58 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
thus avoiding the severe troubles of an extreme change in either. 
The compromise reached in this manner has proved thoroughly satis- 
factory. The operator, however, must be very careful to determine 
the conditions actually existing in the kiln after the shift; he can not 
afford to rely simply upon setting the dry-bulb thermostat back to 
the desired temperature. 
Jn progressive kilns having sluggish circulation it will be necessary 
to modify the general softwood schedules somewhat; special Sched- 
ules 106 to 110, inclusive, of course are not suitable for progressive 
kilns. One-inch B and Better southern yellow pine lumber can be 
dried satisfactorily in a kiln of this type under the extreme condi- 
tions of a temperature of 180° F. and 70 per cent humidity at the 
green end and 210° F’. and 30 per cent humidity at the dry end. 
WESTERN YELLOW PINE 
Under certain conditions, not yet fully understood, western yellow 
pine is subject to a brown stain, sometimes called “kiln burn,” which 
appears at or immediately beneath the surface of the sapwood during 
kiln drying. No entirely satisfactory method of preventing this stain 
has been discovered, but a number of operators report that the use of 
low-temperature schedules, with low humidities, will prevent a large 
part of it. The following schedule (Table 16), designed for 6/4-inch 
stock, is typical of such schedules. 
TABLE 16.—Special kiln-drying Schedule 111 for western yellow pine 
Dry- Wet- 
bulb bulb Relative 
Moisture content at which changes should be made, per cent temper- | temper- | humidity 
ature ature 
2 Re Per cent 
bolN(0) BA Ga (0) Re paeers 8 ceria NEES ued SEOOL yO ytd that DURA and eA Airiely ioe We Teg ee ad te ah he 120 103 
eae nee 5s KEE OT OSI EN oa ete ea I in ete uk peer ab Dene otal el 130 108 48 
QR ce os i AUS Soa a te EB cla te A 0 PPE URE EN Ee ee ee A ee ee Es 140 ST? 41 
BO aN ec SE RU ek ens ie a oo eo Se eee Nee el mee et a AE ene 150 114 33 
DU) eee Leet AL Ls he cer Sl SNL al ine keene ese mene 160 114 24 
SS a 
The schedule given in Table 16 requires about six days’ drying 
time. A final steaming may be given to relieve drying stresses; it 
can well be at 160° F. and 80 per cent relative humidity, lasting 
for five hours. 
Other operators report successful prevention of brown stain by 
maintaining high humidities throughout the run and then stopping 
the drying when the lumber has reached a moisture content of about 
18 per cent. 
EASTERN RED CEDAR 
Eastern red cedar, the southern juniper used for pencils and for 
cedar chests, is dried with difficulty; care must be taken to prevent 
the shelling off of the streaks of sapwood that will result from too 
steep a moisture gradient and from too severe casehardening. A 
special schedule (Tabie 17) has been prepared for the drying of 1- 
inch boards of this species. 
