KILN DRYING HANDBOOK 79 
of the air movement that does exist, longitudinal included, is local 
in character and to a great extent is independent of the direction of 
piling. Hence considered from the circulation standpoint there is 
little to choose between end piling and cross piling; both systems 
are employed with success. In comparatively long kilns for drying 
green softwoods, however, end piling seems to permit the securing of 
a somewhat better local circulation, especially if the heating system 
and other detail parts of the kiln are arranged with this purpose in 
view. Cross-piled progressive kilns are designed to take a definite 
maximum length of stock, making it especially desirable to have all 
the piles in such kilns take up fully the space intended for them, in 
order to allow as little opportunity as possible for the development 
of air currents that counteract the regular air movement. 
SPACING OF BOARDS 
As already suggested, the spacing of the boards or pieces in the 
individual layers of the piles has an important bearing upon circula- 
tion, especially in natural-circulation kilns; manufacturers of this 
type frequently recommend a wide spacing. The amount of space to 
leave in any particular case, depending as it does upon the circula- 
tion, is a matter of judgment. Ordinarily 2 to 3 inches of open 
space to each 12 inches of stock width is sufficient for slow-drying 
species, but for fast-drying stock 4 to 6 inches is desirable. One 
space of 2 inches is better than two spaces of 1 inch, but the distance 
between spaces should not be over 14 inches. In piling narrow stock 
it is desirable to group the boards in widths of 10 to 12 inches with 
92-inch spaces (or more) between the groups. Im so far as possible, 
spaces in successive layers should be in vertical alignment. To 
obtain best results, material dried at the same time should be of the 
same species and the same thickness and with its moisture content 
as nearly uniform as circumstances permit. Stock in the same layer 
should be of one thickness, since otherwise warping will develop in 
thin boards. 
STICKERS 
Stickers should be made from clear, straight-grained stock, entirely 
free from both stain and decay, and should be dressed to a uniform 
thickness. Seven-eighths-inch stickers are common for most classes 
of stock, except in edge stacking, in which the requirements of the 
stacking machine may determine both their width and their thick- 
ness. If stickers are made about one and one-half times as wide as 
they are thick, they will lie flat instead of tending to roll when the 
boards are laid on them. 
SPACING OF STICKERS 
The spacing between tiers of stickers should be reasonably close. 
With green hardwoods it should normally vary from 2 to 4 feet. 
Softwood practice, which depends somewhat on species and grade, is 
variable; as a rule the spacing is from 4 to 6 feet, but it has been 
demonstrated, for southern pine, that a spacing as close as 2 feet re- 
sults in sufficient saving from lessened warping and consequent kiln 
and planer degrade to be economically sound practice. Box piling, 
