36 TIMBER. 



adjoining cells and are greatest at the pith rays. These strains cause 

 warping and checking, but exist even where no outward signs are visi- 

 ble; they are greater if the wood is dried rapidly than if dried slowly, 

 but can never be entirely avoided. 



Temporary checks are caused by the more rapid drying of the outer 

 parts of any stick; permanent checks are due to the greater shrink- 

 age, tangentially, along the rings than that along the radius. This, 

 too, is the cause of most of the ordinary phenomena of shrinkage, such 

 as the difference in behavior of entire and quartered logs "bastard" 

 (tangent) and "rift" (radial) boards, etc., and explains many of the 

 phenomena erroneously attributed to the influence of bark, or of the 

 greater shrinkage of outer and inner parts of any log. 



Once dry, wood may be swelled again to its original size by soaking 

 in water, boiling, or steaming. Soaked pieces, on drying, shrink again 

 as before; boiled and steamed pieces do the same, but to a slightly less 

 degree. Neither hygroscopicity, i. e., the capacity of taking up water, 

 nor shrinkage of wood can be overcome by drying at temperatures below 

 200° F. Higher temperatures, however, reduce these qualities, but 



nothing short of a coaling heat robs wood 

 of the capacity to shrink and swell. Eapidly 

 dried in the kiln, the wood of oak and other 

 hard woods " case-harden," that is, the outer 

 part dries and shrinks before the interior 



f.< ; . 24.- Honeycombed- board, has a chance to do the same, and thus forms 

 The checks or cracks form along a firm shell or case of shrunken, commonly 

 t epi rays. checked wood around the interior. This shell 



does not prevent the interior from drying, but when this drying occurs, 

 the interior is commonly checked along the medullary rays, as shown 

 in fig. 24. In practice this occurrence can be prevented by steaming the 

 lumber in the kiln, and still better by drying the wood in the open air 

 or in a shed before placing in the kiln. Since only the first shrinking 

 is apt to check the wood, any kind of lumber which has once been air 

 dried (three to six months for 1 -inch stuff) may be subjected to kiln heat 

 without any danger. Kept in a bent or warped condition during the 

 first shrinking, the wood retains the shape to which it was bent and 

 firmly opposes any attempt at subsequent straightening. 



Sapwood, as a rule, shrinks more than heartwood of the same weight, 

 but very heavy heartwood may shrink more than lighter sapwood. 

 The amount of water in wood is no criterion of its shrinkage, since in 

 wet wood most of the water is held in the cavities, where it has no 

 effect oil the volume. 



The wood of pine, spruce, cypress, etc., with its very regular struc- 

 ture, dries and shrinks evenly and suffers much less in seasoning than 

 the wood of broad-leafed trees. Among the latter, oak is the most 

 difficult to dry without injury. Small-sized split ware and "rift" 

 boards season better than ordinary boards and planks. 



