﻿30 BULLETIN 1136, I'. S. DSPABXMEN1 OF AGRICULTURE. 



enough to penetrate the entire compression zone, i he surface may have 

 picked up so much moisture that the resultant great shrinkage wili 

 produce a permanent reverse casehardening, which tiie drying down 



to the desired iinal moisture content is not able to eliminate. This 

 state of affair? must be avoided since reverse casehanieniag 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 (loo 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 00 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 diving 

 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. 



