Purified Calcium Carbonate, made of ground limestone from rock quarries in Vermont, is the perfect ingredient for artists who wish to have a thicker, more opaque paint. Calcium carbonate is commonly used as a pigment extender, particularly in matte emulsions. It is often used with Titanium White to create grounds. Historically up to 30% by weight of the paint could be either chalk or marble, both forms of calcium carbonate. Calcium carbonate is the result of prehistoric seashell deposits layed down on the ocean floor. It is the basic ingredient in eggshells, chalk, marble and the human skeletion. It is considered non-toxic by the FDA, although it should not be inhaled. Purified Calcium Carbonate contains less lead and silica than other less refined forms. PCC has a very fine and controlled particle size, on the order of 3 micrometres in diameter. We have 4 oz., 8 oz., and 16 oz. versions.