Wood Colouring with Natural Dye Extracts. New Research and Perspectives

The aims of the present research were to test new types of natural dye extracts for wood as a possible alternative to synthetic dyes and to evaluate the colour changes after a period of one year of exposure to natural light, in indoor conditions. Three types of extracts using raw materials: nutshell from Juglans regia L. (E1), onion peels (E2), and dry plant of Chelidonium majus L. (E3) were employed. Spruce wood (Picea abies L. Karst) samples with dimensions of (120x80x5) mm were used. The dye aqueous solutions with a concentration of 5% (E1, E3) and 2.5% (E2) were obtained by solid-liquid extraction at 100°C for three hours. The extracts E1, E2, E3 were modified by the addition of mordant ferrous sulphate Fe2(SO4)3 3% and coded EM1, EM2, EM3 respectively. The wood was coloured by dipping procedure at 60° C for 30 min. Three replicates were employed for each type of dye solution. After conditioning, two replicates together with the uncoloured samples (M) were further exposed indoors, and one coloured sample remained unexposed, as a control. Colour measurements in the CIELab system were performed before and after each period of exposure (30, 60, and 365 days), and the colour differences ΔL, Δa, Δb, and ΔE were calculated. Generally, all the samples dyed with natural extracts had fewer colour differences compared with the uncoloured wood. The lower value for colour change was registered for the E2 extract, followed by the E3, and E1 extract, respectively. According to the exposure time, the colour differences values ΔE range correspond to a medium colour up to different colours in visual perception. For mordant dye extracts EM1, EM2, and EM3 the results indicated a different colour in visual perception. The ΔE values registered were even higher than the uncoloured samples (M). The results were in accordance with similar research in the field. The present study opens new research opportunities in wood finishing and new colouring perspectives with eco-friendly dyes.