

THE AIR SEASONING OF WESTERN SOFTWOOD LUMBER 5 
sawed faces lose it more rapidly than do vertical-grain or quarter- 
sawed faces. 
For an understanding of the air-seasoning process, this general de- 
scription of the movement of moisture in wood is sufficient. It can be 
assumed that the moisture tends to distribute itself evenly through 
the wood, moving from the moist regions to the drier ones. The 
really important facts are that the temperature and humidity of the 
atmosphere at the surface of the wood are controlling factors, and 
that circulation of the air is of extreme importance in maintaining 
and modifying these. 
EFFECT OF HUMIDITY ON DRYING 
Wood possesses the property of giving off or taking on moisture 
from the surrounding atmosphere until the moisture in the wood 
comes to a balance with that in the air. The humidity or water 
vapor in the air is, therefore, very important in the drying of wood, 
and a general understanding of this relationship between humidity 
and moisture content of wood is essential. 
The weight of the water vapor contained in a cubic foot of air 
is the absolute humidity and is usually expressed in number of 
grains. This does not, however, indicate the drying capacity of the 
air, for the ability of air to hold water, or its saturation point, varies 
greatly with the temperature, as is illustrated by Table 2. This 
ability of air to dry wood, or any other substance, varies according 
to the additional moisture it can hold before becoming saturated. 
The vapor in the air expressed as a percentage of the saturation point 
for the same temperature is called the “ relative humidity ” and indi- 
cates the comparative drying capacity of air. The lower humidi- 
ties represent dry air and the higher ones moist air. As used in this 
bulletin, the term “humidity ” alone refers invariably to relative 
humidity. 
TABLE 2.—Cubic foot moisture capacity of air at different temperatures 
Temperature} Moisture 
STR, Grains 
20 1. 24 
40 2. 86 
60 5. 80 
80 11.10 
100 20. 00 
Marked changes in relative humidity are evident from season to 
season, and also the usual daily fluctuations must be taken into 
account. Because of its tendency to come to definite balance with 
the surrounding air wood is, under ordinary atmospheric conditions, 
practically always undergoing at least slight changes in moisture 
content. This same tendency accounts for the differences in final 
moisture content of thoroughly air-dry wood at different times of 
the year. The pick-up in moisture content of lumber left piled in 
the yard over winter is likewise explained. ‘Table 3 shows the ulti- 
mate moisture content of wood if kept under exact humidity and 
temperature conditions. 
