Bentonite is a general term for clay-like materials composed mainly of montmorillonite metamorphosed from volcanic ash or tuff.
Bentonite is formed by hydrothermal, hot-spring, and weathering effects of groundwater on the volcanic glass materials that make up tuff and volcanic ash. Generally, quartz, cristobalite, boiling stone, feldspar, gelatinous silicic acid, and organic matter are also present, and the color of the raw soil varies from white, yellow, grayish white, reddish brown, greenish blue, yellowish orange, and black. Most swell in water, but some do not. Those that swell are usually called bentonite or swollen clay, while those that do not swell and show acidity in water are called acid white clay in Japan. Bentonite is used as a plasticizer and strength enhancer for ceramic and refractory clay, and is also added to casting mud to increase its drying strength. It should not be added to ceramics in amounts greater than 2 to 5 percent because the iron content affects the firing color. (Ceramic Raw Materials)
