HPM-20 Bentonite powder (left) and National Standard 325 (right) powder samples fired to cone 9 oxidation. These have shrunk alot since cone 6, but are not yet melting.

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Only a small addition of this material, often 1–5%, can dramatically increase the plasticity and dry strength of a porcelain or other clay body. Why not use one of the very white specialty plasticizers instead? Cost: they can be ten or even twenty times more expensive.
This is a typical Wyoming bentonite. It is so plastic that, to make these SHAB test bars without excessive drying cracks and warping, we mixed it 90:10 raw and calcined powders. These fired bars, cone 1 through 7 oxidation (bottom to top), reveal the baggage that comes with that plasticity. The material is iron-bearing and fires progressively from reddish brown to very dark brown as vitrification advances. At the higher temperatures, the surface becomes strongly glassy (because of the soluble salts it also contains). Yet, at an addition of only a few percent, its contribution of iron and other impurities to the total body is small enough that the loss of whiteness can be surprisingly modest. That is why an inexpensive, decidedly non-white bentonite can still be a very practical porcelain plasticizer.
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HPM-20 Volclay Bentonite
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