26 
BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
tree. Thus the knot shrinks away from the wood lengthwise of the 
board, but does not shrink appreciably in the radial direction. End 
checking, which is caused by the very rapid drying from the end 
surfaces, is discussed more fully under " Drying schedules." 
v COLLAPSE. 
One form of seasoning defect which occurs in the green wood 
is the actual collapse of rows of cells, just as a rubber tire collapses 
when the air is let out. This defect occurs only in a few species, 
such as redwood, western red cedar, swamp oak, and red gum. The 
remedy consists in the use of low temperatures at the beginning of 
the kiln run. 
Fig'. 4. — Development of a surface cheek into a honeycomb. 1, 2, and 3 show the cheek 
gradually closing up as the piece dries and shrinks. 4, 5, and 6 indicate how the 
tensile stresses deepen the bottom of the honeycomb as the casehardening becomes 
more severe. The depression along the center of the top, in 5 and 6, is typical of 
honeycomb. 
STRESS DETECTION. 
The detection and relief of the shrinkage stresses causing case- 
hardening, checking, and honeycombing is one of the most im- 
portant of the kiln operator's duties, and one which requires skill 
and close application. 
The usual method of detecting the presence of these stresses, com- 
monly called casehardening stresses, is to cut a stress section from 
an average board. This stress section should be cut at least 2 feet 
from the end of the board, and should be about 1 inch long in the 
direction of the grain. It is then slotted as shown in Figure 5, the 
number of slots depending upon the thickness of the board and upon 
the preference of the individual operator. Often it is desirable to 
