58 



ECONOMIC WOODS OF THE UNITED STATES 



be accommodated by the plasticity of the wood substance, cracks 

 or checks are formed. These are most common along the rays, 

 since there the strains are greatest and most complex. However, 

 when the strength of the rays is greater than the cohesive force 

 <of the cementing substance uniting the two layers of the primary 

 -cell wall, radial fracture passes through the median plane of the 

 primary wall of the wood cells instead of along the ray. 



Variation in moisture content due to irregular drying results 

 in checks, most of which are temporary, and as equilibrium 

 becomes again established gradually close and become imper- 

 ceptible. The more rapidly wood is dried, the greater is the 

 tendency to check, for even if evaporation could be controlled 

 so as to proceed uniformly throughout the specimen, the cells 

 would not be given sufficient time to adjust themselves to the 



Fig. 15. — Effects of shrinkage. A, plank cut from middle of log (boxed 

 heart), showing double-convex surfaces and large season check through upper 

 half. B, log cut in half, showing the flat 'surface becoming convex and the appear- 

 ance of three large season checks. C, half of a log cut into boards showing warping. 



changed conditions. The presence of checks in wood, no matter 

 how imperceptible, always impairs the strength of the material. 



If the outer portion of a piece of wood, especially hard wood, 

 dries much more rapidly than the inner, a hard shell is formed 

 on the outside, while the interior retains most of its original 

 moisture. This condition is known as case-hardening. This dry 

 shell resists the transpiration of the moisture from the interior and 

 retards drying, besides increasing the strains on the fibres. When 

 the interior finally dries, the internal strains frequently become 

 so great that large checks open up, producing a honeycombed 

 condition. 



Checks which result from greater shrinkage along the tangent 

 than along the radius are permanent and increase in size as drying 

 progresses (Figs. 1 ; 15 B). They cause serious difficulty in season- 

 ing large timbers and especially material in the round, such as 



