Bentonite beds formed from volcanic ash that turned to clay underwater. High-grade sodium bentonite comes mostly from between the Black Hills of South Dakota and the Bighorn Basin of Wyoming, and from Turkey. Miners strip off the ground above and dig out the beds.

Freshly dug clay is wet and sticky, so it is dried before grinding. Dry clay breaks cleanly instead of smearing.

Crushers break the clay lumps down, and mills grind them to a fine powder. One patented process grinds every particle to 2 mm or smaller.

Calcium bentonite clumps poorly, so some plants stir in a soluble sodium salt such as sodium carbonate. Sodium trades places with calcium in the clay's layers, and the clay gains much of sodium bentonite's swelling power.

The damp clay goes through a granulating machine that shapes it into round or cylindrical grains. Additives such as wood powder or diatomite can be blended in first to set the weight and whiteness.

Granules are dried until little water is left. Dry granules soak up urine quickly and clump firmly when it hits.

Screens pull out dust and oversized grains. Some brands mix in baking soda to absorb odors, or guar gum to strengthen the clumps, before the litter is bagged.