Natural and Artificial Ultramarine.

Manufacturer and builder 7, 1869

There happen to be a few rare cases in which art has succeeded in equaling and even excelling the productions of nature. Thus the artificial aventurine far exceeds in beauty the natural mineral; and for all practical purposes is superior to it. So, too, the artificial ultramarine produced by the best manufacturers is far superior to the average quality of the natural mineral, but is not at all equal to the very finest specimens. The natural mineral varies in quality quite as much as any artificial product, and indeed more. The steps by which chemists have been enabled so perfectly to simulate this exquisite product of nature are highly interesting, and our readers will no doubt appreciate the following excellent renting of the chemistry of the subject, which we extract from a paper by M. Zuber, who contributed it to Der Papier Fabrikant, an excellent technological journal published in Southern Germany. The beautiful blue pigment now known as ultramarine, and manufactured in France and Germany on a large scale, has become common by the results of scientific and analytical chemistry. The native ultramarine, also called lazulite, lapis lazuli, and zeolite blue, is a rather rare mineral, of which, in pure state, according to Patrin, Central Asia is the native place. This native mineral consists, in 100 parts, according to Varrentrapp's analysis, of: Silica, 45.40; alumina, 31.07; soda, 9.09; lime, 3.52; sulphuric acid, 5.89; sulphur, 0.95; iron, 0.52; chlorine, 0.42; water, 0.12.

The finest specimens of native ultramarine are used as precious stones; and some of it is still used by artists, it being prepared for that purpose, in a peculiar manner, chiefly at Rome and Florence. Artificial ultramarine was first made by a process discovered by M. Guimet, in France, in 1826, but kept secret, and remaining so up to the present day. Although the pigment obtained by artificial means is one of great beauty and permanency, under ordinary conditions it is never made equal to the pigment derived from the native lapis lazuli. Notwithstanding a large number of scientific and practical men have studied this subject, the precise answer to the question, Whence the blue color of this substance is derived? is a perfect mystery; the only fact certainly known is, that a portion of the sodium is combined with the sulphur, to form a certain sulphuret, and that that alkaline sulphuret is chemically combined with a double silicate of alumina and soda. Iron and calcium have been proved to have nothing to do with the cause of the blue color, and to be simply accidental. In the origi-nal paper, from which we condense the results, no less than ten analyses of samples of artificial ultramarine, made by different analysts, and from different makers, aregiven. Varrentrapp's results are, for 100 parts, as follows Silica, 46.60; alumina, 23.30; soda, 21.46; sulphur, 1.68; potassa, 1.75; iron, 1.06; lime, 0.02; sulphuric acid, 3.08; with traces of chlorine, oxide of iron, oxygen, and an accidental trace of clay. The ingredients required for the manufacture of artificial ultramarine are - sulphur, wood-charcoal, sulphate of soda, carbonate of soda, and kaolin; curiously enough, the latter substance is preferred from Cornwall, and fetched thence to be transported abroad, while the manufacture of ultramarine in this country is of little or no importance. The product of ultramarine met with in commerce is rarely, if ever, adulterated, unless it be with the pure sulphates of lime and baryta, to reduce the original deep blue color to a less dark shade. In order to test the coloring value of a sample of ultramarine, it is usual to weigh off 0.1 gramme and to mix it, in a dry state, with 1 gramme of powdered French chalk, and to compare the tinge obtained with other mixtures of the kind and a standard sample of ultramarine. Besides being largely used in oil painting, ultramarine yields blue printing ink, blue for calico, silk, and woolen tissues; for coloring paper, sugar, (it is largely used in sugar refineries to brighten the loaf sugar,) fresco and stucco painting, and other purposes. All acids, especially the more concentrated ones, destroy the color, which, however, stands fire, as well as many other agents. The native ultramarine is not affected by acids, unless heat is at the same time applied.

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