piece of
wood to its surface has not as yet been fully determined, although it
is one of the most important factors which influence drying. This must
involve a transfusion of moisture through the cell walls, since, as
already mentioned, except for the open vessels in the hardwoods, free
resin ducts in the softwoods, and possibly the intercellular spaces,
the cells of green wood are enclosed by membranes and the water must
pass through the walls or the membranes of the pits. Heat appears to
increase this transfusion, but experimental data are lacking.

It is evident that to dry wood properly a great many factors must be
taken into consideration aside from the mere evaporation of moisture.

Losses Due to Improper Kiln-drying

In some cases there is practically no loss in drying, but more often
it ranges from 1 to 3 per cent, and 7 to 10 per cent in refractory
woods such as gum. In exceptional instances the losses are as high as
33 per cent.

In air-drying there is little or no control over the process; it may
take place too rapidly on some days and too slowly on others, and it
may be very non-uniform.

Hardwoods in large sizes almost invariably check.

By proper kiln-drying these unfavorable circumstances may be
eliminated. However, air-drying is unquestionably to be preferred to
bad kiln-drying, and when there is any doubt in the case it is
generally safer to trust to air-drying.

If the fundamental principles are all taken care of, green lumber can
be better dried in the dry kiln.

Properties of Wood that affect Drying

It is clear, from the previous discussion of the structure of wood,
that this property is of first importance among those influencing the
seasoning of wood. The free water way usually be extracted quite
readily from porous hardwoods. The presence of tyloses in white oak
makes even this a difficult problem. On the other hand, its more
complex structure usually renders the hygroscopic moisture quite
difficult to extract.

The lack of an open, porous structure renders the transfusion of
moisture through some woods very slow, while the reverse may be true
of other species. The point of interest is that all the different
variations in structure affect the drying rates of woods. The
structure of the gums suggests relatively easy seasoning.

Shrinkage is a very important factor affecting the drying of woods.
Generally speaking, the greater the shrinkage the m