48 WOOD AND FOREST. 



the pith along the pith rays and widen outward; heart shakes, Fig. 

 41, I, splits crossing the central rings and widening toward the cen- 

 ter; and cup or ring shakes, Fig. 41, c, splits between the annual 

 rings. Honeycombing, Fig. 41, d, is splitting along the pith rays 

 and is due largely to case hardening. 



These are not all due to shrinkage in drying, but may occur in 

 the growing tree from various harmful causes. See p. 232. 



Wood that has once been dried may again be swelled to nearly if 

 not fully its original size, by being soaked in water or subjected to 

 wet steam. This fact is taken advantage of in wetting wooden 

 wedges to split some kinds of soft stone. The processes of shrinking 

 and swelling can be repeated indefinitely, and no temperature short 

 of burning, completely prevents wood from shrinking and swelling. 



Eapid drying of wood tends to "case harden" it, i. e., to dry and 

 shrink the outer part before the inside has had a chance to do the 

 same. This results in checking separately both the outside and the 

 inside, hence special precautions need to be taken in the seasoning 

 of wood to prevent this. When wood is once thoroly bent out of 

 shape in shrinking, it is very difficult to straighten it again. 



Woods vary considerably in the amounts of their shrinkage. The 

 conifers with their regular structure shrink less and shrink more 

 evenly than the broad-leaved woods. 3 Wood, even after it has been 

 well seasoned, is subject to frequent changes in volume due to the 

 varying amount of moisture in the atmosphere. This involves con- 

 stant care in handling it and wisdom in its use. These matters are 

 considered in Handwork in Wood, Chapter III, on the Seasoning of 

 Wood. 



3 The following table from Roth, p. 37, gives the approximate shrinkage 

 of a board, or set of boards, 100 inches wide, drying in the open air: 



Shrinkage 

 Inches. 



1. All light conifers (soft pine, spruce, cedar, cypress) , 3 



2. Heavy conifers (hard pine, tamarack, yew) honey locust, box elder, 



wood of old oaks ) 4 



3. Ash, elm, walnut, poplar, maple, beech, sycamore, cherry, black locust. 5 



4. Basswood, birch, chestnut, horse chestnut, blue beech, young locust. ... 6 



5. Hickory, young oak, especially red oak Up to 10 



The figures are the average of radial and tangential shrinkages. 



