HISTOLOGICAL-CHANGES OF SOFTWOOD SURFACES DURING OUTDOOR WEATHERING

Histological changes after exposure to natural weathering of hinoki (C hamaecyparis obtusa), sugi (Cryptomeria japonica) and western hemlock (Tsuga heterophylla) woods with different surfaces of grain or figure were evaluated. Furthermore, in sugi sapwood, the spectrum of solar li ght wavelengths reaching the test samples was restricted by placing it behind the selected light filters. On the surfaces of hinoki logs, ma ny small checks appeared parallel with each other, acompanying a few l arge and deep cracks during the early stage of weathering. These small checks occurred in the ray tissue as their starting points when both the solar spectrum of less than 500 nm and rainwater acted on the surf aces. Slitlike apertures of checks gradually spread outward by the col lapse of surface cells and cells adjacent to the checked surfaces. Aft er weathering for half a year, the photodegraded zone with grayish col or reached a 0.2 mm depth below the surface. The zone stayed at a cons tant depth even when the weathering period was prolonged. The brown zo ne with 0.5 mm thickness was found underneath the gray zone and kept i ts distinct difference in appearance from the upper zone. On the surfa ces of flat-sawn grain pieces, the occurrence of small checks varied w ith the annual ring direction. Many checks occurred starting from the ray tissue in the latewood on the typical tangential surface. On the s urface of edge grain pieces. the occurrence of checks was small except in western hemlock in which small checks star:ed in the initial part of the earlywood. The nature of the earlywood errosion of sugi was two fold, the initial thinning of cell walls with ultraviolet light follow ed by the collapse of the photodegraded cells. The collapse was caused by the action of the surface tension of water during the drying proce ss of the surface of the wood after rainfall. The most histological ch anges of surfaces were attributed to the cooperation of the solar ligh t less than 500 nm and the rainwater.