288 TIMBERS AND THEIE USES 



water from the inside, the shrinkage, so far as 

 the degree of moisture affected it, would be 

 uniform. In the best type of drying kilns this 

 condition is approximated by first heating the 

 wood thoroughly in a moist atmosphere before 

 allowing drying to begin. 



" In air seasoning and in ordinary kilns this 

 condition is not so often attained, and the result 

 is that a dry shell is formed which encloses a 

 moist interior. Subsequent drying out of the 

 inner portion is rendered more difficult by this 

 ' case-hardened ' condition. As the outer part 

 dries it is prevented from shrinking by the wet 

 interior, which is still at its greatest volume. 

 This outer portion must either check open or 

 the fibres become strained in tension. If this 

 outer shell dries while the fibres are thus strained 

 they become set in this condition and are no 

 longer in tension. Later, when the inner part 

 dries, it tends to shrink away from the hardened 

 outer shell, so that the inner fibres are now 

 strained in tension and the outer fibres are in 

 compression. If the stress exceeds the cohesion, 

 numerous cracks open up, producing a ' honey- 

 combed ' condition, or ' hollow horning,' as it 

 is called. If such a case-hardened stick of wood 

 be resawed, the two halves will cup from the 

 internal tension and external compression, with 

 the concave surface inward. 



