12 BULLETIN 552, TJ. S. DEPARTMENT OP AGRICULTURE. 



WARPING. 



Warping or twisting in lumber is due to unequal shrinkage. Some 

 woods are much more subject to warping than others. The trouble 

 can be prevented to some extent by careful piling, both during 

 drying and afterward. Plate II, figure 2, shows badly warped 

 pieces of lumber. 



COLLAPSE. 



In some woods, notably western red cedar and redwood, when 

 the very wet wood is dried at a high temperature, depressions appear 

 on the surface of the boards, presumably due to the collapse of the 

 plastic cell walls in certain places. If, however, the woods in ques- 

 tion are heated above the boiling point while wet, the steam generated 

 in the nonporous cells causes the wood to bulge on the surface. 

 Plate III shows collapse and bulging, or " explosion," as it is termed 

 by the discoverer of the phenomenon. 1 



AIR-SEASONING. 



Though the use of dry kilns is increasing steadily, the bulk of our 

 wood is still seasoned in the open air. If kept in the air long enough, 

 the moisture content of the wood finally comes into equilibrium with 

 that of the surrounding atmosphere, and the wood is said to be air- 

 dried. The rate of drying varies, of course, with time of year, 

 species of wood, size and form of piece, and method of piling. Certain 

 of these factors may be controlled or utilized in a way to hasten the 

 drying process and lessen the likelihood of defects appearing in the 

 material. 



CROSSTIES, POLES, AND SAWED TIMBERS. 



The data in figures 2 to 12, inclusive, collected by the For- 

 est Service 2 in various parts of the country, show the rate at 

 which crossties, poles, and sawed timbers of several species lose mois- 

 ture when freely exposed to the atmosphere. In some cases it was 

 not possible to weigh the pieces for several days after they were cut. 

 Freshly cut timber loses weight very rapidly in warm dry weather. 

 Ties in some species lose 10 pounds in 24 hours. The rates of season- 

 ing of the various species may be compared by the general trend of 

 the curves. When the curves reach a horizontal position, the ma- 

 terial may be said to be air-dry, unless this happens at a time of 

 year very unfavorable for seasoning. 



The ties were seasoned in piles of 50 each, and were exposed with- 

 out cover. The ties on the top of each pile, however, were placed 

 close together and served as a rough roof. The curves are platted 



1 H. D. Tiemann, in charge Section of Timber Physics, Forest Products Laboratory of Forest Service. 



2 See "The Air Seasoning of Timber," by W. H. Kempfer, Forest Service, in Bui. 161 of the American 

 Railway Engineering Association. 



