KILN DRYING HANDBOOK 4] 
Preliminary steaming of green stock.—The first steaming of stock 
that is put into the kiln in a totally undried condition is solely to 
warm it. The procedure is outhned on page 47. 
RELIEF OF INTERNAL TENSION 
Assume that the stock has safely passed the first stages of drying, 
and that the stress in the outer portion of each piece has changed 
' from tension to compression. During this period of the process the 
stock usually is not lable to injury, and the only surface phenomenon 
is that the checks close up. The surface compression existing will 
naturally be accompanied by a corresponding tension in the cores. 
If surface checks were originally present and have closed up, such 
increasing tension is more likely to produce honeycombing than if 
the stock had never been surface checked. In any event, the greater 
the tension stress across the grain in the center of a piece the greater 
is the danger of the wood failing internally in tension across the 
grain (honeycombing). Now the degree of tension that ultimately 
develops in the interior of a piece of wood depends on the degree to 
which the surface is set in an expanded condition. Honeycombing, 
therefore, can largely be prevented by carrying high relative humidi- 
ties during the period in which set is developing in the lumber. 
The shell of a piece of air-dried lumber, however, is usually set in 
an expanded condition when the stock enters the kiln, so that in 
general it is not the relative humidity used in the kiln schedule 
that governs the extent to which the core of such a piece is stressed 
during subsequent kiln drying; the degree of stress during kiln treat- 
ment depends chiefly on the drying conditions that existed during 
the previous air-seasoning process. The relative humidity specified 
in a drying schedule, therefore, has little to do with the prevention 
of honeycombing in the usual kiln charge. The temperature car- 
ried has a bearing on such prevention, though, because hot wood will 
honeycomb under smaller stress than cool wood. 
With 6/4-inch and thinner partially dried stock, it ordinarily is 
safe practice so to adjust the relative humidity in the kiln that the 
moisture content of the surface of the stock will be raised to the 
value of the core content. This treatment will usually have a desir- 
able effect on the surface, moistening and softening it and tempo- 
rarily increasing the compression in the outer shell, which in the 
semiplastic condition thus caused fails permanently under the in- 
creased load, taking a compression set across the grain. In this way 
the set expanded condition is decreased. Almost immediately after 
the steaming treatment the surface layers will lose most of the mois- 
ture picked up, and if the treatment has been properly conducted, with 
respect to duration, temperature, and relative humidity, the stock will 
then be stress-free. On the other hand, if the relative humidities em- 
ployed have been too high the stresses in the surface layers will be re- 
versed. The outer prongs in a casehardening stress section will then 
turn outward. 
Prevention of reverse casehardening—When stock is reasonably 
dry and the compression shell of each piece is comparatively thick, a 
steaming treatment at too high a relative humidity may readily re- 
sult in too severe an effect on the surfaces without enough effect. to- 
ward the deeper portions of the shells. If the treatment is con- 
