10 DEPARTMENT BULLETIN 1425, U. S. DEPT. OF AGRICULTURE 
shaded to decrease the rate of drying, and the area covered by the 
crosser should be reduced as much as practicable. 
Season checks appearing on the faces of the stock result both 
from uneven drying and from the differential between tangential and 
radial shrinkage. With excessively rapid surface drying, the outer 
layers become much drier than those on the interior and tend to 
shrink before the inside portion is dry enough to do so. Stresses 
are thus set up in the piece which may cause checking either im- 
mediately or when the stock is run through the planer. Other stresses 
are set up when one section begins to shrink before an adjacent one 
is sufficiently dry to do so, and these result in checking in the drier © 
section. Crossers are often responsible for this type of check in the 
stock. In plain-sawed pieces the face of the board nearer the heart 
of the tree is more subject to tangential shrinkage than the other 
face, and although the resultant tendency to cup is met by the weight 
of the pile holding the boards flat, the stresses set up may result in 
checking. The prevention of excessively rapid drying tends to reduce 
any form of checking, and a decrease in the area covered by the 
crosser will also aid materially. 
CUP 
The cupping of lumber may be caused by one side drying and 
shrinking more rapidly than the other, as when stock is piled two 
layers to the course and the exposed faces are dried to a lower final 
moisture content than is the other face. It may also be due to one 
side shrinking more than the other even when uniformly dried, as 
in flat-sawed lumber; in flat or tangentially sawed lumber the side 
toward the center of the tree shrinks less, causing the lumber to cup 
away from the center. In general, cupping may be held to the 
minimum by the prevention of too rapid drying and by allowing both 
faces of the stock to dry evenly. 
WARP-BOW-TWIST 
Warp-bow-twist is usually the result of uneven shrinkage caused 
by structural differences. Spiral or interlocked grain is commonly 
responsible. Minor defects of this class may result from uneven 
drying, which also can aggravate those due to the wood structure. 
Preventive measures are confined to decreasing the rate of drying 
and to the use of piling methods which will hold the stock firmly in 
place and in proper alignment. 
LOOSENING OF KNOTS 
Knots are loosened during seasoning as a result of the drying 
out of the cementing resins and gums and of differences in the 
shrinkage of the knots and the surrounding wood. In a plain-sawed 
board, the axis of the knot being at right angles to that of the tree, 
the knot shrinks away from the wood lengthwise of the board but 
does not do so appreciably in the direction of the board width. As 
shrinkage in the thickness of the board is greater than that along the 
axis of the knot, many knots are loosened when stock is machined. 
The loosening of knots can not be entirely avoided by any method 
of seasoning, since a certain type of knot is not directly connected 
ee 
