The Effect of Hydrothermal Treatment on the Color Stability and Chemical Properties of Birch Veneer Surfaces

The effects of hydrothermal pre-treatment on the color of silver birch veneer were determined. Spectrophotometric and chemical analyses of the veneer by means of extraction were conducted to determine the detailed chemical characteristics of the veneers. Results from spectrophotometric analysis revealed a significant increase in lightness (L*) and a decrease in yellowness (b*) in the veneer produced from a log soaked at 70 °C in short-term. Conversely, the effect of long-term storage resulted in a significant reduction in L* and an increase in b*. Chemical analyses of the veneer extract identified a moderate negative correlation between soluble proanthocyanidins content and treatment temperature. A higher amount of wood extractives, such as lipophilic extractives, free monosaccharides, and other organic substances, were obtained from wood material soaked at 70 °C than at 20 °C, which was most likely due to moderate chemical changes during treatment. Comparison of the results between dried veneers and wet veneers from the water extraction study revealed that the dried veneers had higher amounts of lipophilic matter and less water-soluble organic matter than the wet veneers. Other aspects of the loss of soluble proanthocyanidins in the birch veneer during plywood manufacturing are discussed.