Aside from heavy spar, which is barium sulphate, another form of barium occurs in nature, a carbonate of barium known as witherite. This is an earthy mineral, nearly white, that is insoluble in water, but dissolves in strong acids. It dissolves in hydrochloric acid with effervescence similar to any other carbonate. The solution when evaporated will crystallize and then forms a barium salt known as barium chloride, from which, as we have mentioned before, blanc fixe is precipitated by the addition of sulphuric acid. When witherite is calcined at intense heat it turns into barium oxide, similar to burnt lime or calcium oxide.
Barium carbonate or carbonate of barytes, as it is sometimes called, has a specific gravity of 4.1, and its chemical formula is BaCo3, while its oil absorption and density are very similar to that of blanc fixe, but being a carbonate it is not unaffected by sulphur gases or acids like barytes or blanc fixe. It is therefore, not a safe pigment in paint, and one firm of paint manufacturers in the Middle West has been driven out of business by their indiscriminate use of it in all of their paint products. Some fifteen years ago they made great claims for their goods, and many of the most prominent hotels and large buildings, as well as a great many railway stations and equipments, were painted with the material, with the result that in a very short space of time the coatings of paint either changed color or disintegrated, according to local conditions. The next important of the pigments under consideration is Carbonate of Lime (Calcium Carbonate) the raw material in lump form being known as chalk, while when prepared for the use of the paint grinder or putty maker and the trade in general it is called by the collective term whiting, being classed according to grade as English cliff stone Paris white, American Paris white, gilders' whiting, Spanish whiting and commercial or common whiting. Marble dust also is calcium carbonate.