Bentonite supplies SiO2, Al2O3, Fe2O3, TiO2, and small amounts of several fluxes; bentonite helps to suspend glazes. [Generic bentonite; map to a particular type of bentonite, or use another formula. Typical density 172.0 g/c]
Bentonites contain large amounts of montmorillonite clay minerals, produced by the alteration of volcanic ash in marine environments. Montmorillonites also result from hydrothermal alteration of Mg/Fe-rich igneous rocks. Theoretical formula for montmorillonite is Al2Si4O10 (OH)2 nH2O where n is around 2-4. Mg replaces some of the Al, and Na and Ca ions hang around in the interlayer space, making for a very complex molecular structure. Bentonite consists of extremely small particles, most less than one micron. Bentonites are on the order of ten times finer than ball clays. There are two general classes of bentonites – the CaO-rich bentonites that do not swell much in water, and the CaO-poor, Na2O-rich bentonites that swell considerably.
Bentonite is the most plastic material used in ceramics. Bentonites are used to add plasticity to clay bodies and help to suspend glaze ingredients. Should be mixed with warm water before adding to clay or glaze, or mixed thoroughly with other dry ingredients.
Bentonites are so-called because they were first discovered in 1896 in Fort Benton, WY.
(courtesy of glazy.org)
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