. 
KILN DRYING HANDBOOK 67 
In addition, it changes the color of the stock from white to pink, 
which is desired by some consumers. In patent steaming cylinders 
the period of steaming varies from 30 to 45 minutes and in specially 
built steam boxes from 4 to 24 hours; the temperatures in the boxes 
reach 180° to 212° F. Live steam is used in the cylinder, and a 
gauge on the supply line indicates pressures up to 20 pounds per 
square inch, but the pressure and the temperature within the cylin- 
der itself are not known definitely. In the boxes either live or ex- 
haust steam may be used. High temperatures seem to cause check- 
ing in any red gum present with the sap gum, but below 180° IF. such 
checking is less common. Two hours of steaming at 180° is prob- 
ably sufficient to kill all fungi and all larvae of insects. For the pur- 
poses of seasoning, a 4-hour steaming period at 180° I’. seems to be as 
good as 8 hours or longer, provided the stock is properly handled 
after it leaves the steam box. 
With sap gum up to six quarter inches in thickness, the doors 
_ should be opened immediately after the steaming, and as soon as the 
stock can be handled it should be pulled out and allowed to cool in 
the open for at least 48 hours before being bulk piled on the lumber 
buggies or the trucks that take it to the yard. This treatment is too 
severe for 8/4-inch sap gum and for plain-sawed red gum of all 
thicknesses above four quarter inch, since it is apt to cause surface 
checking. Such stock should be allowed to cool in the kiln or other 
steaming chamber to about 140° F. before the doors are opened; in 
other respects, it is handled as just indicated. Distortion can easily 
be avoided by careful piling. 
Rapid cooling dries the surface of the lumber, and if the evapo- 
rated moisture is carried away quickly the surface will be dried 
below the critical point (the lowest moisture content at which fungi 
ean develop) sufficiently to prevent reinfection by the spores of the 
stain organism, which are always present in the air. When the 
cooled stock is taken from the kiln trucks it should be run out to the 
vard and repiled. with as lttle delay as possible. If it is allowed 
to stand bulk piled on the lumber buggies for several hours after be- 
ine removed from the kiln trucks, the surface of each piece will 
retain the moisture flowing to 1t from the interior of the piece, and 
consequently the beneficial effect on surface drying that should 
result from the steaming process will virtually be lost. Such re- 
moistening of the surface is probably responsible for some of the 
blue or brown spots that are seen on steamed sap gum during subse- 
quent yard drying. In other instances the spotting probably de- 
velops because the surface never became sufficiently dry after 
steaming; this is likely to be the case if stock is pulled from the 
steaming chamber on a damp day or if the kiln trucks stand so close 
together while the stock is cooling that the evaporated moisture can 
not be carried away rapidly. 
SEASONING SPECIFICATIONS 
MOISTURE CONTENT 
Many of the disputes over trouble experienced in the use of lumber 
are caused by a difference of opinion over the meaning of broad and 
loose terms common in the trade, such for instance as “ kiln-dried ” 
