88 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
The hardwood schedules and the softwood schedules together cover 
almost the entire temperature range commonly used in kiln drying. 
They range from an initial temperature of 105° F. in hardwood 
schedule 8, Table 2, to a final temperature of 210° F. for vertical- 
grain flooring strips in softwood schedule 000, Table 4, While most 
drying can be reasonably well done by the use of the proper one of 
these 11 schedules, it has been found advantageous to develop special 
schedules for certain purposes. A number of these follow. 
COMMON GRADES OF DOUGLAS FIR. 
The kiln drying of Douglas fir common is a problem quite different 
in several respects from most seasoning problems. One of the most 
important considerations is to keep the knots from falling out, and 
another is the fact that it is not necessary to dry the stock lower 
than about 15 per cent. This latter makes it much easier to keep the 
knots from falling out, but brings the added complication that it is 
very difficult to dry a load of mixed heart and sap to a uniform 
moisture content as high as 15 per cent. To prevent excessive 
shrinkage of the knots, it is necessary to maintain a high humidity 
throughout the drying. The maximum temperature is more or less 
definitely limited because it is undesirable to use temperatures high 
enough to melt the resin from around the knots, The need for rea- 
sonable uniformity in moisture content at the end of the run and the 
use of high humidity make it necessary to have a very rapid and 
uniform circulation readily reversible in direction, and an accurate 
control of temperature and humidity. 
Douglas fir common schedules were developed in a semicommercial 
unit of the Forest Service internal-fan kiln, and this is the only 
type which can at present be safely recommended for this class of 
work. A constant temperature of 175° F. may be used throughout 
the entire drying period. In the case of 1 by 6, 1 by 8, 2 by 4, and 
2 by 6 inch stock the humidity may be kept constant at 70 per cent. 
For 1 by 10, 1 by 12, 2 by 8, 2 by 10, and 2 by 12 inch stock, it is 
better to use a humidity of 80 per cent for the first half of the run, 
dropping then to 70 per cent. The drying time will vary consider- 
ably with the size and shape of the stock. For 1 by 8 inch material 
it should be about 32 hours, with a final average moisture content of 
15 per cent. 
AIRCRAFT STOCK. 
It has been proved by many kiln runs and by many thousands of 
strength tests that the aircraft schedules given below, if carefully 
followed, will produce stock that is just as strong in every way as 
the most carefully air-seasoned stock. These schedules were pre- 
pared by the Forest Products Laboratory, and since 1917 they have 
been the standard for the Army and Navy air services. They are 
intended to be used on stock 3 inches or less in thickness. For 
thicker stock the temperature is to be lowered 5° F. for each inch 
increase in thickness. 
