

THE AIR SEASONING OF WESTERN SOFTWOOD LUMBER 9 
CAUSES AND CONTROL OF STOCK DEPRECIATION 
The defects in lumber resulting from air seasoning are very 
definitely related to methods used in the drying process. An under- 
standing of their causes will permit a better appreciation of the 
possibility and means of prevention. These defects may be grouped 
as those due to shrinkage and those due to fungi. Shrinkage de- 
fects include check, cup, warp-bow-twist, and loosening of knots. 
Defects caused by fungi include stains and decay. 
SEASON CHECKS 
Lumber checks as the result of uneven shrinkage. This, in turn, 
may be due to one or both of two causes, uneven drying or the in- 
herent difference between radial and tangential shrinkage in wood. 
Uneven drying is due commonly to the end grain of wood giving 
off moisture more rapidly than the side grain, to surface layers dry- 
ing faster than those on the interior, or to the fully exposed portions 
of the board drying before adjacent sections not so exposed are able 
to lose their moisture. 
Tangential shrinkage, or shrinkage in the direction of the rings, 
is on an average about twice as great as radial shrinkage or shrinkage 
across the rings. Table 4 gives this differential in shrinkage for 
each of the commercial western species. 
TABLE 4.—Radial, tangential, and volume shrinkage* of different woods from 
the green to the oven-dry condition, in percentage of green size? 
Shrinkage Ratio Shrinkage Ratio 
ea of of 
1 tan- tan- 
Western species Tan- tot 5 Western species Pine ean * 
sok Radial] gen- | radial pe Radial] gen- | radial 
tial | shrink- tial |shrink- 
age age 
Per Per Per Per Per Per 
cent cent cent cent cent cent 
Incense cedar______-- 7.6 33 Bei HOI AAWan ey The eee See 10. 2 3.4 7.0 2. 06 
Port Orford cedar____| 10.7 5. 2 8.1 1.56 || Western hemlock____} 11.6 4.5 7.9 1.76 
Western red cedar -__- 8.1 74515 Orel! 2.04 || Western larch_______ 116}, 4.2 8.1 1. 93 
Douglas fir, Rocky Lodgepole pine_____- Ti) 4.5 6.7 1.49 
Mountain_________ 10.6 3.6 6.2 R77 Ni shureeehe poyboYe ee 8.4 2.9 5.6 1. 93 
Douglas fir, Pacific Western white pine_| 11.5 4.1 7.4 1.80 
LOOSE Esta AEG. 12.6 5.0 7.9 1.58 || Western yellow pine_} 10.0 3.9 6. 4 1. 64 
Siliverifire 22-22 - 2k 14.1 4.5 10.0 DL ODIN URANO ROO ee 6.3 all 4.2 15 
Lowland white fir___| 10.6 3.2 7.2 2:25 || Sitka spruce_________ 11.2 4.5 7.4 1. 64 
IVObD emir es sai ek 13.6 4.8 9.1 1.90 || Engelmann spruce_-_-_| 10.4 3.4 6.6 1. 94 

1 Radial shrinkage is at right angle to the annual growth rings, tangential shrinkage is in the direction of 
the growth rings. 
2 Data by the Forest Products Laboratory, Forest Service. 
The end checking of lumber during air seasoning is very largely 
due to the uneven shrinkage which results from the exposed ends 
drying more rapidly than the adjacent portion. Not only does the 
end grain normally give off its moisture more rapidly than the side 
grain, but the side grain at or near the end of the board is covered 
top and bottom by the crosser and so is not exposed to the air. It 
follows that, to minimize end checking, ends of stock should be 
109391°—283——2 
