It is exceedingly important that the material be practically at the same temperature throughout if perfect drying is to be secured. It should be the same temperature in the center of a pile or car as on the outside, and the same in the center of each individual piece of wood as on its surface. This is the effect obtained by natural air-drying. The outside atmosphere and breezes (natural air circulation) are so ample that the heat extracted for drying does not appreciably change the temperature.
When once the wood has been raised to a high temperature through and through and especially when the surface has been rendered most permeable to moisture, drying may proceed as rapidly as it can be forced by artificial circulation, provided the heat lost from the wood through vaporization is constantly replaced by the heat of the kiln.
It is evident that to secure an even temperature, a free circulation of air must be brought in contact with the wood. It is also evident that in addition to heat and a circulation of air, the air must be charged with a certain amount of moisture to prevent surface drying or case-hardening.
There are some twenty-five different makes of dry kilns on the market, which fulfill to a varying degree the fundamental requirements. Probably none of them succeed perfectly in fulfilling all.
It is well to have the temperature of a dry kiln controlled by a thermostat which actuates the valve on the main steam supply pipe. It is doubly important to maintain a uniform temperature and avoid fluctuations 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.