e. The reason
for this imperfect knowledge lies in the fact that wood is not a
homogeneous material like the metals, but a complicated structure, and
so variable that one stick will behave in a manner widely different
from that of another, although it may have been cut from the same
tree.

The great variety of woods often makes the mere distinction of the
kind or species of the tree most difficult. It is not uncommon to find
men of long experience disagree as to the kind of tree a certain piece
of lumber was cut from, and, in some cases, there is even a wide
difference in the appearance and evidently the structure of timber cut
from the same tree.

Objects of Kiln-drying

The objects of kiln-drying wood may be placed under three main
headings: (1) To reduce shipping expenses; (2) to reduce the quantity
necessary to maintain in stock; and (3) to reduce losses in air-drying
and to properly prepare the wood for subsequent use. Item number 2
naturally follows as a consequence of either 1 or 3. The reduction in
weight on account of shipping expenses is of greatest significance
with the Northwestern lumbermen in the case of Douglas fir, redwood,
Western red cedar, sugar pine, bull pine, and other softwoods.

Very rapid methods of rough drying are possible with some of these
species, and are in use. High temperatures are used, and the water is
sometimes boiled off from the wood by heating above 212 degrees
Fahrenheit. These high-temperature methods will not apply to the
majority of hardwoods, however, nor to many of the softwoods.

It must first of all be recognized that the drying of lumber is a
totally different operation from the drying of a fabric or of thin
material. In the latter, it is largely a matter of evaporated
moisture, but wood is not only hygroscopic and attracts moisture from
the air, but its physical behavior is very complex and renders the
extraction of moisture a very complicated process.

An idea of its complexity may be had by mentioning some of the
conditions which must be contended with. Shrinkage is, perhaps, the
most important. This is unequal in different directions, being twice
as great tangentially as radially and fifty times as great radially as
longitudinally. Moreover, shrinkage is often unequal in different
portions of the same piece. The slowness of the transfusion of
moisture through the wood is an important factor. This varies with
different woods and greatly