e receiving end of the kiln should always be
kept moist, where the stock has not been steamed before being put into
the kiln. The reason for this is simple enough. When the air is too
dry it tends to dry the outside of the material first--which is termed
"case-hardening"--and in so doing shrinks and closes up the pores of
the wood. As the stock is moved down the kiln, it absorbs a
continually increasing amount of heat, which tends to drive off the
moisture still present in the center of the stock. The pores on the
outside having been closed up, there is no exit for the vapor or steam
that is being rapidly formed in the center. It must find its way out
some way, and in doing so sets up strains, which result either in
checking, warping, or hollow-horning. If the humidity had been kept
higher, the outside of the material would not have dried so quickly,
and the pores would have remained open for the exit of moisture from
the interior of the wood, and this trouble would have been avoided.

Where the humidity is kept at a high point in the receiving end of the
kiln, a higher rate of temperature may also be carried, and in that
way the drying process is hastened with comparative safety.

It is essential, therefore, to have an ample supply of heat through
the convection currents of the air; but in the case of wood the rate
of evaporation must be controlled, else checking will occur. This can
be done by means of the relative humidity, as stated before. It is
clear now that when the air--or, more properly speaking, the space--is
completely saturated no evaporation can take place at the given
temperature. By reducing the humidity, evaporation takes place more
and more rapidly.

Another bad feature of an insufficient and non-uniform supply of heat
is that each piece of wood will be heated to the evaporating point on
the outer surface, the inside remaining cool until considerable drying
has taken place from the surface. Ordinarily in dry kilns high
humidity and large circulation of air are antitheses to one another.
To obtain the high humidity the circulation is either stopped
altogether or greatly reduced, and to reduce the humidity a greater
circulation is induced by opening the ventilators or otherwise
increasing the draft. This is evidently not good practice, but as a
rule is unavoidable in most dry kilns of present make. The humidity
should be raised to check evaporation without reducing the circulation
if possible.

Whil