ULTRAMARINE is a beautiful blue pigment obtained from the variegated blue mineral, called lazulite (la is lazuli), by the following process :—Grind the stone to fragments, rejecting all the co lorless bits, calcine at a red heat, quench in water, and then grind to an impalpa ble powder along with water, in a paint mill, or with a porphyry slab and muller. The paste, being dried, is to be rubbed to powder, and passed through a silk sieve. 100 parts of it are to be mixed with 40 of rosin, 20 of white wax, 25 of linseed oil, and 15 of Burgundy pitch, previously melted together. This resin ous compound is to be poured hot into cold water ; kneaded well first with two spatulas, then with the hands, and then formed into one or more small rolls. MM. Clement and Desormes, who were the first to divine the true nature of this pigment, think that the soda contained in the laznlite, uniting with the oil and the rosin, forms a species of soap, which serves to wash out the coloring-matter. If it should not separate readily, water heated to about 150° F, should be had recourse to. When the water is suffi ciently charged with blue color, it is poured off and replaced by fresh water ; and the kneading and change of water are repeated till the whole of the color is extracted. The first waters afford, by rest, a deposite of the finest ultramarine; the second, a somewhat inferior article, and so on. Each must be washed after wards with several more waters, before they acquire the highest quality of tone ; then dried separately, and freed from any adhering particles of the pitchy com pound by digestion in alcohol. The best ultramarine is a splendid blue pigment, which works well with oil, and is not liable to change by time. Its price in Italy was 26 dollars the ounce, a few years ago, but it is now greatly reduced. The blue color of lazulite had been al ways ascribed to iron, till MM. Clement
and Desormes, by a most careful analy sis, showed it to consist of—silica, 84 ,• alumina, 83 ; sulphur, 3 ; soda, 22 ; and that the iron, carbonate of lime, &c., were accidental ingredients, essential neither to the mineral, nor to the pig ment made from it. By another analyst, the constituents are said to be—silica, 44; alumina, 35; and soda, 21 ; and by a third, potassa was found instead of soda, showing shades of difference in the com position of the stone.
Till a few years ago, every attempt failed to make ultramarine artificially. At length, in 1828, M. Guimet resolved the problem, guided by the analysis of MM. Clement and Desormcs, and by an observation of M. Tasseert, that a blue substance like ultramarine was ocasion ally produced on the sandstone hearths of lns reverberatory soda furnaces. M. Gnimet has kept his process secret. M. Gmelin of Tubingen, has published a prescription for making it ; which con sists in enclosing carefully in a Hessian crucible a mixture of two parts of sul phur, and one of dry carbonate of soda, heating them to redness till the mass fuses, and then sprinkling into it by de grees another mixture, of silicate of so da, and aluminate of soda ; the first con taining seventy-two parts of silica, and the second seventy parts of alumina. The crucible must be exposed after this for an hour to the fire. The ultrama rine will he formed by this time ; only it contains a little sulphur, which can he separated by means of water. M. Per 1407, professor of chemistry at Stras bourg, has likewise succeeded in mak ing an ultramarine. Lastly,' M. Robi quet has announced, that it is easy to form ultramarine, by heating to redness a proper mixture of kaolin (China clay), sulphur, and carbonate of soda.