in different directions. Wood becomes soft
and plastic when hot and moist, and will yield more or less to
internal stresses. As some species are practically impervious to air
when wet, this plasticity of the cell walls causes them to collapse as
the water passes outward from the cell cavities. This difficulty has
given much trouble in the case of Western red cedar, and also to some
extent in redwood. The unequal shrinkage causes internal stresses in
the wood as it dries, which results in warping, checking,
case-hardening, and honeycombing. Case-hardening is one of the most
common defects in improperly dried lumber. It is clearly shown by the
cupping of the two halves when a case-hardened board is resawed.
Chemical changes also occur in the wood in drying, especially so at
higher temperatures, rendering it less hygroscopic, but more brittle.
If dried too much or at too high a temperature, the strength and
toughness is seriously reduced.

Conditions of Success

Commercial success in drying therefore requires that the substance be
exposed to the air in the most efficient manner; that the temperature
of the air be as high as the substance will stand without injury, and
that the air change or movement be as rapid as is consistent with
economical installation and operation. Conditions of success therefore
require the observance of the following points, which embody the basic
principles of the process: (1) The timber should be heated through
before drying begins. (2) The air should be very humid at the
beginning of the drying process, and be made drier only gradually. (3)
The temperature of the lumber must be maintained uniformly throughout
the entire pile. (4) Control of the drying process at any given
temperature must be secured by controlling the relative humidity, not
by decreasing the circulation. (5) In general, high temperatures
permit more rapid drying than do lower temperatures. The higher the
temperature of the lumber, the more efficient is the kiln. It is
believed that temperatures as high as the boiling point are not
injurious to most woods, providing all other fundamentally important
features are taken care of. Some species, however, are not able to
stand as high temperatures as others, and (6) the degree of dryness
attained, where strength is the prime requisite, should not exceed
that at which the wood is to be used.

Different Treatment according to Kind

The