•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com

Sign-up at https://insight-live.com today.

Mineral: 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.

Firing shrinkage variation between various clays

Firing shrinkage variation between various clays

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and diring 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

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.

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).

Out Bound Links

In Bound Links

  • (Materials) Bentonite

    Montmorillonite, Bentonite USA

  • (Materials) Big Horn CE 200 Bentonite - Wyoming Bentonite
  • (Minerals) Attapulgite, Palygorskite

    Attapulgite is a magnesium aluminum silicate clay of very fine particle size. It is also known as Fullers Earth and is closely related to Sepiolite mineral. Unlike bentonite (or montmorillonite) attap...

  • (Materials - Parent mineral) Bentone MA - Glaze Suspender, Hectorite


  • (Minerals - Related) Hectorite

    An uncommon fine grained highly plastic clay mineral related to bentonite. Named after Hector, CA. From volcanic sources.

  • (Minerals - Related) Smectite

    A highly plastic clay mineral related to montmorillonite (bentonite), more correctly, the name of the group on minerals that includes montmorillonite and other similar minerals. The following was o...

  • (Materials - Parent mineral) Gelwhite H - Highly refined bentonite plasticizer
  • (Materials - Parent mineral) Mineral Colloid BP

By Tony Hansen

Feedback, Suggestions

Your email address


Your Name


Copyright 2003, 2008, 2015 https://digitalfire.com, All Rights Reserved