by Rose Schreiber
A fine grey-green powder. A type of clay. Swelling, clumping, thickening in water, becoming viscous. We add bentonite in such small amounts to our clay bodies and glazes—notice how, in a given recipe, it most often comes at the end, alongside stains and oxides. An addition to the whole. In ceramics, we tend to use bentonite not for its chemistry but for its raw, unfired behavior. Bentonite particles are so small, they absorb so much water, that they make our wet clay bodies more plastic and our dry greenware stronger. Like other clays, we add bentonite to our glazes because of the electrolytic relationships it creates: bentonite causes particles to bounce off each other, repel one another, keeping them suspended rather than, as they would otherwise do, sinking and settling at the bottom. A versatile material, widely used in medicine, cosmetics, civil engineering, and industry. The gelling properties we employ in ceramics are even more sought after by the oil and gas industry—most of the bentonite we extract from the earth is used as drilling mud, a lubricant for oil wells. Bentonite is a material extracted to facilitate yet more extraction.
If you are in North America, the bentonite in your ceramic studio almost certainly derives from central and northeastern Wyoming, on Lakota, Crow (Apsáalooke), Cheyenne (Tsis tsis’tas), and Eastern Shoshone (Newe) Indigenous land. Bentonite mining in Wyoming dates to the late 1880s, around the time that American westward expansion consolidated and Wyoming was incorporated as the 44th state. This was barely two decades after the ‘Four Great Surveys’ of the American West, a series of extensive, federally funded land surveys searching for, among other things, mineralogical wealth and other exploitable natural resources. It was also a mere decade after the Black Hills War, a resource war in which the US government sought to expropriate Indigenous land for gold mining and other resource extraction. Today, bentonite mining continues to take place in this region, in the Northern and Southern Black Hills, the Kaycee District, as well as the Eastern Bighorn Basin. It is estimated that as much as 70 percent of the world’s bentonite deposits are located in this region.
Bentonite is formed through the interaction of volcanic ash and seawater. As such, its presence reveals two major features of this region’s geologic history: namely, that this area was once submerged under an inland sea—the Western Interior Seaway which, during the Late Cretaceous period, divided North America in two—and that it has, over tens of millions of years, experienced immense amounts of volcanism from the Idaho batholith. The bentonite we use today is, therefore, the result of 100 million years of geologic history.
Bentonite is extracted today through surface mining. Topsoil is removed and stockpiled, then, with shovels and scrapers, bentonite is removed from relatively shallow pits, no more than about 50 feet deep. Once the bentonite has been removed, the pit is filled with topsoil and other debris, then reseeded. Bentonite is a solid, even with a low water content, so after extraction it is first crushed and then dried to a moisture content of around 15%, and then sieved or milled to its final consistency.
Map location of the American Colloid Bentonite plant outside of Lowell Wyoming
