and a fresh piece of sapwood may
lose weight in boiling water, and can be dried to quite an extent in
hot steam.

In drying chemicals or fabrics, all that is required is to provide
heat enough to vaporize the moisture and circulation enough to carry
off the vapor thus secured, and the quickest and most economical means
to these ends may be used. While on the other hand, in drying wood,
whether in the form of standard stock or the finished product, the
application of the requisite heat and circulation must be carefully
regulated throughout the entire process, or warping and checking are
almost certain to result. Moreover, wood of different shapes and
thicknesses is very differently effected by the same treatment.
Finally, the tissues composing the wood, which vary in form and
physical properties, and which cross each other in regular directions,
exert their own peculiar influences upon its behavior during drying.
With our native woods, for instance, summer-wood and spring-wood show
distinct tendencies in drying, and the same is true in a less degree
of heartwood, as contrasted with sapwood. Or, again, pronounced
medullary rays further complicate the drying problem.

Physical Properties that influence Drying

The principal properties which render the drying of wood peculiarly
difficult are: (1) The irregular shrinkage; (2) the different ways in
which water is contained; (3) the manner in which moisture transfuses
through the wood from the center to the surface; (4) the plasticity of
the wood substance while moist and hot; (5) the changes which take
place in the hygroscopic and chemical nature of the surface; and (6)
the difference produced in the total shrinkage by different rates of
drying.

The shrinkage is unequal in different directions and in different
portions of the same piece. It is greatest in the circumferential
direction of the tree, being generally twice as great in this
direction as in the radial direction. In the longitudinal direction,
for most woods, it is almost negligible, being from 20 to over 100
times as great circumferentially as longitudinally.

There is a great variation in different species in this respect.
Consequently, it follows from necessity that large internal strains
are set up when the wood shrinks, and were it not for its plasticity
it would rupture. There is an enormous difference in the total amount
of shrinkage of different species of wood, varying from a shrink