alls
begin to yield up their moisture. Heat raises the absorptive power of
the fibres and so aids the passage of water from the interior of the
cells. A confusion in the word "sap" is to be found in many
discussions of kiln-drying; in some instances it means water, in other
cases it is applied to the organic substances held in a water solution
in the cell cavities. The term is best confined to the organic
substances from the living cell. These substances, for the most part
of the nature of sugar, have a strong attraction for water and water
vapor, and so retard drying and absorb moisture into dried wood. High
temperatures, especially those produced by live steam, appear to
destroy these organic compounds and therefore both to retard and to
limit the reabsorption of moisture when the wood is subsequently
exposed to the atmosphere.

Air-dried wood, under ordinary atmospheric temperatures, retains from
10 to 20 per cent of moisture, whereas kiln-dried wood may have no
more than 5 per cent as it comes from the kiln. The exact figures for
a given species depend in the first case upon the weather conditions,
and in the second case upon the temperature in the kiln and the time
during which the wood is exposed to it. When wood that has been
kiln-dried is allowed to stand in the open, it apparently ceases to
reabsorb moisture from the air before its moisture content equals that
of wood which has merely been air-dried in the same place, and under
the same conditions, in other words kiln-dried wood will not absorb as
much moisture as air-dried wood under the same conditions.

Difference between Seasoned and Unseasoned Wood

Although it has been known for a long time that there is a marked
difference in the length of life of seasoned and of unseasoned wood,
the consumers of wood have shown very little interest in its
seasoning, except for the purpose of doing away with the evils which
result from checking, warping, and shrinking. For this purpose both
kiln-drying and air-seasoning are largely in use.

The drying of material is a subject which is extremely important to
most industries, and in no industry is it of more importance than in
the lumber trade. Timber drying means not only the extracting of so
much water, but goes very deeply into the quality of the wood, its
workability and its cell strength, etc.

Kiln-drying, which dries the wood at a uniformly rapid rate by
artificially heating it in inclosed rooms,