Bentonite

Bentonite is an off-white montmorillonite clay formed from altered volcanic ash. It has a sheet-silicate structure and is especially notable for the way in which it absorbs and loses water and for its base-exchange properties.

Early uses of bentonite in the 1800s, included lubricant for wagon wheels, sealant for log cabin roofs, and soap. Modern uses are highly variable. In foundry or metal casting, bentonite is mixed with sand to make the molds. The bentonite forms a pliable bond with the sand grains, so the mixture holds its shape well. Molten metal is then poured into the impression to make the casting. When complete, the bentonite and sand mold can then be broken away from the casting and reused.