ECONOMIC WOODS OF THE UNITED STATES 



57 



tests made by Mr. Hugh P. Baker at the Yale Forest School. 

 The figures given represent the average shrinkage resulting 

 from reducing green wood to a kiln-dry condition and are com- 

 puted on the basis of the original measurements. 



TABLE IV 

 Shrinkage of Wood along Different Dimensions 



Irregularities in shrinkage tend to cause- wood to become 

 distorted or warped. In woods with straight grain and uniform 

 texture the tendency to warp is minimum unless the distribution 

 of the moisture content is very unequal. Thus the upper surface 

 of a green board exposed to the hot rays of the sun will dry much 

 more rapidly, and therefore becomes shorter than the lower side, 

 causing the board to curl up at the ends. Woods with interlaced 

 fibres or with cross or spiral grain {e.g., Nyssa, Liquidambar, 

 Eucalyptus) always shrink unequally, and consequently require 

 careful handling in drying to prevent serious deformation. Warp- 

 ing due to unequal distribution of moisture may subsequently 

 be overcome by thorough drying, but the deformation resulting 

 from great irregularity of structure is usually permanent. 



In Fig. 15 is shown in somewhat exaggerated manner the 

 deformation caused by the greater tangential shrinkage. The flat 

 side of a log cut through the middle becomes convex (A). Boards 

 cut from half of a log assume the form shown in (C), while a plank 

 from the middle of a log becomes convex on both sides. This 

 explains most of the difference in shrinkage of timbers and boards 

 of different sizes, shapes, and manner of sawing {i.e., whether 

 plain or quarter-sawed). 



When the strains due to unequal shrinkage can no longer 



