THE SEASONING OF WOOD. 
27 
culation of the air passing over the lumber should give satisfactory 
results. If checking begins during the drying process, the humidity 
should be increased until it stops. Steam jets in a kiln are often useful 
for this purpose. In changing the humidity the circulation should 
not be reduced. A large body of 
moving air is necessary in order 
to keep a uniform temperature 
clear through to the center of 
each piece of wood in the pile and 
at the same time supply the heat 
required for evaporation. If suf- 
ficient circulation is not secured, 
the supply of heat for both pur- 
poses will be lacking and the 
material will not dry uniformly. 
Figure 18 shows the conditions 
in a kiln during a run with refer- 
ence to temperature, humidity, 
and moisture in the wood. It 
will be noted that the humidity 
is kept high at first and lowered 
gradually. The temperature is 
held at a certain level for some 
time and then raised. The mois- 
ture is lowered gradually to a 
final condition of less than 5 per 
cent. 
The maximum rate of drying 
at a given temperature is reached 
when moisture is evaporated 
from the surface of the wood just 
as fast as it is transmitted from 
the interior. This rate is fixed 
by the rate of transmission of 
moisture within the wood and 
varies with different woods. 
The temperature of drying ap- 
parently influences the rate of 
transmission of moisture within 
the wood. The higher the temperature of the wood the more rapid is 
the rate of transmission of the moisture, and hence the rate at which 
the moisture may be evaporated safely. This, of course, applies only 
to temperatures below those which might result in injury to the wood. 
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