False Precious Stones: Artificial

The basis of all false stones of this kind is glass. A fixed alkali (soda or potash) and silica heated to a red heat will combine and produce glass. Alumina, lime, magnesia, &c., may enter into the combination with the silica; but the result in both cases is colourless, or what is ordinarily called white glass. But if to these substances metallic oxides, or metals in a divided state, are added, even in minute quantities, the result is coloured glass.

Chemical analysis shows us that the elements of glass are found in all vegetables. If, then, a fire consumes a certain quantity of wood, gathered together at a single spot, vitrifications will be found in the residuum. When silicious stones are subjected to an intense heat, the bases contained in the stones and in the cinders combine and produce glass. This is what may be seen every day in an examination of the interior walls of a lime-kiln or brick-kiln. It is evident, then, that the discovery of glass belongs to the earliest period of man's existence. If it be remarked, besides, that the glass thus obtained is always coloured, and therefore in harmony with the pronounced taste of primitive people for brilliant objects, we understand how these vitreous substances produced by conflagrations and, above all, by the action of fire upon silicious stones, must have excited, in the most lively manner, the attention of men from the first ages of our species.

Had this book been written a dozen or fifteen years ago, it could have furnished but little information on this head; but, thanks to the researches of archaeologists, and in particular those of M. Boucher de Perthes--for whom, no doubt, history reserves an exceptional place in its annals--humanity beholds its origin almost instantaneously extended far beyond the historic ages, far beyond all traditions! A new period, during which man lived upon our globe, and which has not until our own epoch been suspected, is now revealed in the most incontestable manner; and among the remains of human industry referable to that remote epoch, are found objects of coloured glass. It must be remarked that coloured glass is much more easily obtained than glass without colour, and that the latter has been produced with ease only in quite modern times.

Without departing from historic times, but only reverting to their most ancient ages, we find that the Egyptians understood very early the manufacture of glass, and especially the coloured glasses.

The design of an Egyptian vase of blue glass, ornamented with white and yellow, is given in Fig. 92. In quality of material, in form, in elegance of ornamentation and harmony of colouring, this vase is in no respect inferior to the best productions of the present day, and yet it must have issued from the hands of the Egyptian workman four thousand years ago. In Fig. 91, 93, 94, 95, and 96, the objects represented are of ceramic paste. From an artistic point of view, these objects are of no value, but the delivery of their details is well worth notice, especially when we consider that they must have been moulded when the matter was in a soft state.

In the time of Pliny, the manufacture of false stones was far advanced as a branch of industry among the Romans. There existed several treatises upon the subject; and Pliny declared that it was a difficult task to distinguish between the false and the true. Not only in Rome were false stones in vogue, but, according to Pliny, the Indians counterfeited jewels with success, especially opals.

The processes that Pliny was so careful not to divulge, were not held sacred with the same scrupulousness by the alchemists of the twelfth and thirteenth centuries. Both Albertus Magnus and St. Thomas Aquinas refer openly to this subject; and the latter in his treatise on the Essence of Minerals, states explicitly that there were "men who fabricated artificial jewels." Among the precious stones counterfeited, he instances the hyacinth, sapphire, emerald, ruby, and topaz.

At the commencement of the Renaissance the fabrication of false stones still continued; but it was not yet separated from much hesitation and experiment. Cardan proves this in his curious receipts.

A century later we perceive by the descriptions of Kircher that the industry had greatly advanced. To the unburned "brick" of Cardan, in whose cavity his mixture for precious stones was heated, excellent crucibles had succeeded; special furnaces had replaced the brick-kiln; and in the time of Kircher, that is to say, about the middle of the seventeenth century, false stones were no longer manufactured according to methods differing for each stone, but according to a general formula much the same as that followed at the present day.

No other proof is needed than the writings of Kircher to dissipate the error that has ascribed the invention of strass--a peculiar kind of glass of considerable refractive power, which forms the base of all modern artificial gems--to a workman of that name, towards the close of the last century. This production was perfectly well known in the middle ages; and it was used for exactly the same purposes as it is used for to-day--for decoration, and the counterfeiting of precious stones. It is distinguished from ordinary glass by the presence of about 50 per cent. of oxide of lead among its constituents.

There existed in the middle ages, and probably had existed among the ancients also, a substance called at first amasa, then encausta, and lastly smalta, from which last term our modern email (enamel) is derived. These were generic expressions for substances formed of glass and a metallic oxide; and the basis employed was certainly a kind of strass--that is to say, glass containing a great quantity of oxide of lead.

The improvement made in strass since the middle ages is due to our modern chemistry, which furnishes productions of a perfect purity, otherwise the ingredients, and probably their proportions, remain the same; and the same rule is still observed that the longer the fusion is prolonged, the finer will be the quality of the strass. According to M. Dumas, the strass now employed consists of--

Silica,........................................ 38.2

Red oxide of lead,............................. 53.0

Calcined potash,............................... 7.8

Calcined borax, alumina, and arsenious acid,... Traces.

When the strass is obtained very pure, all the precious stones may be imitated with it. For this purpose it is melted and mixed with substances having a metallic base, generally oxides, which, combining with the elements of the strass, communicate to it the most varied colours. We add a few details to show how the principal gems may be imitated.

Diamond.--The diamond being colourless, pure strass, cut into brilliants and roses, is used to counterfeit it.

Ruby.-- 1000 parts strass, 40 glass of antimony, 1 purple of Cassius, and 1, in excess, of gold.

Sapphire.--1000 parts strass, and 25 oxide cobalt.

Topaz.--Same formula as that of the ruby, without the excess of gold, and heated for a less time.

Emerald.--1000 parts strass, 8 oxide of copper, and 0.2 oxide of chromium.

Amethyst.--1000 parts strass, 25 oxide of cobalt, and a little oxide of manganese.

Garnet.--1000 parts strass, and a variable quantity of purple of Cassius, according to the shade to be obtained.

Aventurine.--For several centuries Venice has had the monopoly of the fabrication of aventurine; and even now, it is a Venetian artist, Bibaglia, who furnishes to commerce the artificial aventurine that is most highly prized.

Aventurine is a glass the base of which is soda ash, lime, and magnesia, coloured yellow by oxide of iron, and holding in suspension a large number of small particles of oxide of copper. The distribution of these particles in a regular manner through the whole vitreous mass appears to be the chief difficulty in its manufacture.

The dexterity requisite to accomplish this must be very difficult to attain, for the profits realized from the manufacture of aventurine are remarkably large. According to its quality, the artificial gem sells for $5 to $15 the pound, while the raw materials that enter into the composition of a pound of it are certainly not worth a quarter-dollar.

French chemists--M. Hautefeuille in 1860, and M. Pelouze in 1865--have published processes by which productions have been obtained equal to that of Venice, and, in the latter case, perhaps superior.