rapidity with which water may be evaporated, that is, the rate of
drying, depends on the size and shape of the piece and on the
structure of the wood. Thin stock can be dried much faster than thick,
under the same conditions of temperature, circulation, and humidity.
Pine can be dried, as a general thing, in about one third of the time
that would be required for oak of the same thickness, although the
former contains the more water of the two. Quarter-sawn oak usually
requires half again as long as plain oak. Mahogany requires about the
same time as plain oak; ash dries in a little less time, and maple,
according to the purpose for which it is intended, may be dried in one
fifth the time needed for oak, or may require a slightly longer
treatment. For birch, the time required is from one half to two
thirds, and for poplar and basswood, from, one fifth to one third that
required for oak.

All kinds and thicknesses of lumber cannot be dried at the same time
in the same kiln. It is manifest that green and air-dried lumber,
dense and porous lumber, all require different treatment. For
instance, Southern yellow pine when cut green from the log will stand
a very high temperature, say 200 degrees Fahrenheit, and in fact this
high temperature is necessary together with a rapid circulation of air
in order to neutralize the acidity of the pitch which causes the wood
to blue and discolor. This lumber requires to be heated up immediately
and to be kept hot throughout the length of the kiln. Hence the kiln
must not be of such length as to allow of the air being too much
cooled before escaping.

Temperature depends

While it is true that a higher temperature can be carried in the kiln
for drying pine and similar woods, this does not altogether account
for the great difference in drying time, as experience has taught us
that even when both woods are dried in the same kiln, under the same
conditions, pine will still dry much faster, proving thereby that the
structure of the wood itself affects drying.

The aim of all kiln designers should be to dry in the shortest
possible time, without injury to the material. Experience has
demonstrated that high temperatures are very effective in evaporating
water, regardless of the degree of humidity, but great care must be
exercised in using extreme temperatures that the material to be dried
is not damaged by checking, case-hardening, or hollow-horning.

The tempera