6 DEPARTMENT BULLETIN 1425, U. S. DEPT. OF AGRICULTURE 
The rate of drying depends largely upon the relative humidity. 
At a low humidity, evaporation is rapid; at a high humidity, it is 
slow. Relative humidity alone, as indicated, does not altogether de- 
termine the rate of drying. The temperature and the circulation of 
the air also influence the rate of evaporation. 
Taste 3.—Ultimate moisture content of wood at different temperatures and 
degrees of relative humidity * 
Moisture content at three 
Rela- temperatures 
tive 
humidity 
70° F. 140 °RR a 21200 Re 




Per cent | Per cent | Per cent | Per cent 
2 : ! 
0 4.5 3.3 2.2 
30 6.0 4.5 2.9 
40 bo 5.9 3.9 
50 9.3 Want 4.9 
60 11.2 8.8 6.2 
70 13.5 10.7 8.0 
80 17.0 14.0 10.5 
90 22. 2 18. 2 14.0 
100 32. 0 26. 2 21.0 
1 Prepared by the Forest Products Laboratory, Forest Service. 
EFFECT OF TEMPERATURE ON DRYING 
The temperature of the air surrounding wood affects drying in a 
number of ways. Heat is always consumed when evaporation takes 
place, and must be continuously supplied by the air, if evaporation is 
to be kept up. Also, as has been pointed out, an increase in tempera- 
ture of the air increases its capacity to hold moisture and thus 
hastens evaporation. Below the fiber-saturation point a greater de- 
gree of heat is required to separate water from wood, this require- 
ment increasing as the wood becomes drier. 
These effects of heat or the temperature of the air upon the drying 
process explain certain conditions encountered in air seasoning. For 
example, even during the coolest months of the year, loss of moisture 
is comparatively rapid until a moisture content of about 30 per cent 
is reached, which corresponds to the fibre-saturation point. Then 
an abrupt decrease in the drying rate takes place. 
EFFECT OF AIR CIRCULATION ON DRYING 
Air circulation plays a big part in the drying process. As wood 
dries and evaporation uses up heat and increases the amount of 
moisture in the surrounding air, circulation of air is required to sus- 
tain the supply of heat necessary for evaporation and to remove the 
evaporated moisture. Circulation is thus a real factor in the drying 
of wood by any method and is particularly important in air 
seasoning. 
CONDITIONS OUTSIDE THE PILE THAT INFLUENCE AIR SEASONING 
Since the air seasoning of lumber is dependent upon the tempera- 
ture, humidity, and circulation of the surrounding air, regional 
climatic conditions, modified as these are locally by elevation, topog- 
OE ~ 
