8 BULLETIN 8&4, U. S. DEPARTMENT OF AGRICULTURE. 
the open flat piling, while taking advantage of the downward cir- 
culation. 
For slant piling the pitch should not exceed 1 inch in 5 inches, on 
account of the tendency of the lumber to slide on the supports. 
Lumber or plank may be laid edge to edge, but heavy dimension 
stock should be piled open, as illustrated in fig. 2. The piles should 
not be over 5 feet wide and 9 feet high. 
PRINCIPLES OF KILN DRYING. 
The successful kiln drying of refractory woods is absolutely 
dependent upon the ability of the operator to control and maintain 
the three essential drying conditions — temperature, humidity, and 
circulation. Heat is required for evaporation of the moisture; a 
circulation of air is necessary to carry the heat from the heating 
coils to the lumber and to carry away the moisture evaporated from 
the wood; and the relative humidity controls the rate of drying from 
the surface of the lumber. 
TEMPERATURE. 
There are two definite reasons for accurately controlling the tem- 
perature: (1) Many species of wood have very exacting temperature 
requirements for successful drying, especially while green; and a 
relatively slight departure from the desired temperature may result 
in damaged stock. As the drying progresses, however, relatively 
higher temperatures can be used without causing damage, and usually 
fluctuations in the temperature at this time may occur without 
serious damage. (2) Any increase or decrease in temperature not 
under the control of the operator, or a difference in temperature at 
opposite ends of the kiln or in different parts of the pile, will affect 
the relative humidity, and the drying will not be under control. 
HUMIDITY. 
In the process of drying the surface of a piece of wood becomes 
drier than the interior, owing to the resistance of the wood substance 
to the transfusion of moisture. If the drying occurs in hot, dry air 
the moisture difference between the surface and center portions of a 
given piece becomes very great. The surface tends to shrink more 
than the center, with the result that the stresses which develop cause 
warping and surface checking in the early stages of the run and honey- 
combing and permanent casehardening during the final period of 
drying. To control the drying, therefore, the operator must be 
able to maintain any humidity desired, independent of the tem- 
perature. 
CIRCULATION. 
Evaporation requires heat. . Air is the medium used for conveying 
this heat to all parts of the lumber pile and for carrying away the 
