50 Trees, Stars, and Birds 



This weakens the timber and lessens its value. Do 

 you think thick or thin pieces are more likely to check? 

 Pieces dried in the sun or in the shade ? 



U. 5. Forest Service 

 FIG. 29. Checking and warping due to shrinking of wood when dried. 



Warping is due to greater expansion or contraction of 

 the wood at one surface of a board than at the other. 

 A board lying on the damp ground absorbs water from 

 beneath, and the wood fibers of the lower part of the 

 board swell so that its width becomes greater on the 

 under surface than above. This causes the board to 

 warp. Which side becomes convex? What would be 

 the effect of turning the board over? Make the above 

 experiment with a thin piece of board laid on damp 

 earth. Some kinds of woods warp more than others. 

 Cottonwood boards have a special tendency to warp, 

 and a board of this kind would be a good one with 

 which to experiment. 



A board changes its length very little when it shrinks 

 or swells, and it shrinks or swells much less in the direc- 

 tion across the annual rings than parallel with them. 

 Will a door swell more sidewise if all the boards run the 

 long way of the door or if some of them run crosswise? 



