12 BULLETIN 552, U. S. DEPARTMENT OF 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 saw^ed timbers of several species lose mois- 
ture w T hen 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. II. Kempfer, Forest Service, in Bui. 161 of the American 
Railway Engineering Association. 
