色彩値変化からみた来待石の風化速度

The Kimachi sandstone, occurred on the southern coast of Shinji Lake, Shimane, Japan, is a stone material composed of medium-grained tuffaceouS sandstone of the Miocene Omori Formation. It has been quarried for processing into various household utensils and stone lanterns since the Burial Mound Period. The sandstone is homogeneous, isotropic, soft, and not brittle in its mechanical properties, and therefore easy to carve with traditional chisels and other tools. However, the sandstone's rapid weathering and deterioration is a serious shortcoming as a stone material.<BR>A distinguishing feature of the weathering of this sandstone is a rapid change of rock color from bluish gray to brownish, reflecting oxidative changes in the iron components of the sandstone. Therefore, the change of color of the rock surface over time can serve as an indicator of the weathering rate of the sandstone.<BR>Rock color values a* and b* (CIE L*b* color measurement system.) of the surface of two samples of the sandstone were repeatedly measured with a colorimeter (Minolta CR-800). The Samples were kept unde natural room temperature and humidity conditions in a laboratory during more than two years, and their color changes with time were analyzed.<BR>The repeated measurements of the color values showed that both a* and b*values initially increased rapidly with time, but their rates of increase slowed with time. Here, the positive b* value, which expresses the yellow color component, was used as the major index of the change of rock color from bluish gray to brownish. The change in the b* value with time t (day) was graphically determined and could be described by the equation b* =2.35logt+4.40.<BR>The empirical relation between uniaxial compressive strength q_u (MPa) of cyliIldrical specimens of the salldstone and the b* values measured on their surfaces was also determined. From this relation and using the equation mentioned above, the decreasing value of q_u (MPa) over time t could be expressed as q_u=40-4.0logt. According to this equation, it takes about 102 years from the time that Kimachi sandstone is exposed to attain the level of weathering resulting in a 50% reduction in its strength.<BR>The changes in color values a* and b* were not independent during the weathering process; the two values were related to each other linearly. The slopeΔb*/Δa* of the line graphically determined from the linear relation was around 3.5, which corresponds to the gradient of color values during the formation of goethite, which is brownish, as a result of oxidation of iron at normal temperature. Therefore, the brownish color of the sandstone may be attributable to the formation of iron compounds by oxidation of iron dissolved from the numerous andesite fragments and chlorite that compose the sandstone.<BR>The rock color changes with time not only at the rock surface, which is in direct contact with the atmosphere, but also within the rock. To obtain information on the rate at which oxidation proceeds inward from the outer surface, color values were compared between the outer surface of a rock specimen and the faee of a cut plane of the same specimen, which was cut 59 days after the first color measurement. Consequently, slight change of color values was recognized an the cut plane. This means that the oxidation attains at the depth of 2 cm from the surface during 59 days.