Thermal Modification of Color in Red Alder Veneer. I. Effects of Temperature, Heating Time, and Wood Type

Abstract

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 orange color following felling, which is undesirable when the wood is used for decorative purposes. 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. This study examined the variation in color of red alder wood samples cut sequentially from the pith to the bark and subjected to heating under isothermal conditions. The aim was to examine whether within-tree variation in the susceptibility of red alder wood to thermal darkening can explain variation in color of veneer sliced from steamed red alder cants, and to determine the optimal thermal treatment (temperature and time) that can impart the tan color to red alder wood that industry is seeking. Results indicated that there was within-tree variation in the color of red alder samples following thermal treatment, but differences were pronounced only when wood was heated at a low temperature. Wood close to the bark tended to be redder than wood close to the pith when heated at 30°C, but such a difference was absent in wood heated at higher temperatures (50-90°C). Heating red alder wood, in vitro, at 70°C for 36 h produced wood that was evenly colored from pith to bark and matched the current industry color preference. It is suggested that the color of thermally modified red alder wood depends on the strength of reactions that produce orange/red chromophores in the wood, thermal darkening of the wood, and destruction of orange/red chromophores.