Bentonite is a natural, clay-like material of volcanic origin, which is geologically a form of montmorillonite, a smectite clay. It is a complex sodium, calcium, and aluminum silicate, and when hydrated it becomes a powerful adsorbent that can be used to clarify beer.

Montmorillonite, a term first used in 1847, was named after the French town Montmorillon, near Poitiers, where it was first mined. Its origin can be traced to ancient volcanic eruptions whereby fine volcanic ash particles were carried by winds and deposited in discrete layers that metamorphosed over time from the “glassy” state to claystone. The term “Bentonite” was first applied in 1898 to a particularly highly colloidal clay found in the Cretaceous beds near Fort Benton, Montana, and the product is still mined in this state. Some of the largest and highest quality deposits of Bentonite occur in neighboring Wyoming, from which the product takes on the alternative name “Wyoming clay.”

Bentonites are usually composed of about 90% montmorillonite, with the residue consisting of feldspar, gypsum, calcium carbonate, quartz, and traces of heavy metals, and it is these metallic impurities that impart any color to the mineral. In the pure state montmorillonite is almost white.

In the brewery, Bentonite is used as a stabilizing (chillproofing) agent and is used to bind and remove haze-forming proteins. Negative charges on Bentonite bind with positively charged proteins, forming a precipitated complex. The reaction yields copious quantities of sediment, which is an inconvenience for brewers, as the sediment has to be removed by filtration. For this reason, the role of Bentonite has been largely superseded by other products, such as silica hydrogels. Bentonite is still widely used in the wine industry, where it will bind with phenols and tannins, clarifying the wine.

See also adsorbent.