Thermal modification of colour in red alder wood
Creators: name: Thompson, Derek W.
Publisher: name: University of British Columbia
Year: 2009

description: 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.