﻿2 BULLETIN 1128, U. S. DEPARTMENT OP AGRICULTURE. 



confusion. Next arc described the various discolorations in airplane 

 woods caused by mechanical injuries to the living trees, chemical 

 reactions, harmless fungi, and decay-inducing fungi in relation to 

 their actual effect on the strength 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- 

 grade lumber which should be most rigidly adhered to in the case of 

 stock for airplanes. The purchaser should be certain that the manu- 

 facturer supphdng 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, 



