Magnesia, when found in the natural state in the form of carbonate, Mg C03, is known as magnesite. It does resemble crude chalk in appearance, but is heavier in specific gravity than the latter, usually 3.06 on the average. It has been used by certain color makers in place of barytes as an extender for chrome green, and the product was known as magnesia green, but it has not come into general use as an extender for white or colored pigments in oil paints, but is ground and levigated with water and offered on the market in pulp form as magnesia white for special trade. Calcined magnesia, however, is the form in which magnesia is used in some paints, not as an adulterant, but as a means of keeping heavy pigments in suspension. It is produced by burning the carbonate, when it changes the carbonate into magnesium oxide, MgO. Oxide of magnesium, called magnesia for short, is a white powder so light in gravity that a gallon of it does not weigh over twelve ounces, and when three pounds of it are mixed with eight pounds of oil it makes a very stiff mix, and a gallon of the paste when ground will not weigh over nine pounds. Only very small portions can be used in paint for suspending the heavier pigments, because of its taking away from their covering or hiding power. Where cost is not against it, it is being used in liquid fillers, either alone or with silica or clay. It can be made into a plastic mass with glue size or gum water and glycerine and formed to imitate various articles. Mixed with asbestine or soapstone and soluble glass in weak solution it serves as a fireproof cement.
There is still another form of magnesia, the sulphate. It occurs in nature as epsomite, and when artificially made we know it as epsom salt. In this form it has no value whatever in paint. When this is calcined, magnesium oxide is produced, but the economical way to produce the oxide of magnesia is to burn the carbonate.