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 

 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." 1 



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. 



!For further discussion see " Problems in Kiln-Drying Lumber," by H. D. Tiemann, Lumber World 

 Review, Sept. 2.5, 1915. 



