Ultramarine - Production

Production

The raw materials used in the manufacture are: (1) iron-free kaolin, or some other kind of pure clay, which should contain its silica and alumina as nearly as possible in the proportion of SiO₂:Al₂O₃ demanded by the formula assigned to ideal kaolin (a deficit of silica, however, it appears can be made up for by addition of the calculated weight of finely divided silica); (2) anhydrous Na₂SO₄; (3) anhydrous Na₂CO₃; (4) powdered sulfur; and (5) powdered charcoal or relatively ash-free coal, or colophony in lumps.

The mixture is heated in a kiln, sometimes in brick-sized amounts. The resultant solids are then ground and washed as per any other insoluble pigment manufacturing process. The chemical reaction produces large amounts of sulfur dioxide meaning that Flue gas desulfurization is an essential part of its manufacture to comply with pollution regulations. Large chimneys were used to disperse sulfur dioxide produced in the process, resulting in ultramarine tinting the surrounding ground surfaces and roof vents with a blue color. (Google Maps offers views of two such synthetic ultramarine manufacturing sites, one near Hull, England (now defunct), another in Comines, France and another in Ambattur, Chennai, Tamil Nadu India Chennai.)

"Ultramarine poor in silica" is obtained by fusing a mixture of soft clay, sodium sulfate, charcoal, sodium carbonate and sulfur. The product is at first white, but soon turns green ("green ultramarine") when it is mixed with sulfur and heated. The sulfur burns, and a fine blue pigment is obtained. "Ultramarine rich in silica" is generally obtained by heating a mixture of pure clay, very fine white sand, sulfur and charcoal in a muffle-furnace. A blue product is obtained at once, but a red tinge often results. The different ultramarines—green, blue, red and violet—are finely ground and washed with water.