FALSE PRECIOUS STONES OF THE ARTIFICIAL KIND
There existed in tine middle ages, and probably had existed among the ancients also, a substance called at first amasa, then encansta, and lastly smalta, from which last term our modern émail (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. 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 of 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 o – 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 i860, 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. The new aventurine of M. Pelouze has a beautiful lustre, and a hardness exceeding that of glass and ordinary aventurine.
It is obtained by melting together 250 parts sand, 100 parts carbonate of soda, 50 parts carbonate of lime, and 40 parts bichromate of potassium. It will be seen that by this formula the spangles with a basis of copper are replaced by spangles with a basis of chrome.
False pearls are little hollow spheres of glass covered internally with a coating imitating the orient of natural pearls. Their fabrication comprehends two series of operations—the production of the sphere, and the introduction of the coating.
The spheres are produced by the glass-blower, who by aid of an enameller’s lamp solders the extremity of a tube having the proper diameter, and blows into the tube when the substance is of the right consistency. In this way very regular little spheres are obtained, that serve for the composition of the ordinary quality of false pearls.
In pearls of great beauty the tube employed is slightly opalescent, and the glass-blower, besides, gives to the little spheres, while they are yet malleable, certain slight perceptible inequalities of surface, by gently tapping them with a small iron bar. This gives them a yet greater resemblance to natural pearls, which are very seldom absolutely regular.
No mention is made in ancient writers of artificial pearls being made, and it is not till we come down to the beginning of the sixteenth century that we find Venice had then established a reputation for this branch of industry.
At first the glass balls were filled with various materials, generally with a base of mercury. But in the year 1680 a rosary-maker named Jacquin conceived the idea of using, in the place of this mercurial mixture, a harmless substance that produced an infinitely more perfect colour.
This substance, the essence of orient, is formed from the scales of the bleak or ablette, a little white fish which abounds in the Seine, the Marne, and the Loiret. The fishes are rubbed rather roughly in pure water, contained in a large basin; the whole is then strained through a linen cloth, and left for several days to settle, when the water is drawn off.
The sediment forms the essence of orient. It requires from 17,000 to 18,000 fishes to obtain 500 grammes (a little over a pound) of this substance. The scaly substance is liable to decompose quickly, and numerous chemical agents are employed by different manufacturers to preserve it.
These means are kept a secret, but it is known that liquid ammonia, or the volatile alkali, is one of the substances most commonly used. The process of colouring the pearl is commenced by lining the interior of the ball with a delicate layer of perfectly limpid and colourless parchment-glue; and before it is quite dr.y, the essence of orient is introduced by means of a slender blow-pipe. It is then allowed to dry; the pearl is filled with wax, and, if intended for a necklace, is pierced.
A number of objects are made at the present day of a composition intended to resemble coral, but this imitation is by no means a success. It is a paste formed of marble dust and isinglass. The colour is given by a mixture of vermilion and minium incorporated with the mass.