香川県に分布する豊島石製石造文化財の風化程度の評価

Weathering grade of Teshima stone, Miocene lapilli tuff, used in lithic cultural heritage in Kagawa Prefecture, Japan was evaluated by using colorimeter, magnetic susceptibility meter, moisture meter and schmidt rock hammer. The stone pagodas have been exposed to weathering for 200 - 600 years, and have developed surface erosion due to weathering. Basaltic rock fragments in Teshima stone are slightly weathered based on necked-eye observation with increasing weathering period. L*-value of color index increased with increasing weathering period, however a*- and b*-values of color index and magnetic susceptibility are constant with increasing weathering period. Increasing of L* value is derived from weathering of basaltic rock fragments in Teshima stone. The results of measurement of a*-, b*- values and magnetic susceptibility suggest that mineralogical weathering such as iron mineral concentration has not occurred. The hardness of excavated cave wall surface on quarry of Teshima stone using schmidt rock hammer decreases in the entrance of the cave. Weathering properties of Teshima stone are characterized by surface erosion and rapid decreasing of rock surface hardness.