Colour change of photodegraded spruce wood by water leaching
Spruce samples (Picea abies Karst.) were irradiated by strong ultraviolet (UV) light emitter mercury lamp and another series of specimens were treated with the combination of UV radiation and water leaching. The total duration of UV radiation for both series of specimens was 50 days. The colour parameters (CIE L*, a*, b*) were measured and evaluated after both UV radiation and water leaching. The increase of redness value was two times greater than the yellowness increase based on the initial value at the end of 50 days treatment. The leaching partly removed the yellow and red chromophore molecules generated by the UV radiation. The samples become slightly lighter after water leaching. The leached samples increased slightly more in yellowness and redness during the first 4-6 days of UV radiation than those of the dry series. The change of surface roughness of the specimens was monitored by diffuse reflectance infrared spectroscopy, because the change of roughness alters the light scattering properties of the surface. The roughness of the samples increased during the UV irradiation, and the tendency of roughness change mirrored that of yellowness change. The leaching reduced the roughness value in all examined cases. Good correlation was found between the hue angle and the lightness in both dry UV treatment and combined UV treatment and water leaching.