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from a leaning tree). Tension wood has an extra layer of material in the cell wall, making the cells more dense and making it more difficult for water to move. This extra layer, however, does not contribute substantially to the strength of the wood, so tension wood is often weaker than would normally be expected. Tension wood also machines poorly, creating abnormally fuzzy surfaces. The fuzz can be easily noted or may be very fine and require special lighting. Tension wood often causes finishing problems, especially with water based stains. Compression wood is the opposite of tension wood in some ways - it develops in softwoods and on the same side of the stem where there are compression forces. The cell walls are very thick, slowing moisture movement, and are weak. Compression wood shrinks along the grain (longitudinally), resulting in bow and crook in lumber during drying. Earlywood The cells formed early in the growing season of a tree are called earlywood or springwood. They typically have thin cell walls and are quite large. Water moves quite easily in the earlywood. Latewood The cells formed toward the later part of the growing season are called latewood or summerwood. They typically are smaller in diameter and have thicker cells walls than earlywood. Water moves more slowly in summerwood. Shrinkage The change in dimension of wood that occurs when it dries is called shrinkage. The individual cells begin shrinking when they are at the fiber saturation point (28% MC) and the shrinkage continues linearly with a decrease in MC to 0% MC. Lumber, because some cells are at fsp very early in drying, begins shrinking very early in drying at very high MCs (within 5% of its "tree green" MC) and continues shrinking until equilibrium is reached. Whenever the relative humidity changes, wood will lose MC and therefore shrink if the humidity is lowered, or gain MC and therefore swell if the humidity is raised. Temperature change has no important effect on shrinking or swelling of wood. (See also Fiber Saturation Point on Page 15.) There is a great deal of variation in the amount of shrinkage. It varies from very little (in teak shrinkage is a maximum of 5.8%) to quite a lot (as much as 12.7% in oak). There are variations in the same species, as well. In addition, wood shrinks differently in the different directions - longitudinally, wood shrinks a maximum of 0.3% which is so small as to be ignored; radially, wood typically shrinks 3 to 8%; tangentially, wood typically shrinks 6 to 16%. Usually, the more dense the wood, the more it shrinks. As an example of the variation of shrinkage, consider some of the eighty species that we classify as red or white oak (Table 3). In general, the greater the shrinkage, the more difficult the wood will be to dry without developing cracks, splits, and checks. 14PDF Image | Principles and Practices of Drying Lumber
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