ULTRAMARINE', a beautiful blue pigment, formerly obtained only from the very valuable mineral, lapis-lazuli; but an artificial kind is now made so cheaply, and is so good, that it is generally used instead. The true ultramarine, from its costly nature, was only used by artists; the artificial sort is, however, extensively used by house and ship painters, and is as cheap as it is beautiful. Many artists still insist upon having the former kind, which is prepared as follows: Fine lapis-lazuli is broken up into very small pieces, so as to enable the operator to see and pick out the small white portions which occur in it. Of the pieces of pure blue which remain, a pound weight is then taken, and in a carefully covered crucible, is heated to redness, and thou thrown into cold water. It is next reduced to an impalpable powder, and mixed with 6 oz. of finely powdered resin, as light in color as it can be obtained, and 2 oz. each of spirits of turpentine, bees-wax, and linseed oil, all previously melted together. When these ingredients are thoroughly worked into a mass, portions of it are taken and kneaded in clean water; as long as any blue color is given out, this is continued, until every portion has been so treated, The blue water is then allowed to rest, and the sediment is collected and washed in water several times. The first washing removes a considerable quantity of dirt and other foreign matters, and is consequently rejected. The second, after being well agitated, is decanted; and from it is obtained the highest quality of the pigment. That which remains usually has.two other washings, each of which gives a product of a less value than the operation which preceded it. The product obtained by sediment from each of the waters used is carefully dried, and is then employed either to make cakes for water-color painting, or a mixture for oil-painting, the value being about Xi per ounce. ARTIFICIAL ULTRAMARINE.—The French chemists Clement and Desormes, in studying the curious process of obtaining ultramarine from lapis-lazuli IT mixing it with resin, etc., were led to an analysis of the coloring matter that suggested to Guimet the idea of composing it artificially. In this lie succeeded, and obtained for his discovery the prize of 0,000 francs offered by the Societe d'Encouragement des Arts. Almost simul
taneously, Gmelin in Tubingen gave an analysis and a synthetic process which also suc ceeded, and artificial ultramarine is now a regular article of manufacture. Chemical skill, however, is necessary to success, and the manufacturers' formulas are very various— differing in the quantities of the ingredients, and the order of mixing tin in. The Ger man manufacturers are very successful, and some of them have recently produced a tine green ultramarine. The following formula is one of the simplest: 100 parts of finely washed kaolin or porcelain clay (silicate of alumina). 100 of carbonate of soda. 60 Of sulphur, and 12 of charcoal are mixed and exposed in a covered crucible to a bright heat for 3i hours, when a green, unfused residue should be left. This residue, alter being well washed and dried, must be mixed with a fifth of its weight of sulphur, and exposed in a thin layer to a gentle heal, so as just to burn off the sulphur. When this is accom plished, more sulphur must be added, and the washing repeated; and so on, until the mass acquires a light blue color, which is usually the case after the third roastinr. In 1872, however, FlIrstenau introduced improvements into the manufacture of raninrine, showing how the blue tint could be produced in one calcination. There is reason to believe, from the experimeuts of Wilkens, that ultramarine is composed of two portions —one of which is constant in its composition, and is the essential coloring muter, •on taining about 40 of silicic acid, 20 of alumina, 13 of sulphur, and 21 of soda. arranged as a mixture of two silicates of alumina, sulphite of soda, and sulphide of sodium—the blue coloring principle being a compound of the latter two; while the other portion differs from the former in resisting the action of hydrochloric acid, and contains a vat Mble amount of sand, clay, oxide of iron, and sulphuric acid. Ultramarine, if heated in the air, gradually assumes a dull green tint; and it is quickly decomposed by the action of the mineral acids and chlorine.
The term yellow ultramanue is sometimes given commercially to chromate of baryta, a yellow insoluble powder used as a pigment.