Wood colour related to kiln drying

Abstract: Raised temperature in artificial drying has led to some changes of wood products such as colour and strength etc. For special treatment of wood-products with clearly modified properties, among them colour, is often high temperature and steam used industrially. Knowledge about the colour rendering from wood that is subjected to heat under humid conditions and/or artificial drying, especially kiln drying, is thus of interest. It is also of interest to understand the chemical background, for the colouring process, for future development of treatment and drying. The stability of the colour of wood exposed to UV/visible-light is also of concern when considering qualitative wood products. The colour response of Scots pine (Pinus sylvestris), Norway spruce (Picea Abies) and birch (Betula pubescens) was studied when exposed to heat and humid air, either as a hydrothermal treatment, where the wood contained capillary (free) water for certain times and temperatures, or a laboratory kiln drying. The wood subjected to hydrothermal treatment was also exposed to UV/visible-light to study the colour stability. The chemical background for the colour change during hydrothermal treatment was briefly investigated by using extraction, thin-layer chromatography (TLC) and UV/Visible-spectroscopy. The results show that multivariate modelling (PLS) of industrial wood-colour is possible as average values for the whole batch, "batch-colour". Quite strong colour changes (DE'ab > 10) were formed using hydrothermal treatment at 80-95ºC for 3-6 days. Furthermore, this quite strong colour formed is similarly stable when exposed to UV/Visible-light for 100 hours as untreated wood. Structure components (cellulose, hemicellulose and lignin) as well as extractive compounds, phenolic types, were both indicated to participate in the colour formation during hydrothermal treatment and the same is expected for kiln drying.