This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Take 1 lb. crystallized commercial alum, heat over a free fire in an earthenware pot, and stir constantly until all becomes a white, porous mass. When cold, powder the mass and keep ready for use in stoppered bottles.
Take 2 lbs. crystallized alum and proceed as before. When the alum is melted add 4 ozs. of calcined or carbonate magnesia. Stir continuously until it has become a white porous mass. When cold, powder and keep ready for use in stoppered bottles. The addition of calcined magnesia may be increased up to 1 lb. These powdered preparations are applied to alcoholic liquids or essential oils that need clarification. A sufficient quantity of the powder is added to the liquid, shaken, and then allowed to subside or is filtered.