Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 46

Reduces chance of attack by insects
Improves the capacity of wood to take preservative and fire-
retardant treatment and to hold paint
It is common practice at most softwood sawmills to kiln-dry
all upper-grade lumber intended for finish, flooring, and cut stock.
Lower-grade boards are often air-dried. Dimension lumber is air-
dried or kiln-dried, although some mills ship certain species without
seasoning. Timbers are generally not held long enough to be consid-
ered seasoned, but some drying may take place between sawing and
shipment, or while they are held at a wholesale or distributing yard.
Sawmills cutting hardwoods commonly classify the lumber for size
and grade at the time of sawing. Some mills send all freshly sawed
stock to the air-drying yard or an accelerated air-drying operation.
Others kiln-dry directly from the green condition. Air-dried stock
is kiln-dried at the sawmill, at a custom drying operation during
transit, or at the remanufacturing plant before being made up into
such finished products as furniture, cabinet work, interior finish,
and flooring.
Air-drying is not a complete drying process, except as preparation
for uses for which the recommended moisture content is not more
than 5 percent below that of the air-dry stock. Even when air-drying
conditions are mild, air-dry stock used without kiln-drying may have
some residual stress and set that can cause distortions after nonuni-
form surfacing or machining. On the other hand, rapid air-drying
accomplished by low relative humidities produces a large amount of
set that will assist in reducing warp during final kiln drying. Rapid
surface drying also greatly decreases the incidence of chemical and
sticker stain, blue stain, and decay.
Air-drying is an economical seasoning method when carried out
in a well-designed yard or shed, with proper piling practices (see
Figure 2-6) and in favorable drying weather. In cold or humid
weather, air-drying is slow and cannot readily reduce wood moisture
to levels suitable for rapid kiln-drying or for use.
In kiln drying, higher temperatures and fast air circulation are
used to increase the drying rate considerably. Average moisture con-
tent can be reduced to any desired value. Specific schedules are
used to control the temperature and humidity in accordance with
the moisture and stress situation within the wood, thus minimizing
shrinkage-caused defects. For some purposes, equalizing and condi-
tioning treatments are used to improve moisture content uniformity
and relieve stresses and set at the end of drying, so the material will
not warp when resawed or machined to smaller sizes or irregular