. Bulletin. Forests and forestry. ^'> F7^. 7. or less deeply from the surface inward (See fig. 6. Or-d.) Straight- grained woods that split easily check deep and wide in a few places. Cross-grained woods become covered with a network of fine "superficial season ; When the interior of the piece begins to dry out, it shrinks in its turn and tries to pull the shell with it. If the wood is one of a loose structure, that does not become very hard and stiff in drying, the shell follows the core and the season checks close sometimes so completely as to show only as fine
. Bulletin. Forests and forestry. ^'> F7^. 7. or less deeply from the surface inward (See fig. 6. Or-d.) Straight- grained woods that split easily check deep and wide in a few places. Cross-grained woods become covered with a network of fine "superficial season ; When the interior of the piece begins to dry out, it shrinks in its turn and tries to pull the shell with it. If the wood is one of a loose structure, that does not become very hard and stiff in drying, the shell follows the core and the season checks close sometimes so completely as to show only as fine ;^ Since the four corners of the shell are stiffer than the flat sides, they shrink less than the faces and these become more or less hollow, as shown in figure 6. If, however, the wood is one that sets very hard and stiff, the shell refuses to follow the core and the latter, trying to shrink and restrained by the stiff shell, has to split internally, as shown in figure 7. This is known as "internal checking'* or "honeycombing.*' It occurs most easily in woods having large pith rays, like the oaks, and in dimension timbers. It is likely to be worse in flat-sawn than in quarter-sawn boards; in the former the pith rays, running at right angles to the faces of the boards, prevent these from following the shrinkage of the core, while in the latter they present no obstruction to shrinking through the thickness of the board. But severe honeycombing sometimes takes place in quarter- sawn oak. The more rapid the process of drying is in the beginning, the more pronounced is the casehardening. It is worst when fresh-sawn wood is exposed to the sun, or placed in a steam or fire heated dry-air kiln. It is on this account that most of the earlier attempts to kiln-dry refractory woods were unsuccessful. In kilns where hot moist air or steam is admitted, casehardening is largely or entirely prevented. Most woods dried in the open air, but not exposed to the sun, will not case- hard
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