arly to the
hardwoods, although it may be required for softwoods also.

Drying at Atmospheric Pressure

Present practice of kiln-drying varies tremendously and there is no
uniformity or standard method.

Temperatures vary anywhere from 65 to 165 degrees Fahrenheit, or even
higher, and inch boards three to six months on the sticks are being
dried in from four days to three weeks, and three-inch material in
from two to five months.

All methods in use at atmospheric pressure may be classified under the
following headings. The kilns may be either progressive or
compartment, and preliminary steaming may or may not be used with any
one of these methods:

1. Dry air heated. This is generally obsolete.
2. Moist air.
_a._ Ventilated.
_b._ Forced draft.
_c._ Condensing.
_d._ Humidity regulated.
_e._ Boiling.
3. Superheated steam.

Drying under Pressure and Vacuum

Various methods of drying wood under pressures other than atmospheric
have been tried. Only a brief mention of this subject will be made.
Where the apparatus is available probably the quickest way to dry wood
is first to heat it in saturated steam at as high a temperature as the
species can endure without serious chemical change until the heat has
penetrated to the center, then follow this with a vacuum.

By this means the self-contained specific heat of the wood and the
water is made available for the evaporation, and the drying takes
place from the inside outwardly, just the reverse of that which occurs
by drying by means of external heat.

When the specimen has cooled this process is then to be repeated until
it has dried down to fibre-saturation point. It cannot be dried much
below this point by this method, since the absorption during the
heating operation will then equal the evaporation during the cooling.
It may be carried further, however, by heating in partially humidified
air, proportioning the relative humidity each time it is heated to the
degree of moisture present in the wood.

The point to be considered in this operation is that during the
heating process no evaporation shall be allowed to take place, but
only during the cooling. In this way surface drying and
"case-hardening" are prevented since the heat is from within and the
moisture passes from the inside outwardly. However, with some species,
notably oak, surface cracks appear as a