnier than others, and some are well-mannered while others are troublemakers. Sawyers do their best to make good, sound lumber, but inevitably some problems will get passed along to the air drying yard or kiln operator. The air-drying yard supervisor has a difficult job: it’s up to him to see that every board gets air-dried properly before it’s sent to the dry kiln. Mother Nature and Human Nature being what they are, though, some lumber just won’t get handled optimally, and some of the problems that air drying yards inherit (or even create) will get passed along to the dry kiln operators. Let’s take a quick look at how wood is put together so you can better understand why lumber dries the way it does. FIBERS. Roughly 90% of the cells in a tree run parallel with the trunk. These cells are what we commonly call fibers, and they give the tree strength, supporting the trunk and the branches above. I like to think of a bunch of soda straws as my model for these cells – they’re hollow like fibers, and they’re strong when they’re in a bundle. The rest of the cells are oriented at 90 degrees to the tree trunk, and groups of these cells are called rays. You can think of rays as each tree’s “moving and storage company” if you like. Rays move the waste products from the living cells in the cambium just below the bark into an area closer to the center of the tree. These compounds are often colored or scented, and the area where they are located is called heartwood; heartwood is the portion of the trunk surrounding the pith (in the center of the tree). The lighter-colored region surrounding the heartwood and just beneath the bark is called sapwood (Figure 7). EXTRACTIVES. Boards from different species are different in both color and odor, and this is due to chemicals in the wood called extractives. The differences are usually most pronounced when heartwood is compared, because the extractive content in heartwood is typically significantly higher than in sapwood. The chemicals are called extractives because they can be removed from wood using solvents such as hot water or alcohol. Extractives aren’t part of the cell structure itself, though they might impregnate the cell wall or fill some cells entirely. 15PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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