Colour change of UV-irradiated and heat-treated wood species during 14 years of storage in total darkness

Wood samples treated with ultraviolet (UV) radiation (200 h, 80 °C) and mild thermal treatment (200 h, 80 °C) were stored under laboratory conditions in complete darkness for 14 years. 15 wood species used by the carpentry industry were involved in the tests. Colour changes were monitored and presented using the CIE L*a*b* colour measurement system. Samples with low extractive content (e.g. spruce, poplar, maple and ash) presented the greatest increase in redness and yellowness during UV irradiation. Cherry, larch and American cherry showed the best stability against photodegradation. These tree species have the highest natural extractive content responsible for redness. Both UV irradiation and the subsequent natural ageing in dark conditions resulted in a greater increase in redness than in yellowness. Long-term storage in total darkness resulted in much greater redness and yellowness increases for UV irradiated samples than for slightly thermally treated samples. Thermal treatment at 80 °C followed by the long-term storage in darkness produced only small alterations in lightness, redness and yellowness.