﻿XiLN DRYING HANDBOOK. 25 



rubber by actually stretching it, whereas the tension is produced in 

 the outer layers of the wood by preventing it from shrinking. The 

 same thing occurs if a piece of wet leather is prevented from shrink- 

 ing as it dries. 



This drying stress will increase as the drying progresses. The 

 outer layers continue drying and shrinking and to them are con- 

 tinually being added other intermediate layers which are reaching 

 the fiber-saturation point and are ready to shrink. Layers once in 

 compression begin shrinking and place themselves in tension. Those 

 layers still near the fiber-saturation point are more or less plastic 

 and able to yield to stress without too much difficulty. The outer 

 layers, however, having yielded at first, much like the rubber band, 

 are now getting dry, and are becoming constantly less yielding. 

 Eventually they become sufficiently stiff and there are enough of them 

 so that they can successfully resist the stresses placed upon them 

 by the drying, and they are in what is known as a " set " state. 

 Further drying results in a reversal of stresses. The shrinkage of 

 the inner layers or core is now opposed by the " set " exterior layers, 

 and the result is that the inner layers are in tension and the outer 

 layers in compression. If no special precautions are taken, it is to 

 be expected that most kiln-dried stock will be in this state of stress 

 when it is removed from the kiln. This condition is usually de- 

 scribed as " casehardened." 



CHECKING AND HONEYCOMBING. 



It has been assumed that the stresses in the board were not suffi- 

 cient to cause visible damage. If, however, the strength of the wood 

 in tension across the grain is not sufficient to resist the tensile stresses 

 in the surface layers during the early stages of drying, it will tear 

 open, forming surface checks of varying size and depth. Likewise, 

 if the inner layers are not strong enough to resist the tension placed 

 upon them during the latter stages, they will rupture, causing 

 " honeycomb " or " hollowhorn." Because radial shrinkage is less 

 than tangential, and because a weak plane is produced where the 

 rays and fibers cross, checks and honeycomb more often run radially 

 than tangentially. It not infrequently happens that surface checks 

 formed during the early stages of drying, or, in the case of partially 

 air-dried stock, before entering the kiln, close up and disappear dur- 

 ing the final drying. In fact, the effect caused by the shrinkage of 

 the core may go still farther and result not only in closing the checks 

 at the surface, but in actually deepening them and opening them 

 up in the center, forming honeycomb. (See Fig. 4.) 



WARPING, LOOSENING OF KNOTS, END CHECKING. 



There are several other drying defects due to uneven shrinkage, 

 such as warping and twisting, which are often caused by spiral or 

 interlocked grain, by a difference in longitudinal shrinkage between 

 sapwood and heartwood, and by various other irregularities in struc- 

 ture and in the drying. (See Pis. VIII and IX.) The loosening of 

 knots is caused by the drying-out or exudation of cementing resins 

 and gums and by the differentials in shrinkage caused by the fact that 

 the axis of the knot or branch is at right angles to the axis of the 



