luctuations in the dry kiln, since a change in temperature will
greatly alter the relative humidity.

In artificial drying, temperatures of from 150 to 180 degrees
Fahrenheit are usually employed. Pine, spruce, cypress, cedar, etc.,
are dried fresh from the saw, allowing four days for 1-inch stuff.
Hardwoods, especially oak, ash, maple, birch, sycamore, etc., are
usually air-seasoned for three to six months to allow the first
shrinkage to take place more gradually, and are then exposed to the
above temperatures in the kiln for about six to ten days for 1-inch
stuff, other dimensions in proportion.

Freshly cut poplar and cottonwood are often dried direct from the saw
in a kiln. By employing lower temperatures, 100 to 120 degrees
Fahrenheit, green oak, ash, etc., can be seasoned in dry kilns without
much injury to the material.

Steaming and sweating the wood is sometimes resorted to in order to
prevent checking and case-hardening, but not, as has been frequently
asserted, to enable the material to dry.

Air Circulation

Air circulation is of the utmost importance, since no drying whatever
can take place when it is lacking. The evaporation of moisture
requires heat and this must be supplied by the circulating air.
Moreover, the moisture laden air must be constantly removed and fresh,
drier air substituted. Probably this is the factor which gives more
trouble in commercial operations than anything else, and the one which
causes the greatest number of failures.

It is necessary that the air circulate through every part of the kiln
and that the moving air come in contact with every portion of the
material to be dried. In fact, the humidity is dependent upon the
circulation. If the air stagnates in any portion of the pile, then the
temperature will drop and the humidity rise to a condition of
saturation. Drying will not take place at this portion of the pile and
the material is apt to mould and rot.

The method of piling the material on trucks or in the kiln, is
therefore, of extreme importance. Various methods are in use. Ordinary
flat piling is probably the poorest. Flat piling with open chimney
spaces in the piles is better. But neither method is suitable for a
kiln in which the circulation is mainly vertical.

Edge piling with stickers running vertically is in use in kilns when
the heating coils are beneath. This is much better.

Air being cooled as it comes in contact with a pile