Modeling color and chemical changes on normal and red heart beech wood by reflectance spectrophotometry, Fourier Transform Infrared spectroscopy and hyperspectral imaging

The use of beech is a key topic for Europe as it is one of the most important and abundant broad-leaf species. Physical, mechanical and esthetical features influence both the value and the usage of this wood in each commercial product. In this sense, the comprehension of the surface color modifications induced by solar irradiation is of crucial importance to define the commercial value of the beech wood. Starting from these general remarks, the aim of this work is to study the surface modifications on beech wood with and without red heartwood by different spectroscopic techniques and to obtain a modeling of the changes validated by rigorous statistical and chemometric methods together with principal component analysis. The artificial photo-irradiation of the wood samples was performed in a Solar Box. Reflectance spectrophotometry, Fourier Transform Infrared spectroscopy and hyperspectral imaging were used to assess artificial sunlight influence. The experimental data were statistically treated in order to evaluate their significance. Color monitoring allowed to find that the chromatic coordinates (L*a*b*) in normal wood and in red heartwood tended to similar values after 504 h of photo-irradiation. Fourier Transform Infrared spectroscopy showed the rate of photo-degradation of wood surface due to lignin oxidation and the statistical analysis allowed to demonstrate that red heart and normal wood have the same behavior. Concerning hyperspectral imaging (HSI), the detected spectral features were correlated to color changes in the Visible-Near Infrared (VIS-NIR) range (400-1000 nm) and to the variations of cellulose and lignin during accelerated aging in the Short-Wave Infrared (SWIR) range (1000-2500 nm). The most important result is that a correlation, validated by statistical analysis, of the color changes may be derived with the photo-degradation of wood components obtained by spectral analysis. This fact suggests the possibility to choose the reflectance spectrophotometry as a non-invasive, simple standard method to monitor the state of preservation of the wood surfaces.