Popular Science Monthly/Volume 13/September 1878/Artificial Precious Stones
|ARTIFICIAL PRECIOUS STONES.|
ONE day, not long ago, the jewelers of Paris were in a high state of excitement, and justly so, for the news had reached them from the Academy of Sciences that two chemists, MM. E. Frémy and Feil, had discovered a process for the manufacture by the pound of certain kinds of precious stones ranking in value next to the diamond, and frequently commanding still larger prices than the latter—namely, the ruby, the sapphire, and the most precious of all, the Oriental emerald. At first the Parisian jewelers consoled themselves with the thought that the genuine stones would always be preferred to the artificial ones, but the excitement increased when it became known that MM. Frémy and Feil did not propose to imitate precious stones, but that their productions would be perfectly equal to the natural ones, and that a watch would run on their artificial rubies as well as on natural ones, because both of them were equally hard. Now the dealers in precious stones asserted that it was sinful to imitate Nature's work in that manner, and that the Government ought to prohibit it. On the other hand, a few enthusiastic feuilletonistes proclaimed that the discovery in question foreshadowed a still more important one—that of making gold and diamonds; that the dreams of the alchemists were about to be realized, and that poverty and wretchedness would be no more.
Of the prospect of poverty and wretchedness coming to an end we say nothing here. As for the transformation of lead and other base metals into gold and silver, we have to declare that this branch of alchemy is something altogether different from the manufacture of precious stones. Most of our modern chemists hold metals to be simple, immutable elements, which have always been what they are now, and which may change their form, but never their peculiar nature. Not so with precious stones, most of which, and especially those that are most highly prized, are of very lowly origin indeed. In the eyes of the chemist the ruby, the sapphire, the topaz, etc., are simply modifications of one substance (alumina), which, as clay, forms the greater portion of the earth's crust; and the diamond, which is the prince of all precious stones, is simply pure crystallized carbon, and so allied to charcoal, lampblack, etc. Other highly-esteemed precious stones, such as the emerald, the aqua-marina, and chrysoberyl, on the one hand, and the hyacinth, on the other, contain "earths" chemically related to argillaceous—earth namely, the former consist of beryl-earth, and the latter of zirconia; but these earths in themselves are neither rare nor precious, so that in some countries the streets are paved with the impurer brothers of the emerald. The same is true of all other precious stones, including pearls; in the main they are formed of substances of no value whatever, and to be found everywhere, such as argillaceous earth, silicic acid, fluor-spar, boracic acid, lime, magnesia, etc. Their only superiority consists in the fact that the common substance in them has reached an extraordinary degree of crystallization, for, aside from their beauty, their rarity enhances their value in the market.
Chemical combinations and simple substances of mineral as well as of organic nature assume their due crystal shapes, which are so well defined as frequently to bear a strong resemblance to those of cut stones, only when they pass from the liquid into the solid state, and they assume a large size only when this transition takes place very slowly. For instance, if we dissolve in hot water as much alum as can be dissolved therein, and suspend in the fluid, while allowing it to cool in a quiet place, a wire vessel—a basket, a rosette, or a crown, wrapped in wool—we shall find next morning that wire vessel covered with glasslike, transparent, more or less large, glittering octahedral crystals. Cold water is unable to hold in solution as large a quantity of the salt as warm water; and the surplus, as the temperature of the water decreases, has to separate slowly from it. In so doing, small crystals are formed. They grow constantly as the separation goes on, and, if we leave the solution exposed to the fresh air so that it slowly evaporates, we shall at last obtain very large crystals. If the alum contained an impure admixture of other salts, they would remain in the water. Crystallization, as a general thing, is also a purification of foreign admixtures.
In all probability, in Nature many precious stones have formed in the same manner; and most mineralogists concur in the opinion that rock-crystals, consisting of nothing but silicic acid, and frequently weighing hundreds of pounds, have originated thus. It is almost certain that this formation from liquids into solid bodies has taken place in a large class of half-precious stones, such as quartz and pyrites, consisting likewise of nothing but silica—namely, agate, jasper, opal, chalcedony, chrysoprase, carnelian, heliotrope, and others.
At the same meeting of the Parisian Academy where MM. Frémy and Feil described their process of manufacturing artificial rubies and sapphires, M. Monnier stated that he had obtained artificial opals by pouring a highly-diluted solution of oxalic acid cautiously upon a solution as thick as molasses of silicate of soda, which brings about a slow separation of the silicic acid. When, in so doing, he used a solution of the sulphate of nickel protoxide, he obtained apple-green stones, such as the chrysoprase. Thus we see that, as long as the process of separation lasts, we may talk of the growth of precious stones; and we perceive, from the laws of crystallization, how by the attraction of similar parts, and the exclusion of foreign ones, the formation of precious stones of perfectly "pure water" among the more impure ones, which are frequently found, becomes more intelligible.
Another process of crystallization is the slow cooling of molten substances. This can be explained very strikingly to students of chemistry if a kettle of sulphur or molten bismuth is cooled slowly, until it is covered with a crust of congealed matter, so to speak. Pierce that crust in the middle, and pour out a portion of the liquid, and there will form on the walls of the cavity thus created crystals of surpassing beauty, and the whole assumes the appearance of a so-called crystal druse, a form often assumed by amethysts and other half-precious stones. It has been thought that, to make artificial diamonds, it was necessary only to melt coal; but, unfortunately, the results thus far obtained are of no value.
Nature's most successful way of producing precious stones was not to dissolve minerals, but to put them into a fiery liquid condition, and to separate the new productions slowly from their former impure parts by chemical and electric influences, as we shall see directly. The earth, like the sun and most fixed stars at present, was undoubtedly formerly in a fiery, liquid condition. Then the elements were commingled; all substances met, and entered the strangest combinations; the whole globe was an immense chemical laboratory. The earthy substances with the light metals, at the last period of those gigantic processes, probably formed the "mother-liquor," from which, under various chemical agencies, there separated now valuable metals, now grains of gold, and still more frequently substances which were ennobled by crystallization. The "mother-liquor," cooled with its productions, we call primitive formations—granite, feldspar, porphyry, etc. It may here be stated that these primitive processes have recently been imitated in part, and that two principal components of feldspar, albite and orthoclase, have lately been obtained from a fiery, liquid mixture of minerals.
Precious stones so formed would be colorless if, in the terrible furnace of the primordial world, fire-proof metals had not taken upon themselves the task performed by aniline in our present dying-works. Long before there were colored plants and animals, metals played the part of pigments in Nature, and thus produced, in stones, colors almost surpassing in brilliancy those to be found in the animal kingdom. Rubies and emeralds are probably colored with chrome, sapphires with cobalt, lapis-lazulis with iron, and other precious stones with copper, nickel, manganese, etc. But we only have to refer our readers to the magnificent windows of Gothic cathedrals, with their gorgeous colors, produced by combinations of metals in the molten state. The false precious stones made in Paris with so much perfection from heavy strass-glass are colored with metallic oxides in as lasting a manner as the genuine stones.
The first precious stone reproduced, not only in its appearance, but its real nature, and in all its component parts, is the lapis-lazuli, the sapphire of the ancients, not to be confounded with the sapphire of our modern jewelers. This untransparent stone, of a magnificent azure-blue color, was most highly prized by the ancient Hindoos, Assyrians, Persians, Jews, Egyptians, Greeks, etc.; and this irrefragably refutes the erroneous theory of some archæologists that the ancients were unable to distinguish the blue color. When pulverized, this stone furnishes the surpassingly beautiful ultramarine color with which the artists of the middle ages delighted to paint the mantle or gown of the Virgin Mary, although they had to pay the most extravagant prices for the pigment, which they always charged in the bills of those who had ordered a sacred picture from them. Some fifty or sixty years ago, Gmelin, the German chemist, discovered that this most beautiful of blue colors could be artificially produced by heating argillaceous earth with soda, sulphur, and carbon; and now that Guimet, the French chemist, has practically introduced this process, Europe manufactures annually about 100,000,000 pounds of this pigment, most of which is produced in Germany.
At a very early period chemists devoted their attention to the artificial reproduction of rubies and sapphires, which, as we have said before, consist of nothing but crystallized argillaceous earth, colored by minute particles of metals. Several decades ago, the chemist Gaudin succeeded in obtaining small ruby pellets from pure argillaceous earth, precipitated from dissolved alum and moistened with chromate of potash. The color of these rubies, according to the quantity of chromate which they contained, was either that of a rose or bordering on purple. The pellets were so hard that they easily cut glass, garnets, and topazes; but they were not crystals, and their transparency was by no means perfect. Similar experiments were made by the chemists De Bray, Sainte-Claire Deville, Caron, Sénarmont, Ebelmann, and others. It was long acknowledged that a crystallization of argillaceous or beryl earth had to be obtained, and to that end it was necessary to reduce them with the requisite quantities of the coloring metallic combinations into a state of fiery liquefaction. Boric acid was selected for that purpose, because when heated it slowly evaporates. It appears as vapor in volcanic countries, and is especially obtained in Tuscany. The belief that this fiery means of reduction had played in Nature a part in the formation of precious stones was perfectly justifiable; and so boric acid was placed in comparatively large quantities with argillaceous or beryl earth in open platinum crucibles, which were subjected to a long-continued heat in porcelain furnaces. In fact, as soon as the larger portion of the boric acid has evaporated, there are evolved from the fiery, liquid mass small rubies, sapphires, or emeralds. This was discovered some twenty years ago, but the crystals were too small to make the process a remunerative one.
Far more satisfactory were the results of Frémy's recent experiments. They are based upon a different principle, namely, that of separating the argillaceous earth slowly from its usual combination with silicic acid, as it is found in Nature everywhere, by bringing to bear upon it a substance of stronger affinity for the acid. In consequence, small crystals of argillaceous earth are formed in the fiery, liquid "mother-liquor," which, in the course of further separation, grow slowly. In the glass-factories of M. Feil, quantities of this "mother-liquor" of precious stones, weighing from twenty-five to fifty pounds, were easily' kept in a fiery, liquid state for two and three weeks, and in this way very favorable results were obtained. The most advantageous process turned out to be the separation of the argillaceous earth from the silicic acid by means of oxide of lead, for which purpose a mixture of equal parts of pure porcelain-clay and red-lead was placed in a large crucible of fire-proof clay and exposed for weeks to an intense red heat. Usually, the lead also extracts the silicic acid which the walls of the crucible contain, and eats holes through them. Hence, to avoid losses, the precious-stone crucible should be placed in another.
After several weeks of patient waiting, vividly recalling the expectant watching of the old alchemists at their crucibles in which the philosopher's stone was to be created, the crucible is taken out and cooled. After destroying the crucible, the contents are found to consist of two strata, above a glassy one, consisting principally of silicate of lead, and below a crystalline one, containing the most beautiful crystals of argillaceous earth in round clusters. If nothing but argillaceous earth and red-lead has been placed in the crucible, these crystals are as colorless as glass. They will cut glass and rock-crystal, nay, even the very hard topaz; in short, they are precious corundums or diamond-spar, so called because, next to the diamond and crystalline boron, it is the hardest of all stones.
Now rubies, sapphires, and Oriental emeralds, are nothing but colored corundums, and the former two can be easily obtained by the addition of the requisite quantities of the coloring metallic combinations. When there was added to the mixture of argillaceous earth and red-lead two or three per cent, of bichromate of potash, the crystals showed the beautiful rose-color of the ruby; when only a small quantity of that salt was used, and simultaneously a still smaller quantity of oxide of cobalt was added, sapphires were obtained. The precious stones thus produced, as a rule, are covered with a firm crust of silicate of lead, which is best removed chemically by melting it with oxide of lead or potash, or by means of hydrate of fluor-spar. Among a number of pounds of such crystals of argillaceous earth which the inventors submitted to the Academy, there were numerous pieces that could not be distinguished at all from natural rubies and sapphires. They possessed their crystalline shape, their weight, hardness, color, and adamantine lustre, although the latter was not altogether faultless.
How completely the imitation of Nature has succeeded, may be inferred from a peculiarity which the artificial rubies have in common with the natural ones: both, upon being heated, lose their rose-color, and do not recover it until they are cooled again. The diamond-cutters who were requested to grind these artificial rubies found them not only as hard as the natural ones, but in many instances even harder; they were not long in blunting their best tools made of the hardest steel. For the use of watch-makers they are, perhaps, better than the natural stones.
But jewelers, too, are certain, sooner or later, to derive a great deal of benefit from these discoveries. The rubies hitherto obtained, although very beautiful, did not equal the first-class natural stones; but they are only the first productions of a new process, and it is decidedly creditable to the inventors that they immediately divulged their method without trying to mystify the public. Now others, too, may follow up this new branch of a promising alchemy. Perhaps more time should be given to the crystals for their formation, for Nature had a great deal of time for such productions, and it was owing to this fact, perhaps, that it achieved such glorious triumphs. There can be no doubt but that, at some future time, these crystals of argillaceous earth will be colored also green, yellow, and purple, and that thus the precious stones, which were hitherto distinguished as Oriental emeralds, topazes, and amethysts, from inferior stones of the same name, will be produced. The addition "Oriental," in this connection, has no geographical meaning, and was applied by jewelers to the harder and better classes of emeralds, topazes, and amethysts. Perhaps these Oriental stones will be cheaper at an early day than the inferior ones, and the middle classes may wear as brilliant stones as princesses do now.
Diamonds, too, were the objects of similar processes, that is, by trying to bring about a slow separation of carbon from its combinations. However, Chemistry has to admit here that it cannot demonstrate, with any degree of accuracy, how Nature really produced the diamond. Some think that it could only have been formed at an enormously high temperature; others consider its very slow formation in a cold condition more probable; nay, there are scientists who regard it as the production of some organic agency, because there are frequently discerned in them green, cellular formations resembling certain algæ. In view of the rapid progress of synthetic chemistry, it might, perhaps, be as well for the diamond to maintain even in the eyes of chemists its time-honored name "adamas"—that is, the indomitable one. For what should the "fine lady" wear in the future if the prince of precious stones should follow the example of those standing closest to its throne, and allow itself to be reproduced for a few shillings?
- Translated from the Gartenlaube.