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KILN DRYING HANDBOOK 43 
sible with this method, by drying to a relatively low moisture con- 
tent and then bulk piling the stock for storage in a suitable space, 
preferably one that can be heated to about 25° F’. above the outside 
temperature. The extent to which these stresses will die out dur- 
ing storage varies greatly among different species. As a rule the 
softwoods react very favorably; but many of the hardwoods, such as 
the oaks, for instance, are quite resistant, and severe drying stresses 
consequently are likely to remain almost indefinitely. On the other 
hand, it undoubtedly is true that wood exposed during service to 
widely varying atmospheric conditions will normally tend to relieve 
itself of casehardening stresses. The relief of such stresses during 
storage and in use depends upon the pick-up of moisture and the 
resultant compression set. Without this pick-up and set, relief can 
not be obtained. (See also “ Storage of kiln-dried stock,” p. 68.) 
KILN DRYING TO KILL FUNGI AND WOOD BORERS 
The kiln operator is frequently confronted with the necessity of 
handling stock showing evidences of decay, mold, or stain, or of 
the action of borers. Under ordinary drying conditions in the kiln, 
some borers will be killed, and the growth of decay, molds, and 
stains of fungous origin will be arrested. When drying is carried 
on at low temperatures and high humidities, however, the conditions 
are favorable to the growth of many of these parasites, and they 
may at times cause trouble in the kiln. The growth of mold on 
semigreen stock during the early stages of drying is not uncommon. 
About 180° F. is required to kill many of the borers, such as the 
Lyctus powder-post beetle, that infest wood, although considerably 
lower temperatures will suffice for some. When wood infested by 
heat-endurant borers has not been subjected to a temperature of 
180° F. or higher during the drying process, the kiln temperature 
should be raised to 180° F. at the end of the run and so held for a half 
hour or longer, the exact time depending upon the thickness of the 
stock. If the moisture content of the wood does not exceed 12 per 
cent and if the relative humidity during the heating period is con- 
trolled so as to prevent any visible damage, it is improbable that 
subjecting the stock to 180° F. for two or three hours will injure the 
strength of the wood. 
The steaming of sap gum before air drying is discussed on 
page 66. 
i DRYING SCHEDULES 
A drying schedule is a set of brief directions for the operation 
of the kiln during the drying period. Such schedules are usually 
presented in the form of curves or of tables showing the temperatures 
and the humidities to be used at various stages of the process, it 
being taken for granted that a kiln of suitable type, with ample and 
uniform circulation, is available; obviously, successful drying can 
not be accomplished if the kiln is incapable of doing the work 
required of it. The temperatures and the humidities in drying 
schedules are based upon either the length of time the stock has 
been in the kiln or the current moisture content of the stock. The 
