Colour in thermally modified wood of beech, Norway spruce and Scots pine. Part 1: Colour evolution and colour changes

Abstract

Colour evolution and colour changes were analysed from surface images of small specimens of three thermally-modified timber species using the CIE L*a*b* colour space. Upon heat exposure, the wood substance became orange and then approached grey irrespective of species; this was accompanied by a steady reduction in lightness. Colour changes were similar in the three woods at any given level of heat-induced weight loss (WL), whilst changes in the three coordinates of the CIE L*a*b* space in function of WL were different regardless of the wood species. For ΔL*, the profile was curvilinear and monotonous, while Δa* and Δb* bear a complex, non-linear profile. In turn, ΔE* was found to be highly influenced by the behaviour of ΔL*. It is proposed that ΔE* in thermally modified wood originates from chemical changes in the main wood polymers, more so in lignin than in polysaccharides, due to the darkening of the lignin itself. This was associated with the generation of chromophoric groups, mainly the increase in carbonyl groups appearing in the Fourier transform infrared spectra of lignin between 1710 and 1600 cm-1, particularly the emergence of quinone species.