Bentonite is a clay-based clarifier that absorbs colloidal substances. Usually added prior to fermentation, sometimes during secondary to wine.
To use, add 3 tablespoons to 1 pint boiling water while stirring. Cover and let sit overnight. Shake until creamy smooth. Mix 1 to 2 tablespoons of this slurry per gallon of wine into a small quantity of wine. Add mixture to fermenter.