414 APPLIED SCIENCE 



fiber saturation point corresponds with a moisture content 

 of from 25 to 30% of the dry weight of the wood.' 



When lumber is kiln-dried too soon, it becomes case-hard- 

 ened; that is, its outside becomes hard before the sap from 

 the center can evaporate (Fig. 195). During the process of 



FIG. 195. Result of Case-Hardening. 



drying, the lumber shrinks across the width of the board 

 and also in thickness, but rarely to any extent in its length. 

 The sap naturally escapes most readily from the ends of the 

 lumber. These ends often become quite dry while the 

 center of the plank still contains a great deal of moisture. 

 The ends of the plank then tend to become narrower than 

 the center and in consequence split or "check." 



477. Warping. Warping is another serious delect which 

 occurs in wood, and must be very carefully guarded against. 



