| Summary: | Red alder has become one of the most widely traded hardwood species in North America, and sliced
red alder veneer is commonly applied as a decorative overlay on composite wood panels used by the
furniture and cabinet industries. Red alder wood, however, acquires a mottled red-orange color
following felling, which is undesirable when the wood is used for decorative purposes. The
discolouration is caused by the enzymatic oxidation of oregonin, a diarylheptanoid xyloside that
occurs naturally in red alder wood. Heating red alder wood remedies this problem to some extent, but
there is still an unacceptable level of variability in the color of veneer sliced from heated veneer cants.
Industry experience suggests that some of the variability in colour is caused by seasonal changes
within red alder wood, the effect of log storage prior to heat treatments, and the type of wood used
(inner/outer sapwood and position in the stem). This study attempted to clarify the causes of
variability in colour of thermally modified red alder wood. First, the effect of various treatment
temperatures (30, 50, 70 and 90°C) and treatment durations (8, 24, 36, 48 and 72 hours) on colour in
red alder was examined. Secondly, the effects of season, storage and wood type on colour in
thermally modified red alder wood were examined. In both experiments, heat was applied under
isothermal conditions. Initial experimentation found that increasing treatment temperature resulted in
wood that was darker and less red. Heating the wood at 70°C for at least 24 hours produced a uniform
colour from pith to bark. Our findings suggest that the final colour of the wood depends on the
strength of reactions that produce red-orange chromophoric groups in the wood, thermal darkening of
the wood, and destruction of red-orange chomophoric groups. Subsequent experimentation revealed a
highly significant interactive effect of season and storage on the colour of heat treated wood. Wood
colour (following heat treatment) in samples harvested in spring and summer was dependent on the
length of storage time prior to processing. When stored for up to two weeks, spring and summer
samples became noticeably redder and darker, followed by an increase in lightness and decrease in
redness when stored for 4 weeks. Fall and winter harvested samples were less affected by the length
of storage and maintained colour within industry preferences. In both experiments, wood type
(inner/outer sapwood) had a significant effect on the colour of heat treated wood and the inner
sapwood was darker than the outer sapwood when heated at 70°C. The position of the sample along
the stem also had a significant effect, but the colour change was small and indiscernible to the human
eye. The findings from this study should provide the basis for further research required to develop
differential heating and storage schedules in order to minimise seasonal variability in the colour of red
alder. Also, alternative heating methods should be tested to reduce thermal gradients throughout the
wood during heat treatments. === Forestry, Faculty of === Graduate
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