On Artificial Ultramarine.

Scientific American 14, 26.12,1846

Till within the last twelve or fifteen years the only source of this beautiful pigment was the rare mineral, lapis lazuli. The price of the finest ultramarine was then so high as 5 guineas the ounce. Since the mode of making it artificially has been discovered, however its price has fallen to a few shillings the ounce. Artificial ultramarine is now manufactured to considerable extent on the Continent, but as far as I can learn, none has yet been made in Great Britain. The chief French manufactories of ultramarine are situated in Paris; and the two largest ones in Germany are those of Messein in Saxony, and of Nuremberg in Franconia. Three kinds of ultramarine occur in commerce, the blue, the green, and the yellow. The first two are only true ultramarines, that is, sulphur compounds: the yellow is merely the chromate of baryta.

Both native and artificial ultramarine have been examined very carefully by several eminent chemists, who, however, have been unable to throw much light upon their true nature. Chemists have undoubtedly ascertained that ultramarine always consists of silica, alumina, soda, sulphur;and a little oxide of iron; but no two specimens, either of the native or artificial ultramarine, contain these ingredients in at all similar proportions. In fact, the discrepancies between the analyses are so great as to render it impossible to deduce from them any formula for the constitution of ultramarine; if indeed it does possess any definite composition. The following are a few specimens of these analyses, and others equally discordant might easily be added.

Lapis Lazuli.
By Clement and Desormes.
Soda : 23.2
Alumnia : 24.8
Silica : 35.8
Sulphur : 3.1
Carbonate lime : 3.1

Lapis Lazuli.
By Varentrap.
Soda : 9.09
Alumnia : 31.67
Silica : 45.50
Sulphur : 0.95
Lime : 3.52
Iron : 0.86
Chlorine : 0,42
Sulp. Ac. : 5.89
Water : 0.12

Artificial Ultramarine.
Parisian.
By C. G. Gmelin.
Soda and potash : 12,863,
Lime : 1,546
Alumina : 22 000
Silica : 47.396
Sulphuric Acid : 4.679
Resin, Sulp. & Loss : 12.218

Artificial Ultramarine.
Meissen.
By Varentrap.
Soda and potash : 21.47
Lime : 1.75
Alumina : 0.02
Silica : 23.30
Sulphuric Acid : 45.00
Resin, Sulp. & Loss : 3.83
Iron : 1.063

The last chemist who has examined ultramarine is Dr. Elsner, who has published a very elaborate paper upon it in the 23d number of Erdmann's Journal for 1841. The first part of Dr. Elsner's paper is historical, and contains an account of the accidental discovery of artificial ultramarine by Tassart and Kuhlman in 1814, and of the labors of subsequent chemists. He then gives a detailed account of his own experiments, which have been very numerous, and from these he deduces the following conclusions: 1st, That the presence of about one per cent of iron is indispensable to the production of ultramarine; he supposes the iron to be in a state of sulphuret. 2d, That the green ultramine is first formed, and that as the heat is increased it passes by degrees into the blue. The cause of this change is, he affirms, that part of the sodium absorbs oxygen from the atmosphere, as tine operation is conducted in only partially closed vessels, and combines with the silica, while the rest of the odium passes into a higher degree of sulphurization. Green ultramarine, therefore, contains simple sulphurets, and blue, polsulphurets.

Dr Elsner's paper does not, however, furnish any details by which ultramarine could be manufactured succesfully in the great scale. Thus, for example, in regard to the necessary degree of heat, perhaps the most important circumstance in the process, he gives no directions whatever. We know, however, from other sources, that it should be a low red heat, as at much higher temperatures both native and artificial ultramarine soon become colorless. Dr. Elsner, indeed does not affirm that he was able to procure ultramarine in quantity of a uniform color. In fact, the process of Robiquet, published nearly ten years ago, is the best which scientific chemists possess, though undoubtedly, the manufacturers have greatly improved upon it. Robiquet's process consists in heating to low redness a mixture of one part porcelain clay, one and a half sulphur, and one and a half parts anhydrous carbonate of soda, either in an earthenware retort or covered crucible, so long as vapors are given off. When opened the crucible usually contains a spongy mass of a deep blue color, containing more or less ultramarine mixed with the excess of sulphur employed, and some unaltered clay and soda. The soluble matter is removed by washing, and the ultramarine separated from the other impurities by levigation. It is to be regretted, however, that the results of Robiquet's process are by no means uniform: one time it yields a good deal of ultramarine of excellent quality, and perhaps, at the very next repetition of the process in circumstances apparently similar, very little ultramarine is obtained, and that of an inferior quality.

The fabrication of ultramarine is a subject which well deserves the attention of English chemical manufacturers, as it could be carried on with peculiar advantage in this country.- The chief expense of the process is the fuel required, which can be purchased in Great Britain for less than half the money it would cost in France or Germany.

- Lon. Mec. Mag.