Heavy calcined: Magnesium oxide by calcination of magnesite (natural magnesium carbonate), and then ground for use as a compounding ingredient for molded goods and hard rubber. (b)Light calcined: Magnesium oxide by calcination of purified magnesium carbonate and/or magnesium hydroxide. It has a fine particle size and a bulk factor of 10 to 30 pds. per cubic ft. Used chiefly in neoprene stocks. (c)Extra light calcined: Prepared similarly by calcination of magnesium carbonate, but with a bulk factor of 4 to 6 pds. per cubic ft. Used chiefly in neoprene stocks.