To this class belong gypsum (or terra alba, as it is often termed by its Latin name, meaning white earth), selenite and calcined plaster or, as it is more familiarly known, plaster of Paris. Gypsum is found in many parts of the globe, the cleanest and least off color variety coming from Nova Scotia, while abundant quantities are found in Michigan, Colorado, California and other States, as well as in Ontario and Quebec, Canada, although the mineral is somewhat discolored from iron oxide. Gypsum is the form preferred for use in paint making. It is chemically a hydrated calcium sulphate, having the formula Ca S04, 2H2o, meaning that it has two molecules of water in combination. Normal gypsum should consist of 46.5 per cent sulphuric anhydride, S03, 32.5 per cent calcium oxide, CaO, and 21 per cent water, H20, or rather 79.04 per cent sulphate of lime and 20.96 per cent water of crystallization. When heated thoroughly to a temperature of a few degrees above 300 degrees F. gypsum loses its water of crystallization and is converted into an opaque white powder that when mixed with clear water has the property of setting into a hard mass, in which state we know it as calcined plaster or more popularly as plaster of Paris and known to chemistry as anhydrous sulphate of calcium with the formula Ca S04. However, gypsum, when heated in a furnace in admixture with sulphate of iron (copperas) may be dead burned so as to render it incapable of taking up or absorbing water again. The U. S. Navy Department in their specifications for Venetian red expressly refer to this feature and will not permit this red to contain sulphate of lime in any other form, while previous to the time of issuing these specifications the provision was that the presence of any sulphate of lime not fully hydrated was sufficient cause for rejection. The specifications issued by railroads whenever sulphate of lime is permitted in connection with oxide of iron paints still adhere to the rule of having it fully hydrated. The writer inclines to the belief, which is based on years of observation, that the U. S. Navy Department is on the safe side with their provision of having the calcium sulphate dead burned and probably more so than the railroad chemists who adhere to the previous rule of permitting no other than the fully hydrated form of calcium sulphate.
It seems like splitting hairs when authorities condemn the use of gypsum in paint for the sole reason that it has been discovered that one part of gypsum is soluble in 500 parts of water.