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BULLETIX 1136, U. S. DEPARTMENT OF AGRICULTURE. 



tree. Thus the knot shrinks away from the wood lengthwise of the 

 board, but docs 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." 



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 check into a honeycomb. 1, 2. nnd 3 show the check 

 gradually closing up as the piece dries and shrinks. 4, 5, and indicate how the 

 tensile stresses deepen the bottom of the honeycomb as the casehardening becomes 

 more severe. The depression along th<- center of the top, in 5 and ft. 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 w T hich 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 



