SHRINKAGE AND CHECKS. 



33 



x 



shares this diminution of size, the distances, a b and c r7, each becoming 

 shorter. Where the cells are very similar in size and in the thickness 

 of their walls, as in the case of piece A, fig. 20, a b and c d become 

 shorter by about the same amount; but if the piece is made up of 

 fibers, some of which have thin and others thick walls, as piece i>, fig. 

 20, then the row of thick-walled cells shrinking much more than the 

 row of thin- walled cells, the piece becomes unevenly shrunk or warped 

 as shown in fig. 20, G. Not only is the piece warped, but the force 

 which led to this warping continues to strain the interior parts of the 

 piece in different directions. 



Since in all our woods cells with thick walls and cells with thin walls 

 are more or less intermixed, and especially as the spring wood and 

 summer wood nearly always differ from each 

 other in this respect, strains and tendencies 

 to warp are always active when wood dries 

 out, because the summer wood shrinks more 

 than the spring wood, heavier wood in gen- 

 eral more than light wood of the same kind. 



If the piece A, fig. 20, after drying, is 

 placed edgewise on moist blotting paper, 

 the cells on the underside, at c d, take up 

 moisture from the paper and swell before 

 the upper cells at a b receive any moisture. 

 This causes the underside of the piece to be- 

 come longer than the upper side and, as in 

 the case of piece G, warping occurs. Soon, 

 however, the moisture penetrates to all the 

 cells and the piece straightens out. A thin 

 board behaves exactly like this minute piece, 

 only the process is slower and more easily ob- 

 served. But while a thin board of pine curves 

 laterally, it remains quite straight length- 

 wise, since in this direction both shrinkage 

 and swelling are small. A thin disk or cross 

 section swells, and when moistened on one side warps as readily in one 

 direction as in another. If a green board is exposed to the sun with 

 one side, warping is produced by removal of water and consequent 

 shrinkage of the upper side, and the course of the process is simply 

 reversed. 



As already stated, wood loses water faster from the end than from 

 the longitudinal faces. Hence the ends shrink at a different rate from 

 the interior parts. 



In a timber, the width A B (fig. 21, X) may have shortened (fig. 21, 



Y), while a short distance from the end c d, the original width is still 



preserved. This should produce a bending of the parts toward the 



center of the piece as shown in exaggeration at Y, but the rigidity of 



3521— No. 10 8 



Fig. 21.— Formation of checks. 



