THE SEASONING OF WOOD. 11 
HOW WOOD MAY BE INJURED IN SEASONING. 
CHECKING. 
Checking is caused by unequal shrinkage. If the outside of a piece 
of wood dries considerably faster than the inside, the surface in time 
will contract until it can no longer extend around the comparatively 
wet interior, and so will be torn apart in checks. Checks often are 
if classified as end checks and face checks. End checking or splitting 
during seasoning causes nearly as much loss as face checking. 
CASEHARDENING. 
Casehardening or surface hardening occurs when the surface of 
wood becomes set in a partially dry condition while the interior is 
still wet. This condition results from too rapid surface drying. 
If the interior of a casehardened piece of wood dries further, it tends 
to shrink, while the "set" condition of the surface tends to prevent 
it from doing so . As a result, stresses are set up in the piece. Plate I , 
figure 1, shows sections cut from casehardened boards, with a strip 
sawed from the center of each section. In A, the stresses cause the 
prongs to curve inward and bind on the saw. If the stresses are 
relieved by treatment with steam, as may be done sometimes, and 
the board dried a second time, the resawed prongs, as shown in B, 
will curve outward, owing to a reversal of the stresses. This is 
termed "reverse casehardening." x 
Plate I, figure 2, shows the form taken by resawed pieces of kiln- 
dry boards steamed for different lengths of time. In No. 1 the 
prongs curve inward, owing to casehardening. No. 2 and No. 3 
also show a casehardened condition as indicated by the strips curving 
inward. In Nos. 4, 5, and 6 the casehardening has been eliminated 
by longer steaming and the resawed strips are straight. No. 7, which 
has been steamed still longer, shows a condition of "reverse case- 
hardening," in which the resawed strips curve outward. 
Sections cut as shown in Plate I may be used also to determine the 
distribution of moisture in lumber, whether casehardened or not. 
If not casehardened, such sections will curve inward as they dry 
if the lumber is wetter on the inside than on the surface, and outward 
if the reverse is the case. If the lumber is uniformly dry, the prongs 
will remain practically straight. 
HONEYCOMBING. 
| Honeycombing or internal checking occurs in casehardened 
pieces when the interior continues to dry and the surface remains 
fixed. In such cases splits appear in the interior. Plate II, figure 1, 
shows examples of honeycombing in casehardened pieces. 
1 For further discussion see " Problems in Kiln-Drying Lumber/' by H. D. Tiemann, Lumber World 
Review, Sept. 25, 1915, 
