elopment of the timber
industry. In the earlier days, before power machinery for the
working-up of timber products came into general use, dry kilns were
unheard-of, air-drying or seasoning was then relied upon solely to
furnish the craftsman with dry stock from which to manufacture his
product. Even after machinery had made rapid and startling strides on
its way to perfection, the dry kiln remained practically an unknown
quantity, but gradually, as the industry developed and demand for dry
material increased, the necessity for some more rapid and positive
method of seasoning became apparent, and the subject of artificial
drying began to receive the serious attention of the more progressive
and energetic members of the craft.

Kiln-drying which is an artificial method, originated in the effort to
improve or shorten the process, by subjecting the wood to a high
temperature or to a draught of heated air in a confined space or kiln.
In so doing, time is saved and a certain degree of control over the
drying operation is secured.

The first efforts in the way of artificial drying were confined to
aiding or hastening nature in the seasoning process by exposing the
material to the direct heat from fires built in pits, over which the
lumber was piled in a way to expose it to the heat rays of the fires
below. This, of course, was a primitive, hazardous, and very
unsatisfactory method, to say the least, but it marked the first step
in the evolution of the present-day dry kiln, and in that particular
only is it deserving of mention.

Underlying Principles

In addition to marking the first step in artificial drying, it
illustrated also, in the simplest manner possible, the three
underlying principles governing all drying problems: (1) The
application of heat to evaporate or volatilize the water contained in
the material; (2) with sufficient air in circulation to carry away in
suspension the vapor thus liberated; and (3) with a certain amount of
humidity present to prevent the surface from drying too rapidly while
the heat is allowed to penetrate to the interior. The last performs
two distinct functions: (a) It makes the wood more permeable to the
passage of the moisture from the interior of the wood to the surface,
and (b) it supplies the latent heat necessary to evaporate the
moisture after it reaches the surface. The air circulation is
important in removing the moisture after it has been evaporated