2015 年 79 巻 8 号 p. 404-412
The Takamatsuzuka tumulus was constructed in the 7th to 8th centuries at Asuka in Nara prefecture. The mural and wall surfaces are soiled and deteriorated by rainwater, mold, and other causes. The mural is painted on the stone wall that is coated with stucco. The stone walls are jointed with masonry joint stucco. The purpose of this investigation is to clarify the composition, crystal structure, and other material factors of the surface contaminant layer on the joint stucco. An X-ray computer tomography, an X-ray diffractometer, a scanning electron microscope, and a transmission electron microscope are utilized to examine the microstructure. The contaminant layer thickness is 0.2-0.3 mm. Kaolinite, muscovite, montmorillonite, hematite and a few silicates are detected from the contaminant layer. The compounds detected are constituents of soil-like clay, and most of them contain Fe. By transmission electron microscopy observation, the contaminant layer is found to consist of fine mineral grains, as mentioned above, most of which are acicular. The acicular grain size is approximately 0.2-1 μm in length and 0.015-0.2 μm in width. The grains are in a row parallel to the layer surface. The hematite grains are circular with diameters of 15-250 nm. In addition, iron titanate is observed. The yellowish brown color of the stucco surface is mainly due to iron ions contained in the above-mentioned compounds. It is thought that the grains deposited on the stucco surface from colloidal-like water containing the above-mentioned fine grains.
