Bentonite

Bentonite is an aluminum phyllosilicate clay formed from weathered volcanic ash. Its microscopic plate-like particles have a strong affinity for water, allowing bentonite to absorb several times its own weight in water and swell into a gel-like suspension.

This swelling behavior is what makes bentonite so valuable in ceramics. Added to a glaze bucket at just 1 to 2 percent, bentonite holds all the heavier materials in suspension, preventing them from settling into a hard pack at the bottom. This gives potters time to apply the glaze evenly without constant stirring.

In clay bodies, bentonite dramatically improves plasticity. Even 1 to 3 percent bentonite can transform a short, crumbly clay body into something smooth and workable on the wheel. This is particularly useful for porcelain bodies, which naturally lack plasticity.

Bentonite deposits were first commercially mined in Fort Benton, Montana, giving the material its name. The best bentonite for ceramics is sodium bentonite, which swells the most dramatically.

Typical usage: 1 to 2 percent in nearly every glaze recipe for suspension. 1 to 3 percent in clay bodies for added plasticity.

Mixing tip: Always dry-mix bentonite thoroughly with other dry materials before adding water. If added directly to water, it forms stubborn lumps that are difficult to break up.