Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 55

exactly the same results on the strength of the wood. Wood increases
in strength with the elimination of moisture content. This may ac-
count for the claim that kiln-dried lumber is stronger than air-dried
lumber. This has little significance because, in use, wood will come to
practically the same moisture content whether it has been kiln-dried
or air-dried.
The same kiln-drying process cannot be applied to all species
of wood. Consequently, it must be remembered that lack of certain
strength properties in wood may be because of improper kiln-drying.
Similar damage also may result from air seasoning under unsuitable
conditions.
Sapwood Versus Heartwood
The belief is common that in some species the heartwood is stronger
than the sapwood, and that the reverse is the case in such species
as hickory and ash. Tests have shown conclusively that neither is
the case, sapwood is not necessarily stronger than heartwood or
heartwood stronger than sapwood, but that density (rather than
other factors) makes the difference in strength. In trees that are ma-
ture, the sapwood is frequently weaker, whereas in young trees the
sapwood may be stronger. Density, proportion of springwood and
summerwood, then must be the basis of consideration of strength
rather than whether the wood is sapwood or heartwood. Under un-
favorable conditions, the sapwood of most species is more subject
to decay than the heartwood.
The cells in the heartwood of some species are filled with vari-
ous oils, tannins, and other substances (called extratives ) that make
these timbers rot-resistant. There is practically no difference in the
strength of heartwood and sapwood if they weigh the same. In most
species, only the sapwood can be readily impregnated with preser-
vatives, a procedure used when the wood will be in contact with the
ground.
Defects and Deterioration
The defects found in manufactured lumber have several causes.
Those defects found in the natural log include the following:
Shakes
Knots
Pitchpockets
Those caused by deterioration include the following:
Rot
Dote