By shrinkage is meant the amount of weight which a piece of wood loses in
passing from the green to the dry condition. Newly cut, or u green " wood, is
full of moisture; one-half, or sometimes more, of its weight is water, which
is held in the walls of the cells and between the cells. A large proportion
of this water must be removed before the timber is in shape to use, as green
wood is likely to decay.
The process by which this moisture is removed is known as " seasoning/* There
are two general methods of doing this:
Natural drying by air or Artificial or kiln drying.
Natural drying is done at the saw mill. The sawed boards are piled in such
a way that there is good circulation of air between them, and the pile sloped
at the top so that the water will run off quickly. The length of the process
depends upon the time of year, the weather, and the kind of lumber. In the dry
climate of the southwest it takes only two months for pine to dry in summer,
while in the damper climate of the Gulf coast cypress takes a year to dry. Lumber
dried in the natural way contains from 15 to 30 per cent of moisture.
In the artificial process of kiln drying, the work is carefully regulated
by principles which have been worked out, and the lumber is usually superior
to the air-dried. The two processes are often combined.
Besides losing moisture and consequently weight, seasoned wood is different
from green in other respects. It is stronger, stiffer, and harder, but not so
tough. It is less liable to shrink in subsequent usage.
Among the soft woods, cedar and white pine shrink the least, spruce somewhat
more, and long-leaf pine and tamarack the most.
Among the hard woods, locust, butternut, and black cherry shrink little, maple
somewhat more, and white oak, hickory, and birch the most. The hard lumber
requires more care in seasoning to prevent warping and checking or cracking.