2 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 
contusion. Next are described the various discolorations in airplane 
woods caused by mechanical injuries to the living trees, chemical 
reactions, harmless fungi, and decay-inducing fungi in alate to 
their actual effect on the streneth of wood. In the case of those 
defects and properties which it is not within the province of this 
bulletin to discuss in detail, references to available literature are 
given. 
GENERAL CONSIDERATIONS. 
There are certain basic principles in the manufacture of high- 
gerade lumber which should be most rigidly adhered to in the case of 
stock for airplanes. The purchaser should be certain that the manu- 
facturer supplying his requirements is both willing and able to fulfill 
these conditions, so that defects very difficult to detect are not intro- 
duced. 
When trees are felled the logs should be removed from the woods 
with reasonable promptness, because as soon as the timber is down 
it becomes subject to decay, sap-stain, checking, and the attacks of 
wood-boring insects. Leaving logs in the woods over winter is par- 
ticularly poor practice. If the logs must be stored for any consider- 
able length of time they should be kept in the pond, where the defects 
mentioned will be largely prevented. 
After the logs are sawed the lumber should be carefully inspected 
and those pieces unsuitable for use in airplanes diverted to other 
uses. Next comes seasoning. Drying with artificial heat in dry kilns 
is preferable. The kilns should be of proper construction, so that the 
temperature and relative humidity can be completely controlled and 
the lumber brought to an average final moisture content of about 8 
per cent, within “the limits of 5 to 10 per cent (based on oven-dry 
weight), without checking or other injury. If it is necessary to store 
the dry lumber at the mill it should be placed in a dry shed. com- 
pletely protected from the weather. The shed should have a board 
floor. Concrete, particularly if new, or dirt floors may give off con- 
siderable moisture. The stock should be shipped in box cars com- 
pletely protected from moisture. When it reaches the factory the 
lumber should be shop seasoned; that is, placed in a room under 
uniform shop conditions, for about two weeks. During the entire 
process of manufacture the stock should be carefully protected from 
the absorption of moisture. Piling lumber or partly fabricated parts 
on damp floors or under the drip from steam or water pipes are two 
not uncommon offenses. 
In case it is impossible to kiln-dry the stock, air drying must be 
resorted to. As a rule it is not possible to get the moisture content 
below 11 per cent by this process, except in arid regions. When 
the lumber comes from the saw it may be necessary to dip it in a 
chemical solution to prevent sap-stain in regions where lumber is 
especially subject to this discoloration; but “under any conditions 
the stock should be carefully open-piled on elevated foundations to 
assure a circulation of air throughout and only sound, bright, 
thoroughly seasoned stickers used between courses. The piles should 
be properly slanted and roofed, so that rain will run off and not soak 
the lumber. To pile lumber closely, without proper circulation of air 
throughout the piles, results in some cases in warping, sap-stain, - 
