lation or, at
best, on the basis of incomplete data.

Until these problems are solved, kiln-drying must necessarily remain
without the guidance of complete scientific theory.

A correct understanding of the principles of drying is rare, and
opinions in regard to the subject are very diverse. The same lack of
knowledge exists in regard to dry kilns. The physical properties of
the wood which complicate the drying operation and render it distinct
from that of merely evaporating free water from some substance like a
piece of cloth must be studied experimentally. It cannot well be
worked out theoretically.

SECTION X

HOW WOOD IS SEASONED

Methods of Drying

The choice of a method of drying depends largely upon the object in
view. The principal objects may be grouped under three main heads, as
follows:

1. To reduce shipping weight.

2. To reduce the quantity necessary to carry in stock.

3. To prepare the wood for its ultimate use and improve its
qualities.

When wood will stand the temperature without excessive checking or
undue shrinkage or loss in strength, the first object is most readily
attained by heating the wood above the boiling point in a closed
chamber, with a large circulation of air or vapor, so arranged that
the excess steam produced will escape. This process manifestly does
not apply to many of the hardwoods, but is applicable to many of the
softwoods. It is used especially in the northwestern part of the
United States, where Douglas fir boards one inch thick are dried in
from 40 to 65 hours, and sometimes in as short a time as 24 hours. In
the latter case superheated steam at 300 degrees Fahrenheit was forced
into the chamber but, of course, the lumber could not be heated
thereby much above the boiling point so long as it contained any free
water.

This lumber, however, contained but 34 per cent moisture to start
with, and the most rapid rate was 1.6 per cent loss per hour.

The heat of evaporation may be supplied either by superheated steam or
by steam pipes within the kiln itself.

The quantity of wood it is necessary to carry in stock is naturally
reduced when either of the other two objects is attained and,
therefore, need not necessarily be discussed.

In drying to prepare for use and to improve quality, careful and
scientific drying is called for. This applies more particul