Discolouration of Artificially Irradiated Fir (Abies alba L.) Wood Extractives

This study investigates the influence of thermal treatment and subsequent artificial irradiation on the colour stability of fir (Abies alba L.) wood extractives. Wood samples were heat-treated at temperatures ranging from 100 °C to 280 °C for varying durations, and their extractives were isolated using Soxhlet extraction with an ethanol–toluene solvent mixture. The extract solutions were subjected to accelerated ageing under xenon light irradiation for up to 400 h, and colour changes were systematically monitored using a spectrophotometer in transmission mode. The amount and colour of extractives varied with treatment temperature. The extractive yields decreased up to 150 °C, increased between 150 °C and 240 °C, and declined again above 250 °C. The initial colours of the extract solutions ranged from yellow to brown, depending on the treatment conditions. During irradiation, colour differences (ΔEab) progressively decreased, and after 400 h, the solutions became nearly colourless, with lightness values approaching 100. The decline in b* values indicated a loss of yellow hue, while a* values shifted from red or green towards neutrality. These results confirm that fir wood extractives are highly sensitive to both heat and UV exposure, leading to significant discolouration and reduced colour saturation. These findings enhance our understanding of the mechanisms underlying fir wood colour changes and contribute to the broader knowledge of wood ageing processes.