e thin stock, such as cooperage and box stuff is less inclined to
give trouble by undue checking than 1-inch and thicker, one will find
that any dry kiln will give more uniform results and, at the same
time, be more economical in the use of steam, when the humidity and
temperature is carried at as high a point as possible without injury
to the material to be dried.

Any well-made dry kiln which will fulfill the conditions required as
to circulation and humidity control should work satisfactorily; but
each case must be studied by itself, and the various factors modified
to suit the peculiar conditions of the problem in hand. In every new
case the material should be constantly watched and studied and, if
checking begins, the humidity should be increased until it stops. It
is not reducing the circulation, but adding the necessary moisture to
the air, that should be depended on to prevent checking. For this
purpose it is well to have steam jets in the kiln so that if needed
they are ready at hand.

Kiln-drying

There are two distinct ways of handling material in dry kilns. One way
is to place the load of lumber in a chamber where it remains in the
same place throughout the operation, while the conditions of the
drying medium are varied as the drying progresses. This is the
"apartment" kiln or stationary method. The other is to run the lumber
in at one end of the chamber on a wheeled truck and gradually move it
along until the drying process is completed, when it is taken out at
the opposite end of the kiln. It is the usual custom in these kilns to
maintain one end of the chamber moist and the other end dry. This is
known as the "progressive" type of kiln, and is the one most commonly
used in large operations.

It is, however, the least satisfactory of the two where careful drying
is required, since the conditions cannot be so well regulated and the
temperatures and humidities are apt to change with any change of wind.
The apartment method can be arranged so that it will not require any
more kiln space or any more handling of lumber than the progressive
type. It does, however, require more intelligent operation, since the
conditions in the drying chamber must be changed as the drying
progresses. With the progressive type the conditions, once properly
established, remain the same.

To obtain draft or circulation three methods are in use--by forced
draft or a blower usually placed outside