It is at once evident that if drying does not take place uniformly throughout an entire piece of timber, the shrinkage as a whole cannot be uniform.  The process of drying is from the outside inward, and if the loss of moisture at the surface is met by a steady capillary current of water from the inside, the shrinkage, so far as the degree of moisture affected it, would be uniform.  In the best type of dry kilns this condition is approximated by first heating the wood thoroughly in a moist atmosphere before allowing drying to begin.

In air-seasoning and in ordinary dry kilns this condition too often is not attained, and the result is that a dry shell is formed which encloses a moist interior. (See Fig. 26.) Subsequent drying out of the inner portion is rendered more difficult by this “case-hardened” condition.  As the outer part dries it is prevented from shrinking by the wet interior, which is still at its greatest volume.  This outer portion must either check open or the fibres become strained in tension.  If this outer shell dries while the fibres are thus strained they become “set” in this condition, and are no longer in tension.  Later when the inner part dries, it tends to shrink away from the hardened outer shell, so that the inner fibres are now strained in tension and the outer fibres are in compression.  If the stress exceeds the cohesion, numerous cracks open up, producing a “honey-combed” condition, or “hollow-horning,” as it is called.  If such a case-hardened stick of wood be resawed, the two halves will cup from the internal tension and external compression, with the concave surface inward.

[Illustration:  FIG. 26.—­Progress of drying throughout the length of a chestnut beam, the black spots indicating the presence of free water in the wood.  The first section at the left was cut one-fourth inch from the end, the next one-half inch, the next one inch, and all the others one inch apart.  The illustration shows case-hardening very clearly. Photo by U. S. Forest Service.]

For a given surface area the loss of water from wood is always greater from the ends than from the sides, due to the fact that the vessels and other water-carriers are cut across, allowing ready entrance of drying air and outlet for the water vapor.  Water does not flow out of boards and timbers of its own accord, but must be evaporated, though it may be forced out of very sappy specimens by heat.  In drying a log or pole with the bark on, most of the water must be evaporated through the ends, but in the case of peeled timbers and sawn boards the loss is greatest from the surface because the area exposed is so much greater.

The more rapid drying of the ends causes local shrinkage, and were the material sufficiently plastic the ends would become bluntly tapering.  The rigidity of the wood substance prevents this and the fibres are split apart.  Later, as the remainder of the stick dries many of the checks will come together, though some of the largest will remain and even increase in size as the drying proceeds. (See Fig. 27.)