49 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
tinued long enough to penetrate the shells entirely, the surfaces may 
pick up so much moisture that the resultant great shrinkage oc- 
curring during redrying will produce a permanent reverse case- 
har dening, which the drying down to the desired final moisture con- 
tent will not eliminate. ‘This state of affairs must be avoided, since 
reverse casehardening in dry stock can be removed only through 
softening up the entire piece again—a tremendously long and unsat- 
isfactory process. It is better, therefore, to employ relative humidi- 
ties no higher than those necessary to eliminate the casehardening. 
GENERAL RULES FOR STEAMING 
Tnfiexible rules for steaming treatments to relieve caseharden- 
ing can not be laid down, largely because it is impossible to express 
the degree of casehardening itself with numerical exactness; each 
operator will have to learn by experience just what can and must 
be done with each individual kiln charge. Steaming at the satura- 
tion point (100 per cent relative humidity) may, in general, be done 
satisfactorily only with green stock (as a preliminary treatment) or 
with stock in which the moisture in the core is above the fiber-sat- 
uration point and the moisture content of the surface is not below 18 
per cent. For drier stock lower humidities should be used. The 
relative humidity of the air should be adjusted to correspond, during 
the steaming treatment, to the desired final surface moisture con- 
tent. Suppose, for instance, that the surface moisture content is 
10 per cent, and it is desired to raise it to 13 per cent, the ten nperature 
of the treatment being 180° F. At this temperature, a relative hu- 
midity of about 82 per cent is in balance with a moisture content of 
13 per cent (fig. 4) and the treatment, therefore, should be given at 
that humidity. Comparatively high temperatures are usually car- 
ried for final steaming treatments; for hardwoods they customarily 
range up to 165° F. and for softwoods they may rise as high as 200° 
F. Steaming at high temperatures will be discussed further i in con- 
nection with the special softwood drying schedules (p. 56). 
Casehardening is not in itself a serious defect during the drying 
process, but is undesirable because it leads to various other difficul- 
ties. In the finished stock, however, matters are different; casehard- 
ening then is of itself a serious defect, one that results in warping, 
unequal shrinkage, and similar trouble, especially in resawing or 
in working deep patterns. Tt is almost essential, therefore, that case- 
har dening i in such stock be remedied before the stock is taken from 
the kiln, and accordingly provision for a final conditioning treatment 
should be made in the drying schedule. While final conditioning i is 
not customary in the drying of most softwoods, it has repeatedly 
been shown that, especially for resaw stock, final relief of case- 
hardening is highly advantageous even in woods like the pines. 
There are, on the other hand, many cases, such as drying simply to 
reduce shipping weight, where the financial advantage 1s questionable. 
RELIEVING CASEHARDENING DURING STORAGE 
Sometimes a suitable final steaming treatment can not be given— 
for instance, in a typical progressive kiln. Under such circumstances 
it is desirable to relieve casehardening stresses, to the extent pos- 
