ARTIFICIAL JEWELS.
The trade in artificial jewels has become very extensive during the last half-century, and the chemical experiments in which various qualities of imitation diamonds, rubies, sapphires, and emeralds are produced have been recently carried on with an astonishing amount of success. It is becoming more and more difficult, even to the eye of the expert, to distinguish readily between the real and the false gem, when they do not shine in too close proximity.
The most distinctive feature of the real stone is its hardness, though even this quality has been imitated with considerable success. The term ‘hardness’ is used by the lapidary and mineralogist to denote the power of one stone to scratch another; it must not be considered as the power of resisting a blow, for many crystalline stones which are very hard are also easily fractured. The diamond, which will scratch any other stone, can be more easily broken than many stones which are less hard. After the diamond come the ruby and sapphire, which are the next hardest stones; then emeralds, topazes, and quartz or rock-crystal; and finally, a number of other stones, and glass or artificial stones.
The beautiful ‘French paste’ which imitates the diamond so well, is a kind of glass into which a certain quantity of oxide of lead is introduced. The more lead it contains the more brilliant is the artificial stone; but the lead gives softness—so much so, that we have known such artificial gems to become, by friction with other harder substances, quite dull on the surface after being worn for some time.
But the latest chemical experiments on the production of artificial stones for use in jewellery point very clearly to the fact that further success in this direction is likely to be forthcoming before long. The imitation of the natural gems by means of various silicates and oxides has already attained to a great degree of perfection, and no doubt this ingenious branch of industry must interfere considerably with the trade of the dealer in real precious stones. We can already purchase a capital ‘diamond’ for about half-a-crown; and the imitation of the ruby and the emerald is far easier, and more successful, than that of the diamond.
Careful choice in the substances to be melted together, good and effective cutting, and careful artistic setting, have gone a long way to reproduce, artificially, the brightness, brilliancy, and colour of the real stone. Chemical analysis shows the sapphire to be pure alumina, as it has shown the diamond to be pure carbon; but it does not account for its colour, which is partly due to an optical effect, and depends upon a peculiar molecular arrangement. This stone possesses the singular property known as dichroism—that is, it shines with two colours, blue and red. In a well-cut stone, a red cross often appears in the midst of the sapphire blue. The ruby is also pure alumina, and its vivid red colour, like the blue of the sapphire, is thought by some to be due to a peculiar optical effect. In fact, no chemical analysis has been able to account quite satisfactorily for the red colour of the ruby or the blue colour of the sapphire, for pure alumina is quite white, and the sapphire, as we have seen, shows two colours. This peculiar optical effect noticed in the ruby and sapphire has, strange to say, been accidentally reproduced not long since by a French chemist, M. Sidot, who has been making some experiments on artificial stones. He has produced a kind of glass by melting phosphate of lime at a great heat, and the product possesses the blue colour of the sapphire with the remarkable dichroism before alluded to. The experiment is so curious, that a few lines may be devoted to it here.
By the action of heat on what is termed ‘acid phosphate of lime,’ it is transformed into ‘crystallised pyrophosphate;’ and when heated to a still higher temperature, it passes into the vitreous or glassy state. It is supposed that in this condition it loses some of its phosphoric acid by volatilisation, and passes into the state of ‘tribasic phosphate.’ Such is the technical explanation of the changes which occur. The phosphate of lime glass is produced by taking this substance in a moist acid state, and heating it in an iron pot to a dark red heat. During this operation it is worked about with an iron rod, in order to prevent it swelling up and passing over the edge of the iron crucible. The dark red heat is continued until the whole mass has become glassy and transparent. At this moment it is run into another crucible, in which it is heated to a white heat that is kept up for about two hours, being stirred rapidly with a rod the whole time. At the end of this period the molten mass is allowed to remain perfectly quiet for about an hour, and is then run out of the crucible, either on to a metallic slab or into a metal mortar. It is necessary to avoid too rapid a cooling. The product may thus be run out into a sheet, like plate-glass. A small sheet of such a nature was obtained by M. Sidot in one of his experiments: it measured about three inches across, by a quarter of an inch thick, and was large enough to be cut into a considerable number of beautiful artificial sapphires.
The ruby and sapphire have also been closely imitated in another way by Fremy and Feil, two French chemists; and the chief interest in this process is the fact that the artificial stones possess essentially the chemical composition of the real ones. To produce these, equal weights of alumina and red-lead are heated to a red-heat in an earthenware crucible. A vitreous substance is formed, which consists of silicate of lead, and crystals of white corundum. To convert this corundum into the artificial ruby, it is necessary to fuse it with about two per cent. of bichromate of potassium; whilst, to obtain the sapphire, a little oxide of cobalt, and a very small quantity of bichromate of potassium, must be employed. The stones so produced possess at least very nearly the hardness of the real stones, as they scratch both quartz and topaz.
The French ‘paste’ which imitates the diamond so closely is a peculiar kind of glass, the manufacture of which was brought to a great degree of perfection some fifty years ago by Donault-Wieland of Paris. The finest quality of paste demands extreme care in the choice of materials and in melting, &c. The basis of it, in the hands of the expert manufacturer just named, was powdered rock-crystal or quartz. The proportions he took were—six ounces of rock-crystal; nine ounces two drams of red-lead; three ounces three drams of pure carbonate of potash; three drams of boracic acid; and six grains of white arsenic. The product thus manufactured was extremely beautiful, but rather expensive, compared with the prices now charged for artificial jewels. It has never been surpassed in brilliancy. But of late years the greater purity of the potash and lead oxide used, and the improvements in the furnaces and methods of heating them, have all tended to reduce the price of the ‘diamonds’ thus manufactured.