"Kawara" roof tile originated in China approximately 3000 years ago and was introduced with Buddhism into Japan in the Asuka period of 588. The fabrication process was improved by being acclimated to the higher Japanese temperature and humidity. The Japanese smoked roof tile "Ibushi-Kawara" then consisted of sintered basal clay plate and carbon (C) films deposited on it by furnace smoke from pine tree fuel. Although the smoking process is currently accomplished with hydrocarbon fuel gas in a computer-controlled furnace, the basic fabrication process has been handed down within the Kawara industry from the distant past.
The Ibushi-Kawara (hereafter Kawara) has the outstanding features of "metallic color" and "durability," as shown in Fig. 5-15. These features are due to the several-micro m-thick C films deposited on the basal clay. However, these features have not been investigated from a nano-structural point of view, and a state-of-the-art analysis of the C films has been needed by the Kawara industry to improve the Kawara quality. Therefore, an investigation of the C films on Kawara using synchrotron-radiation soft X-ray spectroscopy in collaboration with the Hyogo Prefectural Institute of Technology has been undertaken.
The investigation confirmed that the C films are essentially composed of nano-sized carbon black particles from the C K X-ray emission spectra shown in Fig. 5-16. Although typical carbon black does not show such dependence, the Kawara clearly shows the angle-dependent spectral feature. Such angle dependence means that the carbon black particles in Kawara partially form an oriented structure that is parallel to the basal clay surface. From the quantitative analysis of the angle-dependent spectral feature, the degree of the orientation is estimated to be 50% that of highly oriented pyrolytic graphite (HOPG). The structural models of Kawara C films are illustrated in Fig. 5-17; half of the C atoms form layer-structured clusters that give it the metallic color, and the remaining C atoms form random-structured clusters that make it durable.
C films on Kawara can be recognized as functional materials, and the traditional smoking process can be regarded as the Nano-Technology. The investigation of Kawara is being continued to develop an advanced method to evaluate its quality, aiming at industrial applications by synchrotron radiation science.
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