. Bulletin. Forests and forestry -- United States. 36 TIMBER. adjoining cells and are greatest at the pith rays. These strains cause warping and checking, but exist even where no outward signs are visi- ble; they are greater if the wood is dried rapidly than if dried slowly, but can never be entirely avoided. Temporary checks are caused by the more rapid drying of the outer parts of any stick; permanent checks are due to the greater shrink- age, tangentially, along the rings than that along the radius. This, too, is the cause of most of the ordinary phenomena of shrinkage, such as the differen
. Bulletin. Forests and forestry -- United States. 36 TIMBER. adjoining cells and are greatest at the pith rays. These strains cause warping and checking, but exist even where no outward signs are visi- ble; they are greater if the wood is dried rapidly than if dried slowly, but can never be entirely avoided. Temporary checks are caused by the more rapid drying of the outer parts of any stick; permanent checks are due to the greater shrink- age, tangentially, along the rings than that along the radius. This, too, is the cause of most of the ordinary phenomena of shrinkage, such as the difference in behavior of entire and quartered logs "bastard" (tangent) and "rift" (radial) boards, etc., and explains many of the phenomena erroneously attributed to the influence of bark, or of the greater shrinkage of outer and inner parts of any log. Once dry, wood may be swelled again to its original size by soaking in water, boiling, or steaming. Soaked pieces, on drying, shrink again as before; boiled and steamed pieces do the same, but to a slightly less degree. Neither hygroscopicity, i. e., the capacity of taking up water, nor shrinkage of wood can be overcome by drying at temperatures below 200° F. Higher temperatures, however, reduce these qualities, but nothing short of a coaling heat robs wood of the capacity to shrink and swell. Eapidly dried in the kiln, the wood of oak and other hard woods " case-harden," that is, the outer part dries and shrinks before the interior ;;. Honeycombed- board, has a chance to do the same, and thus forms The checks or cracks form along a firm shell or case of shrunken, commonly t epi rays. checked wood around the interior. This shell does not prevent the interior from drying, but when this drying occurs, the interior is commonly checked along the medullary rays, as shown in fig. 24. In practice this occurrence can be prevented by steaming the lumber in the kiln, and still better by drying the wood in
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