Colour responses of two fast-growing hardwoods to two-step steam-heat treatments

Colour changes by thermal treatments are a good alternative to increase wood quality, especially for non-structural uses. In this study, rose gum (Eucalyptus grandis) and Sydney blue gum (Eucalyptus saligna) woods from Southern Brazil were thermally treated under different conditions (steam pre-treatment in an autoclave at 127 °C for 1 hour and heat treatment in an oven at 180, 200, 200 and 240 °C for 4 hours) and colour changes were investigated using CIEL*a*b* technique. The main findings show that thermal treatments resulted in darkening of the wood, which was more intense at high temperatures. Colour difference after two-step steam-heat treatments was higher than 45 for both species. In general, the influence of the steam pre-treatment was more visible in lower temperatures. Lightness (L*) was the parameter that best described the colorimetric behaviour of rose gum and Sydney blue gum woods. In conclusion, thermal treatments are a great method to improve the market value of wood, especially for aesthetic uses, but it requires a control of process conditions.