sapwood more nearly the color of the heartwoods. The
method of application in kiln-drying green gum we believe to be new,
however. Other methods for kiln-drying this green stock are to be
tested until the proper process is developed. We expect to have
something interesting to report in the near future.[1]

[Footnote 1: The above test was made at the United States
Forestry Service Laboratory, Madison, Wis.]

SECTION XII

TYPES OF DRY KILNS

DIFFERENT TYPES OF DRY KILNS

Dry kilns as in use to-day are divided into two classes: The "pipe" or
"moist-air" kiln, in which natural draft is relied upon for
circulation and, the "blower" or "hot blast" kiln, in which the
circulation is produced by fans or blowers. Both classes have their
adherents and either one will produce satisfactory results if properly
operated.

The "Blower" or "Hot Blast" Kiln

The blower kiln in its various types has been in use so long that it
is hardly necessary to give to it a lengthy introduction. These kilns
at their inauguration were a wonderful improvement over the old style
"bake-oven" or "sweat box" kiln then employed, both on account of the
improved quality of the material and the rapidity at which it was
dried.

These blower kilns have undergone steady improvement, not only in the
apparatus and equipment, but also in their general design, method of
introducing air, and provision for controlling the temperature and
humidity. With this type of kiln the circulation is always under
absolute control and can be adjusted to suit the conditions, which
necessarily vary with the conditions of the material to be dried and
the quantity to be put through the kiln.

In either the blower or moist-air type of dry kiln, however, it is
absolutely essential, in order to secure satisfactory results, both as
to rapidity in drying and good quality of stock, that the kiln be so
designed that the temperature and humidity, together with circulation,
are always under convenient control. Any dry kiln in which this has
not been carefully considered will not give the desired results.

In the old style blower kiln, while the circulation and temperature
was very largely under the operator's control, it was next to
impossible to produce conditions in the receiving end of the kiln so
that the humidity could be kept at the proper point. In fact, this was