Ultramarine, a beautiful blue pigment originally made from lapis lazuli (see Lapis Lazuli), but now prepared in large quantities artificially. Its fabrication was suggested by the discovery of blue masses on taking down soda furnaces and lime kilns; and the societe d'encouragement at Paris in 1824 offered a prize of 6,000 francs for its production, which was awarded to Guimet of Toulouse in 1828. A modfe of preparation had previously been published by Christian Gmelin, in the beginning of the same year. Guimet's process was first applied on the manufacturing scale, but was kept secret. Gmelin's process was long followed, but many others have finally come into use. The following process, given by Prof. Miller of London, answers well upon a small scale: An intimate mixture of 100 parts of finely washed kaolin, 100 of sodic carbonate, 60 of sulphur, and 12 of charcoal is exposed in a covered crucible to a bright red heat for three hours and a half. The residue, which should not be in a fused condition, is green.
The product after grinding is well washed, dried, and mixed with one fifth of its weight of sulphur, and exposed in a thin layer to a gentle heat a little above that required to burn off the sulphur, which being accomplished the process is repeated two or three times, until the mass becomes bright blue. The green modification, the product of the initial process, is also manufactured for the market. There is some doubt as to the nature of the coloring matter of ultramarine. According to the experiments of Wilkens, ultramarine is composed of two portions, one constant, containing the coloring matter and soluble in hydrochloric acid, while the other contains a variable amount of clay, ferric oxide, and sulphuric acid.