eed uniformly at all points, otherwise
extra stresses are set up in the wood, causing warping, etc.

3. Heat should penetrate to the interior of the piece before
drying begins.

4. The humidity should be suited to the condition of the
wood at the start and reduced in the proper ratio as drying
progresses. With wet or green wood it should usually be held
uniform at a degree which will prevent the surface from
drying below its saturation point until all the free water
has evaporated, then gradually reduced to remove the
hygroscopic moisture.

5. The temperature should be uniform and as high as the
species under treatment will stand without excessive
shrinkage, collapse, or checking.

6. Rate of drying should be controlled by the amount of
humidity in the air and not by the rate of circulation,
which should be made ample at all times.

7. In drying refractory hardwoods, such as oak, best results
are obtained at a comparatively low temperature. In more
easily dried hardwoods, such as maple, and some of the more
difficult softwoods, as cypress, the process may be hastened
by a higher temperature but not above the boiling point. In
many of the softwoods, the rate of drying may be very
greatly increased by heating above the boiling point with a
large circulation of vapor at atmospheric pressure.

8. Unequal shrinkage between the exterior and interior
portions of the wood and also unequal chemical changes must
be guarded against by temperatures and humidities suited to
the species in question to prevent subsequent cupping and
warping.

9. The degree of dryness attained should conform to the use
to which the wood is put.

10. Proper piling of the material and weighting to prevent
warping are of great importance.

Requirements in a Satisfactory Dry Kiln

The requirements in a satisfactory dry kiln are:

1. Control of humidity at all times.
2. Ample air circulation at all points.
3. Uniform and proper temperatures.

In order to meet these requirements the United States Forestry Service
has designed a kiln in which the humidity, temperature, and
circulation can be controlled at all times.

Briefly, it consists of a drying chamber with a partition on either
side, making two narrow side chambers open top and bottom.

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