Montmorillonite, Bentonite

A clay mineral of extremely small particle size and high plasticity. Raw bentonite is generally a pale green, buff, cream, or grey material composed of the clay mineral montmorillonite. It is used to plasticize clay and porcelain bodies, often used in amounts of 1-5%.

Its origin can be traced to ancient volcanic eruptions where fine volcanic ash particles were carried by winds and deposited in discrete layers which altered over time from the glassy state to claystone. In North America, bentonites are mined in Montana, Wyoming, South Dakota and Saskatchewan and used in things like porcelain, toothpaste, tablets, cosmetics, oil drilling mud, oven cleaners, insecticides, putty, paint, ink, paper, polishes, cleansing agents, explosives, detergents, plastics and rubber.

Related Information

Firing shrinkage variation between various clays

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and drying shrinkages vary widely. Materials normally acting as fluxes (like dolomite, talc, calcium carbonate) are refractory here because they are fired in the absence of materials they react normally with.

An example of a DFAC drying test of a bentonitic clay

This disk has dried under heat (with the center part protected) for many hours. During that process it curled upward badly (flattening back out later). It is very reluctant to give up its water in the central protected section. Obviously it shrinks alot during drying and forms a network of cracks. When there are this many cracks it is difficult to characterize it, so a picture is best.

A few drops of water on top of a tiny pile of bentonite powder.

Notice the water just sits there in a little lake. It does not soak in because the bentonite gels in contact with the water and that gel acts as a barrier. This water-barrier property of bentonite is a key to its use in many products but can be a problem in ceramics (because it slows down the drying speed of bodies and glazes that contain it).

Links

Materials	Bentonite Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
Materials	Big Horn CE 200 Bentonite
Materials	Bentone MA
Materials	Gelwhite H
Materials	Mineral Colloid BP
URLs	http://en.wikipedia.org/wiki/Montmorillonite Montmorillonite at Wikipedia
URLs	http://www.minersoc.org/pages/gallery/claypix/smectite/A-rose.html Montmorillonite micrograph at minersoc.org
URLs	http://www.handbookofmineralogy.org/pdfs/Montmorillonite.pdf Montmorillonite mineralogy
URLs	http://www.sorptive.org/ Sorptive Mineral Institute for producers of absorbent clay mineral products
Minerals	Attapulgite, Palygorskite Attapulgite is a magnesium aluminum silicate clay of very fine particle size. It is also known as Fu
Minerals	Hectorite An uncommon fine-grained highly plastic clay mineral related to bentonite. Named after Hector, CA. F
Minerals	Smectite A highly plastic clay mineral related to montmorillonite (bentonite), more correctly, the name of th
Typecodes	Clay Minerals The property of plasticity is evident in a wide array of materials of different mineralogy. In addition, many minerals are considered parent minerals to clays, they weather down to become these clays.
Glossary	Clay What is clay? How is it different than dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.

By Tony Hansen
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