This study deals with the investigation of impact of temperature and ultraviolet (UV) radiation on spruce wood (Picea abies (L.) H. Karst.) and fir wood (Abies alba Mill.) colour changes. Samples of investigated woods species were loaded by temperatures of 110, 130, and 150°C and UV radiation (with 253.7 nm wavelength and 40 W m-2 intensity) during 72, 168, 336 and 672 hours. Colour changes were evaluated in the CIE Lab colour space. The neural network for prediction of both colour coordinates and total colour difference of spruce and fir wood was trained by data regarding exposure conditions (temperature, UV radiation and time) and by obtained results. Coefficient of determination (R2) of the neural network was above 0.99 for training, validation and testing. Average colour coordinates (± standard deviation) of the spruce and fir wood before exposure were L* = 80.08 ± 3.70, a* = 7.55 ± 2.13, b* = 21.56 ± 1.79, L* = 80.46 ± 1.91, a* = 6.84 ± 0.97, and b* = 18.90 ± 1.26, resp. Total colour differences after thermal loading were in the interval from ΔEab* = 3.76 ± 1.95 (spruce wood at 110°C) to ΔEab* = 45.37±1.46 (fir wood at 150°C). Total colour differences of both wood species exposed by UV radiation were approximately in intervals from ΔEab* = 12 to 13 (after 72 h) up to ΔEab* = 16 to 20 (after 168 to 672 h). Obtained results proven that both temperature and UV radiation have significant impact on the colour changes of the investigated woods.
In this study, each of the commonly used two types of woods (softwood and hardwood) from five species was studied. All wood species show a systematic trend to change to higher values of surface roughness with natural weathering progress. The Black pine, Calabrian pine and beech wood samples show a more or less smooth trend, whereas basswood gives the highest surface roughness changes under all conditions. However, the hardwood species, except basswood, have higher hardness properties both initially and at the end of weathering process when compared to softwoods. The surface discoloration that occurs is clearly visible as a natural texturing. However, the degree of, and the pattern of texturing, may vary with different kinds of woods; the color changes also vary to some extent. It was revealed that the discoloration is strongly dependent on the botanical origin of wood species. The lower lightness changes (ΔL %) were found for all three pine species (16.2 to 37.2%) when compared to fir (54.9%) and spruce samples (91.8%). The Scotch pine wood showed highest values for the contribution of red color initially and low redness change on the surface after the weathering process, among the other softwood species.