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center will shrink more than the barkside edge, resulting in side bend. Another type of defect, known as twist, can be caused by the sawyer cutting a log with naturally-occurring spiral grain (Figure17), but twist is one of those defects that occurs more in some species than in others. Figure 17. This log with spiral grain was found in the woods. INTERLOCKED GRAIN Try to imagine a bundle of cells twisted around itself like a three-strand piece of rope, with some cells twisting clockwise while others go counter-clockwise. Now imagine this combined with two other similar pieces, making up an even bigger (wood) rope. Obviously the fibers aren’t going to be aligned in a straight direction, and when you attempt to dry wood with grain like this it’s harder to keep the lumber flat. This is called interlocked grain, and it’s mostly found in em, black gum, sweet gum, sycamore and beech. Not every board of every species with interlocked grain will have a tendency to twist, but it seems especially difficult to dry the gums and sycamore and keep them flat. Interlocked grain tends to keep long continuous drying checks from forming. This isn’t usually an issue in boards but it can affect the quality of larger dried timbers such as railroad ties. Species such as the oaks and maples that don’t have interlocked grain may develop long, linear, continuous checks unless these are prevented by incising the green ties at the tie plant. Species with interlocked grain usually develop shorter, curved checks. MOISTURE CONTENT. One of the most important chemicals making up wood cells is cellulose. This is the chemical that gives wood a lot of its strength, but it takes up water readily. You probably know that cotton is almost pure cellulose, so think of how a cotton t-shirt behaves on a clothesline. T-shirts take up water on a damp day and release it when the sun comes out and the relative humidity drops. Wood behaves in the same way, so once wood is dried you have to make sure that it’s kept in an environment that will keep the wood at the moisture content (MC) to which you just dried it. If conditions are stable, wood will come to an equilibrium with the 24PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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