well seen when drying rough lumber
but becomes obvious when planing.
This discoloration bleeds through
some water-based finishes, especially
noticeable on light colored paint or
Causes. Naturally occurring starches
and sugars in the wood will begin to
oxidize as soon as the tree is cut down.
The oxidation colors are different than
the natural wood color. This oxidation is fairly slow, but is accelerated by
heat (roughly, each 20F warmer means
twice as fast). In order for the oxidation to occur, an enzyme in the wood
(enzymes are protein molecules and
are not alive) facilitates the chemical
reaction. The oxidation reaction is a
multi-step reaction that occurs mainly
at high moisture contents, with the
final step being the development of a
The apparent requirements for
oxidation are warm temperatures
within the wood (70F or warmer, but
not above 130F, as the hot temperature deactivates the enzyme. (High
temperatures cause another darkening
reaction, however, so that is why we do
not use high temperatures in drying.);
moisture over 45 percent MC; which
means the humidity in the air is likely
over 80 percent RH; time (at least
one day) at high moisture and warm
Special note: Rain water on drying
lumber also causes an appearance
similar to oxidation stain. This effect is
likely the result of migration and concentration of water soluble chemicals
at or near the surface of the lumber
when drying is slow.
Cures. The initial oxidation reactions
can occur within the log, especially if
the log is stored in warm conditions.
Sprinkling water should be cold. Rapid
drying of lumber can control stain by
eliminating the necessary high moisture. However, fast drying does not
reverse reactions that occur previously;
that is, the further the reaction has
proceeded before fast drying starts,
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