This paper presents the differences in the color changes of unsteamed and steamed beech wood (Fagus sylvatica L.) caused by long-term exposure to sunlight on the surface of wood in interiors for 36 months. The light white-gray color of the yellow tinge of native beech wood darkened under the influence of sunlight, and the wood took on a pale brown color of yellow tinge. The degree of darkening and browning is quantified by the value of the total color difference ∆E* = 13.0. The deep brown-red color of steamed beech under the influence of sunlight during the exposure brightened, and the surface of the wood took on a pale brown hue. The degree of lightening of the color of steamed beech wood in the color space CIE L*a*b* is quantified by the value of the total color difference ∆E* = 7.1. A comparison of the color changes of unsteamed and steamed beech wood through the total color difference ∆E* due to daylight shows that the surface of steamed beech wood shows 52.2% smaller changes than unsteamed beech wood. The lower value of the total color difference of steamed beech wood indicates the fact that steaming of beech wood with saturated water steam has a positive effect on the color stability and partial resistance of steamed beech wood to the initiation of photochemical reactions induced by UV-VIS wavelengths of solar radiation. Spectra ATR-FTIR analyses declare the influence of UV-VIS components of solar radiation on unsteamed and steamed beech wood and confirm the higher color stability of steamed beech wood.
Keywords: ATR-FTIR spectroscopy; beech wood; natural aging; saturated water steam; thermal treatment; wood color.