THE SEASONING OF WOOD. 25 
or through water sprays. The temperature of the pipes or sprays 
governs the amount of water that condenses from the air, and con- 
sequently regulates the humidity of the air when reheated before 
being passed over the lumber again. The circulation of air may be 
either natural or forced. Condensing kilns are generally of the com- 
partment type. 
Kilns operating with superheated steam are used only where the 
species to be dried are not injured by high temperatures, and where 
fast drying is essential. 
Lumber may be piled on the trucks which carry it into the kiln in 
any one of three ways (fig. 17): (a) flat or horizontal, (b) edge or 
vertical, and (c) inclined. Flat piling is best for longitudinal circu- 
lation. It is not so well adapted for transverse circulation, and is not 
economical for downward circulation. Vertical piling increases the 
truck capacity, as there are no vertical spaces between the boards. 
Probably it is the best method for downward or any fast circulation. 
Provision has to be made, however, for keeping the boards in place in 
the stack. Inclined piling allows for a definite movement of air either 
downward or upward (forced draft). It is an improvement, as 
regards circulation, over horizontal or flat piling. 
Some kiln operators using the flat or horizontal method of piling 
report excellent results from the construction of a V-shaped opening 
in the center of the truck pile. Such openings are from 2 \ to 3 feet 
wide at their base, and from 3 \ to 4 feet high. Where this practice 
is followed it is customary also to place the boards in the layers closer 
together as the top of the stack is reached, to force greater lateral 
circulation. 
In loading lumber on kiln trucks by any one of the three methods 
mentioned, the stickers should be of a uniform thickness and arranged 
in the piles in alignment. 
It is advisable also not to attempt to dry various thicknesses of 
lumber together. Thick lumber takes longer to dry than thin 
lumber, and when different thicknesses are mixed the operation has 
to be governed by the thick stock, to the possible detriment, or at 
least the unnecessarily long drying, of the thin stock. 
PRELIMINARY TREATMENTS. 
Lumber to be kiln-dried is sometimes steamed in a separate com- 
partment before being placed in the kiln proper, especially where the 
kiln is not designed for moist-air treatment. In transferring lumber 
from a compartment for preliminary treatment to the kiln proper, 
every care must be used to avoid a sudden change in humidity. The 
object of the steaming is to heat the lumber and thus make easier 
the transmission of moisture from the interior to the surface, and also 
to moisten the surface in case it has become casehardened or "set" 
