Effects of thermal and chemical treatments on colourimetric characteristics of young African mahogany and eucalypts wood for sawn timber production

Abstract

This study aimed to evaluate the effect of boron treatment and thermal modification on the wood colour of African mahogany species (Khaya ivorensis, K. senegalensis and K. grandifoliola) and eucalyptus (Eucalyptus grandis × E. urophylla) intended for sawnwood production. For this purpose, four logs measuring 50 cm in length were sampled along the tree trunk of the selected species, and 3-cm-thick discs were collected at the base of each log, to evaluate the basic density and its variation along the stem. The logs were converted into samples of 2.3 cm × 10.0 cm × 47.0 cm (thickness × width × length), which underwent treatments consisting of the control (no treatment), chemical treatment (boric acid and borax) using a 4% solution in which the samples were immersed until reaching a minimum retention of 0.5%, expressed as boric acid equivalent, and thermal modification (180 and 200 °C), applied for 120 min at the maximum temperature. The basic wood density varied along the trunk of the trees, particularly in K. grandifoliola (0.450–0.410 g cm^-3), with K. senegalensis (0.542 g cm^-3) showing density values similar to those of E. grandis × E. urophylla (0.586 g cm^-3). Colourimetric analysis revealed that the treatments (chemical and thermal) promoted few variations in the colours (ΔE*) of the tangential (T) and radial (R) faces of the wood, except for the treatment at 200 °C, which caused darkening of the wood (K. ivorensis (ΔE*T = 10.01 − ΔE*R = 18.03), K. senegalensis (ΔE*T = 15.70 − ΔE*R = 21.25), K. grandifoliola (ΔE*T = 7.99 − ΔE*R = 11.76), and eucalyptus (ΔE*T = 9.50 − ΔE*R = 9.82)). However, the heat treatments standardised the color of the heartwood and sapwood.