Lapis Lazuli.

Scientific American 21, 23.11.1867

The name of this mineral is derived from the Persian language, and means blue color, or, with the Latin prefix, blue stone. The ancients were well acquainted with it, and have employed it as a substitute for other gems. The Greeks and Romans are said to have called it by the name of sapphire, denominating that with specks of iron pyrites the sapphirus regilus; Pliny called it the cyanus. It was formally used as a strengthening medicine.

Lapis lazuli very seldom occurs crystallized; its regular form is the oblique foursided prism; it mostly occurs compact, and in grains and spects, with an uneven and conchoidal fracture; it is translucent on the edges; its luster is nearly vitreous and shining; structure foliated; its color is fine azure blue, with different shades, often interspersed with spots and veins of pyrites. It scratches glass, but is attacked by quartz and by the file; its specific gravity is 2.3; before the blowpipe and on charcoal it with difficulty runs into a white glass, but with borax it fuses with effervescence into a limpid glass, It consists of lime, magnesia, and silex, with soda, protoxide of iron, and sulphuric acid.

It is generally called in trade, the Armenian stone.

It is found in gangues of the older formations, and in Bucharia; it exists in granite rocks, and is disseminated in all veins of thin capacity; on the Baikal Lake it is found in solid pieces; also, in Siberia, Thibet, China, Chili, and Great Bucharia. Lapis lazuli is much used for jewelry, such as rings, pins, crosses, ear-rings, etc. The best pieces are generally cut out from larger lumps by means of copper saws and emery, then ground with emery on a lead wheel, and polished with rotten stone on a tin wheel. The rocks which yield lapis lazuli, where it is contained in specks, are likewise cut for ornamental purposes, such as snuff-boxes, vases, candlesticks, cups, columns, caneheads, etc.; also, for architectural ornaments and stone mosaic; the larger specimens, having specks regularly disseminated on a white ground of the rock, are those selected for cutting. The most important use of this mineral is that of furnishing the celebrated and beautiful pigment called ultramarine blue, used by painters in oil, and said never to fade. The lapis lazuli takes a very high polish, but becomes dull again after being used for some time. It is sometimes imitated by lazulite (azure stone), or blue carbonate of copper, which, however, is not near so hard, and effervesces on testing with nitric acid. Those specimens having Iron pyrites inclosed are difficult to polish well, on account of the unequal hardness of the two minerals.

Lapis lazuli has lately been discovered in California, but the color of the mineral from this locality is very indifferent, and its price is therefore much inferior to that from Persia.

The value of lapis lazuli, although depending upon its purity, intensity of color, and size, has nevertheless much diminished when compared with its former prices.

The Chinese, who have for a long time employed lapsis lazuli in their porcelain painting, call the pure and skyblue stone zuisang, and the dark blue, with disseminated iron pyrites, the tchingtchang, preferring the the latter to the former; they work the samelfor many ornaments, such as vases, snuff-boxes, buttons, and cups. In the palace which Catharine II. built for her favorite, Orlof, at St. Petersburg, there are some apartments entirely lined with lapis lazuli, which forms a most magnificent decoration.

The process of preparing ultramarine was known as early as the fifteenth century. The color is now mostly prepared at Rome, in the following manner: those pieces which are free from pyrites specks, are first calcined and pulverized; the powder is then formed into a mass with a resinous cement (pastello), and fused at a strong heat; this is then worked with the hands in soft water, whereby the finest coloring particles are disengaged in the water, which will soon be impregnated with the blue color; a fresh portion of water 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 llose 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, etc., 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 stuffs, several manufacturers have already been induced to engage largely in its preparation; and there is now a very extensive establishment in full operation by M. Guimet, three leagues from Lyons, who likewise claims the priority of its discovery: the royal porcelain manufactory at Meissen, in Saxony, also prepares it. The process for making the artificial ultramarine, as it was first described by Gmelin, is here given, as it was published in the Annales de Chimie The whole process is divided into three parts:

1. The pure hydrate of silica is prepared by fusing fine pulverized quartz or pure sand with four times its own weight of salt of tartar, dissoling the fused mass in water and precipitating by muriatic acid; also the hydrate of alumina is prepared from alum in solution, precipitated by ammonia.

2. Dissolve the silex so obtained in a hot solution of caustic soda, and add to seventy parts of the pure silex seventytwo parts of alumina; then evaporate these substances until a moist powder remains.

3. In a covered Hessian crucible, a mixture of dried sal soda, one part to two parts of sulphur, is heated gradually, until it is fully fused, and to the fused mass add small quantities of the earthy precipitate, taking care not to throw in fresh quantities until all the vapors have ceased; after standing for an hour in the fire, remove the crucible, and allow it to cool. It now contains the oltramarine, mixed with an excess of sulphuret, which is to be removed by levigation; and if the sulphuret is still in excess, it is to be expelled by moderate heat. Should the color not be uniform, levigation is the only remedy.


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