PASTES, or FACTITIOUS GEMS. The general vitreous body called Stress (from the name of its German inventor,) preferred by Fontanier, is prepared in the following manner :-8 ounces of lure rock-crystal or flint in powder, mixed with 24 ounces of salt of tartar, are to be baked and left to cool. The mixture is to be afterwards poured into a basin of hot water, and treated with dilute nitric acid till it ceases to effervesce • and then the frit is to be washed till 'the water comes off tasteless. This is to be dried, and mixed with 12 ounces of fine white lead, and the mixture is to be levigated and elutriated with a little distilled water. An ounce of calcined borax being added to about 12 ounces of the preceding mix ture in a dry state, the whole is to be rubbed together in a porcelain mortar, melted in a clean crucible, and poured out into cold water. This vitreous mat ter must be dried, and melted a second and a third time, always in a new crucible, and after each melting poured into cold water, as at first, taking care to separate the lead that may be revived. To the third frit, ground to powder, 5 drachms of nitre are to be added; and the mix ture being melted lbr the last time, a mass of crystal will be found in the cru cible, of a beautiful lustre. The diamond may be well imitated by this Mayence base. Another very fine white crystal be obtained, according to M. Fon tamer, from g ounces of white lead, 2 ounces of powdered borax, .1- grain of man ganese_, and 3 ounces of rock-crystal, treated as above.
The colors of artificial gems are obtain ed from metallic oxides. The oriental topaz is prepared by adding oxide of an timony to the base; the amethyst, by manganese with a little of the purple of Cassius ; the beryl, by and a very little cobalt ; yellow artificial dia mond and opal, by horn-silver (chloride of silver „.) blue-stone or sapphire by cobalt. The following proportions have been given:— For the yellau9 diamond. To 1 ounce of stress, add 24 grains of chloride of sil ver, or 10 grains of glass of antimony.
For the sapphire. To 24 ounces of strass, add 2 drachms and 26 grains of the oxide of cobalt.
For the oriental ruby. To 16 ounces of stress, add a mixture of 2 drachms and 48 grains of the precipate of Cassius, the same quantity of peroxide of iron prepar ed by nitric acid, the same quantity of golden sulphurct of antimony and of man ganese calcined with nitre, and 2 ounces of rock crystal. Manganese alone, com
bined with the base in proper quantity, is said to give a ruby color.
For the emerald. To 15 ounces of stress, add 1 drachm of mountain blue (carbonate of copper), and 6 grains of glass of antimony • or, to 1 ounce of base, add 29 grains of glass of antimony, and 8 grains of oxide of cobalt.
For the common opal. To 1 ounce of stress, add 10 grains of horn-silver, 2 grains of calcined magnetic ore, and 26 grains of an absorbent earth (probably chalk-marl.) M. Douault Wi6land, in an experiment al memoir on the preparation of artificial colored stones, has offered the following instructions, as being more exact than what were published before.
The base of all artificial stones is a col orless glass, which he calls fondant, or flux ; and lie unites it to metallic oxides, in order to produce the imitations. If it be worked alone on the lapidary's wheel, it counterfeits brilliants and rose dia monds remarkably well..
This base or stress is composed of silex, potash, borax, oxide of lead, and some times arsenic. The silicious matter should be perfectly pure; and if obtained from sand, it ought to be calcined and washed, first with dilute muriatic acid and then with water. The crystal or flint should be made redhot, quenched in water, and ground, as in the potteries. The potash should be purified from the best pearl ash ; and the borax should be refined by one or two crystallizations. The oxide of.
lead should be absolutely free from tin, for the least portion of this latter metal causes milkiness. Good red lead is pre ferable to litharge. The arsenic should also be pure. Hessian crucibles are pre ferable to those of porcelain, for they are not so apt to crack and run out. Either a pottery or porcelain kiln will answer. and the fusion should be continued 24 hours ; for the more tranquil and contin uous it is, the denser is the paste, and the greater its beauty.
For rubies, the proportions are :—Paste 2,800; oxide manganese 70.
For emerald :—Paste 4,608; green ox ide copper 42; oxide crome 2.