PASTES

or FACTITIOUS GEMS. ( Pierres précieuses art if i ciel les , Fr.; Glaspasten , Germ.) The general vitreous body called Stra , (from the name of its German inventor,) preferred by Fontanier in his treatise on this subject, and which he styles the Mayence base, is prepared in the following manner:—8 ounces of pure 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 tastele . 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 mixture 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 matter 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 mixture being melted for the last time, a ma of crystal will be found in the crucible, of a beautiful lustre. The diamond may be well imitated by this Mayence base. Another very fine white crystal may be obtained, according to M. Fontanier, from 8 ounces of white-lead, 2 ounces of powdered borax, 1 ⁄ 2 grain of manganese, and 3 ounces of rock crystal, treated as above. The colours of artificial gems are obtained from metallic oxides. The oriental topaz , is prepared by adding oxide of antimony to the base; the amethyst, by manganese with a little of the purple of Ca ius; the beryl, by antimony and a very little cobalt; yellow artificial diamond and opal, by horn-silver (chloride of silver); blue-stone or sapphire, by cobalt. The following proportions have been given :— For the yellow diamond . To 1 ounce of stra add 24 grains of chloride of silver, or 10 grains of gla of antimony. For the sapphire . To 24 ounces of stra , add 2 drachms and 26 grains of the oxide of cobalt. For the oriental ruby . To 16 ounces of stra , add a mixture of 2 drachms, and 48 grains of the precipitate of Ca ius, the same quantity of peroxide of iron prepared by nitric acid, the same quantity of golden sulphuret of antimony and of manganese calcined with nitre, and 2 ounces of rock crystal. Manganese alone, combined with the base in proper quantity, is said to give a ruby colour. For the emerald . To 15 ounces of stra , add 1 drachm of mountain blue (carbonate of copper), and 6 grains of gla of antimony; or, to 1 ounce of base, add 20 grains of gla of antimony, and 3 grains of oxide of cobalt. For the common opal . To 1 ounce of stra , add 10 grains of horn-silver, 2 grains of calcined magnetic ore, and 26 grains of an absorbent earth (probably chalk-marl) Fontanier . M. Douault-Wiéland, in an experimental memoir on the preparation of artificial coloured stones, has offered the following instructions, as being more exact than what were published before. The base of all artificial stones is a colourle gla , which he calls fondant , or flux; and he 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 diamonds remarkably well. This base or stra is composed of silex, potash, borax, oxide of lead, and sometimes arsenic. The siliceous 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 pearlash; 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 milkine . Good red-lead is preferable to litharge. The arsenic should also be pure. He ian crucibles are preferable 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 continuous it is, the denser is the paste, and the greater its beauty. The following four recipes have afforded good stra :— | Grains. | | No. I. | | Or, | Rock crystal | 4056 | | Minium | 6300 | | Pure potash | 2154 | | Borax | 276 | | Arsenic | 12 | | No. II. | | Sand | 3600 | | Ceruse of Clichy (pure carbonate of lead) | 8508 | | Potash | 1260 | | Borax | 360 | | Arsenic | 12 | | No. III. | | Rock crystal | 3456 | | Minium | 5328 | | Potash | 1944 | | Borax | 216 | | Arsenic | 6 | | No. IV. | | Rock crystal | 3600 | | Ceruse of Clichy | 8508 | | Potash | 1260 | | Borax | 360 | Topaz. | Grains. | | Very white paste | 1008 | | Gla of antimony | 43 | | Ca ius purple | 1 | | Or, | | | Paste | 3456 | | Oxide of iron, called saffron of Mars | 36 | Ruby. M. Wiéland succeeded in obtaining excellent imitations of rubies, by making use of the topaz materials. It often happened that the mixture for topazes gave only an opaque ma , translucent at the edges, and in thin plates of a red colour. 1 part of this substance being mixed with 8 parts of stra , and fused for 30 hours, gave a fine yellowish crystal-like paste, and fragments of this fused before the blowpipe, afforded the finest imitation of rubies. The result was always the same. The following are other proportions for rubies :— | Grains. | | Or, | Paste | 2880 | | Oxide of manganese | 72 | Emerald. | Or, | Paste | 4608 | | Green oxide of pure copper | 42 | | Oxide of chrome | 2 | Sapphire. | Grains. | | Or, | Paste | 4608 | | Oxide of cobalt | 68 | This mixture should be carefully fused in a luted He ian crucible, and be left 30 hours in the fire. Amethyst. | Grains. | | Paste | 4608 | | Oxide of manganese | 36 | | Oxide of cobalt | 24 | | Purple of Ca ius | 1 | Syrian Garnet, or Antient Carbuncle. | Grains. | | Paste | 512 | | Gla of Antimony | 256 | | Ca ius purple | 2 | | Oxide of manganese | 2 | Beryl, or Aqua Marina. | Grains. | | Paste | 3456 | | Gla of antimony | 24 | | Oxide of cobalt | 11⁄2 | In all these mixtures, the substances should be mixed by sifting, fused very carefully, and cooled very slowly, after having been left in the fire from 24 to 30 hours. M. Lançon has also made many experiments on the same subject. The following are a few of his proportions :— Paste. | Grains. | | Litharge | 100 | | White sand | 75 | | White tartar, or potash | 10 | Amethyst. | Grains. | | Paste | 9216 | | Oxide of manganese | from 15 to 24 | | Oxide of cobalt | 1 | Emerald. | Grains. | | Paste | 9216 | | Acetate of copper | 72 | | Peroxide of iron, or saffron of Mars | 1·5 |