KILN 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 
M 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 
