DEFECTS OF WOODS 



413 



of this hole make a sort of collar about the base of the dead 

 branch, and, as a new layer is added each year, they press 

 it more and more tightly. So strong does this compression 

 by the living wood become that at last what remains of the 

 dead tissue has so little strength that the branch is broken 

 off by a storm or even falls of its own weight. Then in a 

 short time the hole closes and after a while little or no exterior 

 trace of the knot remains. 



476. Shrinkage of Wood Checks. Water exists in wood 

 in two conditions: (1) as water absorbed in the cell walls, 

 and (2) as free water contained in the 

 cell cavities. When wood contains j ust 

 enough water to saturate the cell walls, 

 it is said to be at the "fiber satura- 

 tion point." Any water in excess of 

 this which the wood may contain is 

 in the form of free water in the cell 

 cavities. The removal of the free 

 water has no apparent effect upon the 

 properties of the wood except to re- 

 duce its weight, but as soon as any 

 of the absorbed water is removed the 



FIG. 194. Result of 

 Shrinkage. 



wood begins to shrink. Shrinkage (Fig. 194) is due to the 

 contraction of the cell walls, and sets up stresses which tend 

 to cause the wood to " check" or crack. " Check" is a term 

 used to denote cracks extending radially and following the 

 pith rays ; lumber splits lengthwise only very slightly. 



Since the free water is the first to be removed, shrinkage 

 does not begin, as a general rule, until the fiber saturation 

 point is reached, though in the case of some of the oak woods, 

 shrinkage begins above this point. For most woods, the 



