ess shrinkage than slow drying at
high temperatures, but is apt to cause case-hardening and
honeycombing, especially in dense woods.

6. Case-hardening, honeycombing, and cupping result directly
from conditions 1, 4, and 5, and chemical changes of the
outer surface.

7. Brittleness is caused by carrying the drying process too
far, or by using too high temperatures. Safe limits of
treatment vary greatly for different species.

8. Wood absorbs or loses moisture in proportion to the
relative humidity in the air, not according to the
temperature. This property is called its "hygroscopicity."

9. Hygroscopicity and "working" are reduced but not
eliminated by thorough drying.

10. Moisture tends to transfuse from the hot towards the
cold portion of the wood.

11. Collapse of the cells may occur in some species while
the wood is hot and plastic. This collapse is independent of
subsequent shrinkage.

Theory of Kiln-drying

The dry kiln has long since acquired particular appreciation at the
hands of those who have witnessed its time-saving qualities, when
practically applied to the drying of timber. The science of drying is
itself of the simplest, the exposure to the air being, indeed, the
only means needed where the matter of time is not called into
question. Otherwise, where hours, even minutes, have a marked
significance, then other means must be introduced to bring about the
desired effect. In any event, however, the same simple and natural
remedy pertains,--the absorption of moisture. This moisture in green
timber is known as "sap", which is itself composed of a number of
ingredients, most important among which are water, resin, and albumen.

All dry kilns in existence use heat to season timber; that is, to
drive out that portion of the "sap" which is volatile.

The heat does not drive out the resin of the pines nor the albumen of
the hardwoods. It is really of no advantage in this respect. Resin in
its hardened state as produced by heat is only slowly soluble in water
and contains a large proportion of carbon, the most stable form of
matter. Therefore, its retention in the pores of the wood is a
positive advantage.

To produce the ideal effect the drying must commence at the heart of
the piece and work outward, the moisture being removed from the
surface as fast as it exudes from the pores of