Weather-induced colour change in wood: material response in different European locations

This study investigates the colour evolution of Norway spruce wood during one year of natural weathering across multiple European locations as part of a Round Robin experiment. Colour measurements ( CIE Lab ) were performed on samples with monthly-resolved exposure duration to quantify discolouration and determine total colour change ( ΔE ). The results show that weathering induces rapid colour modification, with the most pronounced changes occurring within the first month of exposure. A clear two‑stage behaviour was identified: an initial colour‑enrichment phase, characterised by increases in CIE a and CIE b , followed by progressive decolouration and greying, reflected primarily through decreases in CIE L . Although the fundamental mechanisms of photodegradation were consistent across all sites, the rate and extent of discolouration varied substantially with local climatic conditions. Samples exposed in Turkey exhibited the highest ΔE values, whereas those in the UK showed the lowest. This difference is consistent with expected climatic variations in solar radiation, moisture conditions, and temperature fluctuations between the two locations, which are known to influence weathering kinetics. The dataset provides a robust basis for linking colour change and its dynamics with site‑specific climate parameters enabling the development of a dedicated dose-response model.