Clay stoneware pottery. Unglazed kibisho (kyusu) is common. Stoneware made of plastic clay containing relatively large amounts of iron oxide and fired at the 6th to 8th firing temperature of a Zegel pyramid often changes color depending on the nature of the fire in the kiln. Normally, this type of product is not glazed, but due to the strong solubility of the raw material, the surface of the product begins to melt to a certain degree, giving it a lustrous appearance and a beautiful appearance. There are two types of coloring: red and blackish red or purple-brown. The former is called “red mud” and the latter is called “purple mud. The Chinese Yixing kilns have long been famous for their vermilion and purple muds, and other varieties such as white mud, oolong mud, and pear-skin mud have also been seen, but most of these are used for teapots. The famous kyusu are not wheel-thrown, but rather are made of thin sheets of clay and finished in a ten-thousand-yen shape. In Japan, vermillion-dyed kyusu are produced by Owari Tokoname ware, Bizen Ibe ware, Ise Banko ware, Mino Onkyo-Shai ware, and Sado Mumyoi ware. In 1878, Tokoname invited a Chinese potter, Jin Shiheng, to learn Yixing’s techniques, and Sugie Jumon and others produced good pieces. All the others are mainly wheel-thrown work.
The raw materials used to make vermilion mud and purple mud are all natural yellow, red, reddish brown, or dark rat-colored clays produced in lowlands, sometimes containing wood chips and other impurities, but rarely containing sand. Therefore, it is not impossible to mold the clay as it is naturally produced, but it must be watered when it comes to refined products. Because the plasticity of this type of raw material is very strong, fine powder settles very slowly in water. Also, after production, the product must be dried as gradually as possible due to its high shrinkage rate, or it may warp, and must be heated slowly even during firing in the kiln. If heated too quickly by mistake, the substrate can quickly rupture and shatter, or the air bubbles within it can expand, causing convexities on the surface or distorting the entire shape. If the firing is mainly oxidizing and the fire is relatively weak, the color of the product will be red, so-called vermilion mud, and if the fire is slightly strong and the reduction is moderate, purple mud will be produced.
Shudetuchi (red clay)
Shudetuchi is the clay used for Tokoname and Sado Shudo ware, which is mined from rice field soil and carefully watered. Shudo clay is unglazed, so the surface of the clay must be smooth. For this reason, it is carefully watered. In particular, it is essential that the clay contain about 5% pure iron hydroxide in order for the iron content in the clay to be oxidized and fired to produce vermilion mud. If the clay contains two kinds of iron, the vermilion color will not be good, and if it contains even a small amount of other metals in addition to iron, it will have a blackish color. Since clay containing iron hydroxide is alkaline, the firing temperature is low, and watered red clay is characterized by being fired at temperatures of up to 1,000 degrees Celsius.
