﻿10 BULLETIN 1128, l T . S. DEPARTMENT OF AGRICULTURE. 



Too high temperatures in the steam box will make wood brittle, 

 seriously weakening it. Steaming should be a< complished at atmos- 

 pheric pressure and for a period not to exceed six horn's. Higher 

 pressure means higher temperatures and weakened wood. Most 

 hardwoods are more or less discolored by this process, assuming a 

 dead-gray color, but this does not indicate injury. White oak may 

 change to a blackish brown. White ash becomes a dead-gray color, 

 on which a bluish gray discoloration may appear. Elm also takes 

 on this gray shade to some extent. The change in color is very much 

 less noticeable in the soft woods. 



Soft woods should not be steamed and bent, because they are very 

 susceptible to injury by this process. When tested, the bent portion 

 will be very weak and brash. A close examination will reveal numer- 

 ous slight compression failures on the inner curve of the bend. 

 Spruce is particularly subject to this type of injury. 



SEASONING. 



It is well established that a decrease in the moisture content of 

 wood after the fiber saturation point is reached results in marked 

 progressive increase in the strength of wood, accompanied by a de- 

 cided shrinkage (63) . The fiber saturation point is the condition at 

 which the cell walls are completely saturated, or, in other words, 

 have absorbed the maximum percentage of water which they can 

 hold, but the cell cavities are empty. For two reasons, then, to in- 

 crease the strength and to prevent subsequent shrinkage when the 

 pieces have been worked to size or even assembled, it is essential 

 that airplane timber be dried or, as it is commonly termed, seasoned. 

 This may be done by air drying, that is, natural seasoning in the 

 air, or by kiln drying, that is, seasoning with artificial heat (4, 12, 

 64,65,66,70). 



As a result of improper seasoning, particularly that which occurs 

 unevenly or too rapidly, checks, which are small longitudinal splits, 

 may occur in the wood. Almost invariably these are on the tangen- 

 tial face, since wood as a rule shrinks about twice as much in the 

 direction of the annual rings as it does radially or across them. The 

 longitudinal shrinkage (with the grain) is so slight that it usually 

 has no effect. Checks are decidedly weakening, but fortunately are 

 easy to recognize. 



Airplane wood is usually kiln dried, because the seasoning process 

 can be better controlled than in air drying; it is more rapid, a lower 

 moisture content can be attained, and there is less tendency for kiln- 

 dried wood to shrink and swell with subsequent changes in the humid- 

 ity of the air. Extensive tests have been made on the effect of arti- 

 ficial seasoning (73). Kiln drying when not properly done is a 

 source of serious injury. Temperatures that are too high or proper 

 temperatures that are combined with humidity that is too low may 

 markedly weaken a charge of lumber, particularly if these conditions 

 are maintained for some time. The detection of such injury, when 

 not severe, is very difficult. Hence, it is highly important that self- 

 recording instruments showing temperatures and relative humidities 

 at all times be properly installed in the kilns and that these be cali- 

 brated from time to time. In pronounced cases the lumber will 

 readily reveal its brittle nature when picked with a knife blade. 



