Bentonite

A common clay soil called bentonite is used as a fining agent in wine. The main application for bentonite is to remove unstable proteins that can lead to haze formation in finished wine. Negative charged particles in the clay react with the positive charges on proteins and the two bind together, dropping out of solution. Bentonite leaves some residue of either sodium or calcium depending on the form used along with some other trace metals. This normally is not a concern in wine. Bentonite is also occasionally used as a settling aide.