TECHNICAL NOTE NUMBER
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN ISE December 1952
COMPARATIVE STRENGTH, OFIAIR-DRIED ^, I
AND KILN-DRIED WO D
Some wood users claim that kiln-dried wood is brash and not equaI
strength to wood that is air dried. Others advance figures purporting
to show that kiln-driedwood is much stronger than air-dried wood. But
some 150, 000 comparative strength tests, made by the Forest Products
Laboratory, of the U. S. Forest Service, on kiln-dried and air-dried
specimens of 28 common species of wood show that good kiln drying and
good air drying have the same effect upon the strength of the wood.
The belief that kilndrying produces stronger wood than air drying is usu-
ally the result of failure to consider differences in moisture content. The
moisture content of wood onleaving the kiln is generally from 2 to 6 per-
cent lower than that of thoroughly air-dried wood. Since wood rapidly
increases in strength with loss of moisture, higher strength values may
be obtained temporarily from kiln-dried wood that has not come into equi-
librium with the atmosphere than would be obtained from air-dried wood.
Such a difference in strength has no real significance, however, since
in use apiece of wood will come to practically the same moisture condi-
tion whether it is kiln dried or air dried.
Itmustbe emphasizedthat the appearance of thedriedwoodis nota reli-
able criterion of the effect the drying process has had upon its strength.
The strength properties maybe seriously injured by the use of excessive
temperatures without visible damage to the wood. Also, it has been
found that the same kiln-drying process cannot be applied with equal suc-
cess to all species. To insure uninjured kiln-dried material, an effici-
ent kiln, a skilled operator with a knowledge of the correct kiln condi-
tions to use with a stock of a given species, grade, and thickness, and
a record showing that no more severe treatment has been employed, are
SCHOOL Of FISTY LtRY
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