BENTNS
Smectite clay, mostly montmorillonite, from weathered volcanic ash · A clay body plasticiser, added in very small amounts · Sub-micron particles, about ten times finer than kaolin · Raises dry strength as well as plasticity · Disperses when blended dry; clumps if stirred into water · Carries some iron — can show as specks in a light-coloured body
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Bentonite is a smectite clay formed from weathered volcanic ash, with particles about ten times finer than kaolin. That fineness is why such a small addition makes a clay body noticeably more plastic to throw and handle.
Bentonite is a smectite clay, mostly montmorillonite, formed from weathered volcanic ash. Its particles are sub-micron, about ten times finer than kaolin, and that fineness accounts for most of what it does. There are far more particles in a given weight of it than in the same weight of kaolin or ball clay, so a recipe calls for a fraction of the amount, and it takes up a great deal more water. A very small addition raises a clay body's plasticity and its dry strength. More than that makes the body sticky to work and raises its drying shrinkage, which is what keeps the additions small. It disperses when it is blended with the dry materials before any water goes in; stirred into water, or into a glaze that is already mixed, it clumps into lumps that are very hard to break up again. It carries some iron, which can show as specks in a light-coloured body.
