30 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
enough to penetrate the entire compression zone, the surface may have 
picked, up so much moisture that the resultant great shrinkage will 
produce a permanent reverse casehardening, which the drying down 
to the desired final moisture content is not able to eliminates This 
state of affairs must be avoided since reverse casehardening in dry 
stock can not be removed without softening up the entire piece 
again — a tremendously long and unsatisfactory process. It is better, 
therefore, to employ milder means as the stock becomes drier. In- 
stead of steaming (100 per cent humidity), the humidity is kept at 
some lower point ranging usually between 60 and 85 per cent. The 
time required is considerably more, and the effect is correspondingly 
milder and more uniformly distributed through a deeper zone. 
GENERAL RULES FOR STEAMING AND HIGH-HUMIDITY 
TREATMENTS. 
It is not possible to lay down hard and fast rules for steaming and 
high humidity treatments; each operator will have to learn by ex- 
perience just what can and must be done. The Forest Products 
Laboratory usually recommends that high humidity treatments be 
used when the core of the stock contains less than 18 per cent mois- 
ture. Above this point steaming at from 160° to 185° F, may be 
safely used, the period of steaming varying from one-half to three 
hours. These temperatures can be used advantageously also in high 
humidity treatments, The relative humidity will vary with the dry- 
ness of the stock. It may well be between 75 and 90 per cent when 
the core is between 15 and 18 per cent and between 65 and 75 per 
cent below that. The duration of high humidity treatments may be 
from 10 to 30 hours, sometimes shorter but seldom longer. 
The degree to which steaming and high-humidity treatments 
should be used depends entirely upon the stock being dried and the 
purpose for which it is to be used. It may be laid down as a gen- 
eral rule that better results will be secured, and at less risk of dam- 
age, principally from honeycomb, if the stresses are relieved fre- 
quently by short, mild treatments than infrequently through long, 
severe treatments. In any event, the treatment given should be 
determined by the condition of the stock at the time. 
Casehardening is not in itself a serious defect during the drying 
process, though, of course, it is undesirable and leads to various 
difficulties. In the finished stock, however, matters are different, 
and casehardening is of itself a serious defect, which results in cup- 
ping and warping, unequal shrinkage, and similar trouble, espe- 
cially in resawing or in working deep patterns. It is essential, there- 
fore, that casehardening be removed before the stock is taken from 
the kiln, and provision for a final treatment should be made in the 
drying schedule. While it is not customary to do this in the dry- 
ing of most softwoods, it has been repeatedly shown that, especially 
for resaw stock, final relief of casehardening is very advantageous 
even in woods like the soft pines. There are. on the other hand, 
many cases, such as drying simply for shipping weight, where the 
financial advantage is questionable. 
