normal size when saturated with water.
The cell walls may be considered as made up of little particles
with water between them. When wood is dried the films of water
between the particles become thinner and thinner until almost
entirely gone. As a result the cell walls grow thinner with loss
of moisture,--in other words, the cell shrinks.

It is at once evident that if drying does not take place
uniformly throughout an entire piece of timber, the shrinkage as
a whole cannot be uniform. The process of drying is from the
outside inward, and if the loss of moisture at the surface is
met by a steady capillary current of water from the inside, the
shrinkage, so far as the degree of moisture affected it, would
be uniform. In the best type of dry kilns this condition is
approximated by first heating the wood thoroughly in a moist
atmosphere before allowing drying to begin.

In air-seasoning and in ordinary dry kilns this condition too
often is not attained, and the result is that a dry shell is
formed which encloses a moist interior. (See Fig. 26.)
Subsequent drying out of the inner portion is rendered more
difficult by this "case-hardened" condition. As the outer part
dries it is prevented from shrinking by the wet interior, which
is still at its greatest volume. This outer portion must either
check open or the fibres become strained in tension. If this
outer shell dries while the fibres are thus strained they become
"set" in this condition, and are no longer in tension. Later
when the inner part dries, it tends to shrink away from the
hardened outer shell, so that the inner fibres are now strained
in tension and the outer fibres are in compression. If the
stress exceeds the cohesion, numerous cracks open up, producing
a "honey-combed" condition, or "hollow-horning," as it is
called. If such a case-hardened stick of wood be resawed, the
two halves will cup from the internal tension and external
compression, with the concave surface inward.

[Illustration: FIG. 26.--Progress of drying throughout the
length of a chestnut beam, the black spots indicating the
presence of free water in the wood. The first section at the
left was cut one-fourth inch from the end, the next one-half
inch, the next one inch, and all the others one inch apart. The
illustration shows case-hardening very clearly. _Photo by U. S.
Forest Service._]

For a given surface area the loss of water from wood is always
greater from the ends than from the