Drift clay coprecipitates with lignite, lignite, silica sand, etc. The clay with a large amount of the former is called wood-bond clay, and the clay with a large amount of the latter is called frog clay. It sometimes contains uncarbonated woody debris, such as wood knot, among lignite fragments. This is why it is called “Mokubushi clay,” and there are three types: Shirokibushi, Aokibushi, and Kurokibushi. Shirokibushi is a type with little black color, aokibushi is a moistened and bluish version of shirokibushi, and kurokibushi is a blackish-gray version due to organic matter. The term “Mokubushi clay” usually refers to Kurokibushi, and what is called “Mochi” in the Owari region is a well-sticky, watery version of Kurokibushi. Mugi clay, Umekawa and Sakuragawa clay are all a type of wood-bonded clay. Ball clay in England is almost similar to wood-bond clay in properties. The major production areas of Mokujushi clay are Hai-do, Motoyama, and Akazu in Seto City, Aichi Prefecture; Hara, Ohata, and Ikuta in Tajimi City, Gifu Prefecture; Shimagahara in Ayama County, Mie Prefecture; Nagata in Ueno City; and Ikkiryeong in North Hamgyong Province, Korea. In terms of chemical structure, there is little difference between wood and frog-grain, but wood has greater plasticity and is more refractory. However, it has the disadvantage of large shrinkage during firing, which causes warping and cracking. The main constituent is kaolin minerals, but montmorinite and illite are also present in many cases. When these minerals are included, the hardening temperature decreases, but the plasticity improves. Wood-bonded clay with little lignite or silica sand is used as fireclay or binding clay in its raw state, but as a raw material for ceramics, hydrolyzed clay is used. (Ceramic Raw Materials Explanation Book)
