26 BULLETIN 552, IT. S. DEPARTMENT OF AGRICULTURE. 
during partial air drying. Other effects, also, are produced, which to 
a greater or less extent change the properties of the wood. The 
"sap "'in the wood is changed by " cooking," as indicated by a darken- 
ing of the wood, the degree of coloring depending upon the tempera- 
ture and duration of the process. Other changes of a chemical 
nature apparently also take place in the wood during steaming. 
The pressure and duration of steaming desirable in kiln-drying 
have not yet been thoroughly worked out. Durations of from 5 
minutes to 24 hours or longer, and pressures ranging from atmospheric 
to 50 pounds gauge, have been used in practice. The higher the 
pressure the greater is the effect produced, and the longer the time 
the more thoroughly the treatment penetrates the wood. Experi- 
ments have shown that a pressure slightly above atmospheric for 24 
hours will slightly darken 2-inch maple clear through, and a pressure 
of 40 pounds will turn oak and probably other hardwoods almost 
black. Even where the strength of the wood is not the primary 
consideration, it probably is not safe to exceed 15 pounds gauge 
pressure (250° F.), except for special purposes. 
THE PROCESS OF DRYING. 
After the wood has been heated thoroughly in a humid atmos- 
phere, either in the kiln proper or in a separate compartment, it is 
ready to have the moisture removed by evaporation from the sur- 
face. In kiln-drying uniform circulation apparently is the most 
important thing to be secured. The fact that air when it enters the 
drying chamber will be cooled, and therefore will tend to fall, should 
govern the method of piling and the direction of circulation. 1 This 
means that the air should be allowed and assisted to pass downward 
through the pile, either by entering at the top of the pile or by an 
adaptation of this principle to other methods of piling. The rate of 
evaporation may be controlled best by regulating the amount of 
moisture in the air (relative humidity) circulating about the lumber 
in the kiln; it should not be controlled by reducing the air circula- 
tion, since a large circulation is needed at all times to supply the 
necessary heat. Air at 100 per cent relative humidity contains all 
the water it can carry and has no effect in drying wood. If, how- 
ever, the humidity is reduced to 90 per cent and the air then passed 
through a pile of wet lumber, the air can take up a certain amount 
of moisture. If drying does not progress rapidly enough with the cir- 
culating air at 90 per cent humidity, it may be reduced still further. 
This may be accomplished by ventilation, by condensers, by water 
or steam sprays, or in a number of other ways. Any well-made kiln 
which will allow the control of the humidity, temperature, and cir- 
i See "The Circulation in Dry Kilns," by H. D. Tiemann, Lumber World Review, May 10 and June 10, 
1916. 
