This occurs in nature in a crystalline mass, known as heavy spar, and is mined same as iron ore, and is more or less contaminated with foreign material. When the crude article comes to the mill it is sorted and as much as possible freed from the foreign matter, which is usually sand or clay, but sometimes it is colored with oxide of iron running through the lumps in veins. The lumps are broken into small pieces through a crusher, and formerly these pieces were ground to a coarse powder in an edge runner mill or chaser, and then mixed with water in the hopper of a buhr stone mill, which ground it fine. Some well-equipped barytes mills did not use a chaser, but had double sets of mills, grinding from one into the other, thus saving handling or conveying apparatus. In addition to this the material so ground to pulp was allowed to run into levigating tanks, where it was washed and floated. After settling the top layer was drawn off and dried, then pulverized and put up for the market as floated barytes. This refers, however, only to material which was white enough when the levigation had taken place. Of course, the heavy or coarser portion that settled to the bottom of the levigation tank was once more returned to the mill in order to bring it to the standard degree of fineness. When the material is "off color," due to presence of iron, it is necessary to bleach it. This is done by running the water-ground barytes into tanks lined inside with lead and fitted with steam coils of lead pipe to enable the operator to heat the pulp and water. When the water is near the boiling point sulphuric acid is added, which dissolves the iron without affecting the pigment. When a sample taken shows that the bleaching process is complete, the acidulated water is drawn off and the pigment washed with fresh water, until every trace of acid has disappeared. The pigment so treated is usually whiter than the material that did not require bleaching. Water floated barytes is without question the best form in every respect, while the many attempts at air floating have not succeeded in producing its equal. Barytes is a very heavy pigment, its average specific gravity being 4.6, and a one-gallon can of the dry powder weighs anywhere from 15 to 18 pounds, according to its fineness. It is the most permanent white pigment known, being unaffected by sulphuretted hydrogen, acids and alkalies.
It does not combine with oil, and the mixture of barytes and oil is simply a mechanical one. When mixed and ground in a chaser a stiff paste may be made with 93 pounds of barytes and 7 pounds of linseed oil, while running the mixture through a stone mill 92 pounds of barytes and 8 pounds of oil will produce a fairly stout paste, providing the barytes is pure spar and not mixed with appreciable percentages of silica or aluminum silicate. One gallon of paste barytes in oil mixed stiff on the chaser will weigh 27 to 27 1/2 pounds, while that run through the mill will not exceed 26 pounds to the gallon.
While barytes is or has been considered mainly as an adulterant or make-weight in paint, its function is now better understood, and so long as it does not parade in the guise of another higher priced pigment, there should be no objection to its use when not overdone. In the manufacture of commercial chrome greens it is practically indispensable.