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Ch. 6: Opal

Ch. 6: Opal Page of 515 Ch. 6: Opal Text size:minus plus Restore normal size   Mail page  Print this page
326                   A POPULAR TREATISE ON GEMS.
which will soon be impregnated with the blue color * a fresh portion of water is then taken, and the same operation is continued until the remains are colorless. The ultramarine, after a short time, settles to the bottom of the vessels, and is carefully separated and dried. If the lapis lazuli be of the best quality, the product will be from two to three per cent. That color which remains yet in the mass is of an inferior quality, and is called the ultramarine ashes; it is of a paler and more reddish color.
Good ultramarine has a silky touch, and its specific gravity is 2.36. It does not lose its color if exposed to heat, but is soon discolored by acids, and forms a jelly. In order to distinguish the pure ultramarine from numerous spurious and adulterating coloring materials, such as indigo, Prus­sian-blue, mineral-blue, &c, it is only necessary to test the article in question with some acid, when after a few minutes the real ultramarine is discolored, yielding a clear solution and a white residuum. The real ultramarine has always been at a very high price, on account of the small product obtained from the mineral. An ounce of the purest ultra­marine is sold in France for two hundred to two hundred and fifty francs, which is not within the reach of all painters.
In the year 1828, the discovery was made by Professor Gmelin, in Tubingen, that sulphuret of soda was the proper material for imitating this precious and valuable pigment. By his experiments he succeeded in preparing this substance from silex, alumina, soda, and sulphur, producing a color in • every respect corresponding with the true color of the lapis lazuli, and bearing the same relation to acids as the genuine ultramarine. This, for economy, has become a great object to painters and color-men, since a whole pound of it may be purchased in France for twenty francs. As it bids fair to meet with a great consumption, being even substituted for cobalt in bluing paper, thread, and other
Ch. 6: Opal Page of 515 Ch. 6: Opal
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