KILN DRYING HANDBOOK 35 
size. Wood seldom either shrinks or swells without some restraint, 
and therefore set is of great importance in practically all drying. 
The extreme outer layers of any piece of wood of commercial size 
approach equilibrium moisture content shortly after drying com- 
mences. They are then in a state of tension, with a certain amount 
of tension set present. As the wood continues to dry, more and more 
of the deeper layers reach the fiber saturation point and start to 
shrink, thus first relieving themselves of any compression under 
which they may have been and afterward putting themselves in ten- 
sion and as a result increasing, to a corresponding amount, the com- 
pression on the core. The entire piece shrinks, the core, which is 
still wet and plastic, yielding under the stress. The outer layers, now 
dry and quite stiff, assist in producing the shrinkage until they have 
relieved themselves of their tension stress. At this point of stress- 
freedom they set, because of the initial tension still in a condition 
of expansion somewhat beyond that which otherwise would be their 
then natural state. As the drying and the shrinking continue these 
outer layers become compressed, in contrast to their former condi- 
tion of tension, and strongly resist further shrinkage. Accordingly 
the core, which is now below the fiber-saturation point and is trying 
to shrink, is put in tension. All of the wood in such a piece is under 
tension during part or most of its drying below the fiber-saturation 
point and each layer sets in a more or less expanded condition. 
When the wood is finally uniformly dry, the outer layers in conse- 
quence are in compression and the core 1s in tension. Under these 
circumstances the wood is said to be casehardened. Casehardening 
is of Importance in most hardwoods and in many softwoods. Methods 
for relieving it will be considered later. 
CHECKING AND HONEYCOMBING 
It was assumed in the preceding discussion that the stresses in a 
casehardened board are insufficient to cause visible damage. Tf, how- 
ever, the strength of the wood in tension across the grain is not great 
enough to resist the tensile stresses in the surface layers during the 
early stages of drying, they will tear open, forming surface checks 
of varying size and depth. (Pl. 13.) Wikewise, if the inner layers 
are not strong enough to resist the tension placed upon them during 
the latter stages, they will rupture, causing honeycomb. (PI. 14, 
Band C.) Both because radial shrinkage is less than tangential and 
because weakness occurs throughout the planes where rays and fibers 
cross, checks and honeycomb more often run radially than tangen- 
tially. It not infrequently happens that surface checks formed dur- 
ing the early stages of drying or, in the case of partially air-dried 
stock, before entering the kiln, close up and disappear during the 
final drying. In fact, the effect of 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 of the 
piece of wocd, thus again forming honeycomb. (Fig. 7.) 
WARPING, LOOSENING OF KNOTS, END CHECKING 
Uneven shrinkage results in several other drying defects, such as 
bowing and twisting, which are often caused by either spiral or 
interlocked grain, by a difference in longitudinal shrinkage between 
