40 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
checked during air seasoning, and particularly if it is so dry that the 
checks have closed up, great care must be exercised to see that the sur- 
face is not moistened too much during any high-humidity treatment. 
If it is, checks that would otherwise be closed when the drying process 
is over will open up glaringly. 
RELIEF OF SURFACE TENSION 
Among kiln operators it is common practice to steam air-dried or 
partially air-dried lumber initially for the purpose of reducing! the 
tension set in the outer layers of the individual pieces. ‘This practice 
has degraded a large amount of such lumber, not because steaming at 
the initial stage of the drying process is in itself fundamentally wrong, 
but because the customary manner of doing it is wrong. Initial 
steaming is often unessential, and it is far better not to steam air-dried 
lumber than to give the treatment improperly. 
The first evidence of stress in fresh stock in the kiln is a tension in 
the outer portion of a piece (the shell), which appears in the stress 
section in an outward turning of the prongs. Such tension may be 
considered a normal condition, more or less unavoidable. If it be- 
comes too severe, however, surface checks will result. The condition 
of the stock in the early part of the run is usually judged by the 
presence or the absence of surface checks. Excessive tension in the 
surface and resulting surface checks are caused by too steep a moisture 
eradient; in other words, the moisture content of the surface layers, 
under this condition, is too low in comparison with that of the core. 
The remedy is to raise the moisture content at the surface by raising 
the relative humidity of the air in the kiln, thereby reducing the mois- 
ture gradient. (Refer again to fig. 6 to consider the effect of raising 
to a higher position the ends of any one of the curves.) 
Although successful methods of steaming lumber have for the most 
part been worked out empirically, there is one basic principle that 
must be grasped before steaming treatments can be given intelli- 
gently, namely, before a set expanded condition in the outer layers of 
a piece can be reduced, this portion of it must be made to fail perma- 
nently in compression across the grain, that is, these layers must take 
a compression set. Such failure necessitates the following condi- 
tions: (1) An interior below the fiber-saturation point, (2) an exterior 
tending to swell across the grain because of the raising of its moisture 
content above that of the interior, and (38) a resultant increase in 
stress in the wood sufficient in value and in duration to cause a per- 
manent compression deformation across the grain in the exterior of 
the piece. 
A general rule is that the humidity during the initial steaming of 
air-dry stock should correspond to a moisture content 2 or 3 per cent 
higher than that of the surface layers of the stock and that the treat- 
ment should continue until the surface quarter inch has absorbed 
moisture enough to bring it into equilibrium with these humidity con- 
ditions. ‘The time required for such a steaming, once the conditions 
proper for it have been attained, will vary for 1-inch stock from a 
few hours to a day, depending upon both species and individual con- 
ditions. ‘To hasten the treatment and to warm up the stock most 
satisfactorily, it is customary to use a temperature about 15° F, 
higher than that at which drying is to commence. 
