KILN DRYING HANDBOOK. 45 
always enough, however, since there may be a vast difference as to 
seasoning between two lots of stock dried down to the same moisture. 
The specification should include a clause concerning the presence 
of drying stresses, based upon the use of stress sections. To be com- 
plete and accurate such a clause would be quite lengthy and cumber- 
some, and therefore more or less impractical. However, a simple 
statement that the wood shall be free from injurious drying stresses, 
while very broad, affords reasonable protection to the purchaser. 
STORAGE OF KILN-DRIED STOCK. 
Whenever possible, the stock should be cooled before it is removed 
from the kiln, since exposure of the hot stock to the cool air is liable 
to cause checking. All of the boards in the kiln are not of the same 
moisture content at the end of the drying period. It is therefore 
necessary that they be held in storage until both dry and moist 
boards have the same moisture content. The required time for storage 
varies with conditions. Where little accuracy is needed, as with soft- 
wood, the stock need be stored but a short time. One week is con- 
sidered long enough for furniture stock, and two weeks are specified 
for aircraft stock. Careful conditioning in the kiln reduces the re- 
quired time of storage. 
Dimension stock and finished wood products which have to be 
stored should be held in the proper atmospheric conditions, or they 
will absorb or lose too much moisture. Later, when the stock is 
manufactured and put into actual use, this loss or gain, may damage 
its serviceability. Stock taken from damp, unheated storerooms into 
heated shops is too moist for the best utility. The moisture is un- 
evenly distributed not only in the individual pieces of stock but in 
the entire pile. The boards on the sides and top have a different 
moisture content from those in the pile. Products made from such 
stock may be end-checked or distorted. Short stock with large end 
surfaces warps when stored in a damp atmosphere. Such stock used 
in chair seats of the common saddle style, if dried too rapidly, shows 
end checks and open glue joints. 
KILN TYPES. 
• 
Dry kilns for wood may be grouped in two general classes, com- 
monly known as progressive and. compartment. Progressive kilns are 
sometimes called " continuous " kilns, and compartment kilns are 
known as " box " or " charge " kilns. The differences between the two 
types depend on the method of handling the stock through the kiln. 
In the progressive kiln (Fig. 6), the stock enters at one end and moves 
progressively through to the other end, emerging, presumably dry, 
at the proper time. The stock is fed in and removed periodically, 
and the process is continuous. In the compartment kiln the entire 
kiln is loaded at one time, and the charge remains in place throughout 
the drying period. In the progressive kiln the temperature and 
humidity at any point remain constant, but the kiln is hotter and 
drier at the discharge end than at the receiving end ; in the compart- 
ment kiln the temperature and humidity are as nearly uniform as 
possible throughout the kiln at any given time, and are changed from 
time to time as the stock dries. 
