KILN DRYING HANDBOOK 47 
USE OF THE DRYING SCHEDULES PRESENTED HERE 
Drying schedules must meet the conditions of actual service, and 
since these conditions are quite variable it 1s impossible to set up a 
single series of exact schedules that will have universal application. 
The condition of the stock as the result of previous seasoning deter- 
mines to a large degree the preliminary steaming treatment and the 
initial stages of the drying proper for 1t, and the purpose for which 
the dry stock is to be used largely governs the severity of the schedule 
and the final treatment for stress relief. In addition, the kind of kiln 
has an important bearing upon the suitability of a schedule and upon 
the manner of its application. For instance, if it 1s desired to dry 
green hardwoods in a kiln having sluggish circulation, the use of 
high humidities at the beginning is impractical, and it becomes 
necessary to start the drying at a somewhat lower temperature, with 
lower humidity, than would otherwise be needed. Again, in drying 
softwoods, such as pine, in a natural-circulation progressive kiln, a 
typical high-humidity schedule will fail to give satisfactory results 
because it will not induce circulation adequate to carry the heat to 
the lumber and to remove the evaporated moisture. ‘To secure good 
drying in such circumstances requires a lower humidity at the green 
end and a higher temperature at the dry end of the kiln. Under 
ordinary commercial conditions the humidity at the dry end can be 
comparatively low since the lumber will not be exposed to it long. 
enough to be damaged. 
In most progressive kilns and in many softwood compartment 
kilns operating at high temperatures the use of kiln samples is more 
or less impractical, and it become necessary to operate the kilns on a 
time basis. When stock of the same general character is being dried 
right along, it is possible to secure very satisfactory results in this 
way. 
CHARACTER OF THE SCHEDULES 
Most of the drying schedules presented on the following pages are 
intended for use with kiin samples, the indicated changes in tem- 
perature and in humidity being made as the moisture content of the 
samples passes the various stages. All of these schedules are safe. 
It is possible to obtain good results with faster drying, but the use of 
schedules more severe than those recommended will require most 
careful judgment on the part of the kiln operator. 
The schedules of widest application are the hardwood, which were 
originally intended for furniture stock, and the softwood, which 
provide for drying at temperatures higher than those of the hardwood 
schedules. ‘These two series, which supplement each other, are 
numbered consecutively; No. 000 of the softwood schedules is the 
most severe, and No. 8 of the hardwood schedules is the mildest. 
Preliminary steaming is recommended for green stock, not to re- 
lieve stresses but to warm the stock thoroughly before the drying 
operation begins. It is not necessary to steam green stock so long as 
partly seasoned stock, one hour per inch of thickness being sufficient. 
The temperature during steaming may be from 10° to 15° F. above 
