Colour Changes of Wood Veneer as a Function of Heat Treatment

Heat treatment is applied to wood to improve various properties of the material. The present study focuses on the colour changes of wood veneer samples due to heat treatment. Native wood species from Japan and Europe, such as Japanese oak (Quercus mongolica var. crispula), field maple (Acer campestre) and Scots pine (Pinus sylvestris) were used in the experiments. A laboratory-type oven was used to apply the heat at a temperature of 190°C, in the presence of oxygen, for different periods, gradually increasing from 5 to 40 min. The CIELab system (a colour space defined by the International Commission on Illumination) and Near Infrared Spectroscopy (NIR) were employed to evaluate the colour modifications on the samples. As expected, the heat treatment affected the colour of the samples. The lightness index decreased across the three wood species during the treatment. The chroma coordinates changed for pine and maple, while little change occurred in Japanese oak. The overall total colour differences reached their maximum at the final 40-min interval for all wood types. Based on the NIR evaluation, it was found that drastic thermal denaturation of cellulose was unlikely to occur, and the changes in the intermolecular interaction of water affected the colour of the specimens. The data and information of this study could be useful for industrial applications where the veneer of such species is desired. Such heat-treated veneers can be considered as value-added products in furniture manufacturing as well as restoration of furniture units where such veneer is used as an overlay.