The practical part of the art of making fictitious gems, in so far as relates to colour and consistence, is amply ex plained by M. Fontanieu, from whose work we shall quote a few observations, as his principles have received gene ral approbation. M. Fontanieu's primary object was the discovery of a colourless base, constituting the elements of all gems, which, by the addition of metallic oxides, would imitate their respective tinge; and he found it might be obtained from different siliceous substances, pure sand, flint, pebbles, or rock crystal. Any of these, such as crys tal or pebbles, being pounded, are put into a crucible and heated red hot ; the contents are emptied into a wooden bowl filled with clean cold water, and being shaken from time to time, the lighter particles of extraneous matter es cape and rise to the surface, while the rest, together with any metallic particles, remain below. The water is then decanted, and the mass being dried and pounded, is sifted through a sieve of the finest silk, after which the powder is digested four or five hours in the muriatie acid, shaking the mixture frequently. Having poured off the acid, a pure vitrifiable earth remains, which, being frequently washed, is again dried and sifted, when it becomes fit for use. From the earth now obtained, M. Fontanieu formed six different bases, of which the fifth seems to he that, which, in respect of quality, is preferred by himself; and his proportions are all calculated in parts of eight ounces each. The first base is formed by 20 ounces of lead in scales, 12 ounces of prepared rock crystal or flint, 4 ounces of nitre, 4 ounces of borax, and 2 ounces of arsenic, which being well pulverized and mixed, are melted in a Hessian crucible, and poured into cold water. The mass is melt ed again a second and a third time, always in a new cruci ble, and after each melting poured into cold water as at first, taking care to separate the lead that may be revived. This forms the first base. The second is obtained from a mixture of 20 ounces of white ceruse, 8 ounces of prepar ed flint, 4 ounces of salt of tartar, and 2 ounces of calcined borax, all melted in a Hessian crucible, and poured into cold water. The melting must be repeated, and the mass washed a second and third time with the same precautions as before. A compound of 16 ounces of minium, 8 ounces of crystal, 4 ounces of nitre, and 4 ounces of salt of tartar, constitutes the third base, being treated as in the preced ing examples. The fourth is formed by 8 ounces of rock crystal, 24 ounces of calcined borax, 8 ounces salt of tar tar mixed and melted together, and poured into warm wa ter. The mass being dried, an equal quantity of minium is to be added, and the whole repeatedly melted and wash ed as before. Fifthly, a base judged by the author as one of the finest crystalline compositions, and called by him the Mayence base, is procured in the following manner : Eight ounces of rocs crystal, or flint pulverised, is baked along with 24 ounces of salt of tartar, and the mixture left to cool. It is afterwards poured into a crucible of hot water, to dis solve the frit, and the solution is received into a stone ware pan, and the nitrous acid added until it no longer effer vesces. The water being decanted, the frit must be wash ed in warm water until it ceases to have any taste ; and the frit being then dried and mixed with 12 ounces of fine ce ruse, or white lead in scales, the mixture is to be well le vigated with a little distilled water. An ounce of calcined borax is now to be added to twelve ounces of this powder when dried, the whole well mixed in a marble mortar, then melted and poured into cold water, in the same manner as in the rest of the bases already described. After repeating these fusions and lotions, and the mixture being dried and pulverised, five drachms of nitre are to be added, and the whole being melted for the last time, a mass of crystal will be found in the crucible with a beautiful lustre. Lastly, a very fine white crystal may be obtained from eight ounces of ceruse, two ounces of borax finely powder ed, half a grain of manganese, and three ounces of rock crystal treated as above.
The colour of artificial gems is obtained from metallic oxides, a fact that must have been almost coeval with the discovery of vitrification; but the opinion of chemists has differed as to their proportions and combinations. The
diamond being colourless, is imitated simply by the \lay ence base ; and M. Fontanieu gives receipts for making all other fictitious gems, of which the following are exam ples. The oriental topaz is prepared by adding five drachms of antimony to colour 24 ounces of the first and third base : the amethyst, by taking 2-1 ounces of the Mayence base, to which are added for colour, four drachms of manganese, prepared by being exposed to a red heat, and quenched in distilled vinegar ; then dried and powdered, and passed through a silk sieve, and also four grains of precipitate of cassias. The hyacinth is obtained from a base of prepared rock crystal, and two drachms 48 grains added for colour. To imitate the beryl, 96 grains of antimony, and four grains of the calx of cobalt, are added to 24 ounces of the third base. Precautions, however, arc necessary in preparing the oxides ; and these the author has detailed at length in his original work, of which there is an abstract in the Jour nal de Physique, 1778. A fictitious turquoise, from a com position by M. Suaviac, has lately appeared in France, which is a perfect imitation of the original, and the colours penetrate the substance of the mass. It is now ascertained that the genuine turquoise is not what may be called an original product of nature, but the petrified bone of an ani mal. The same artist, M. Sauviac, has likewise made fic titious malachite.
The quality of artificial gems consists in their absolute uniformity of texture, compactness, hardness, and lustre ; and it is surprising to what perfection repeated practice has brought the art. Fictitious gems may be detected from their softness, which always yields to the file, and from their small specific gravity.
The artist having chosen his natural gem, it is put into the hands of the lapidary to give it surface and figure. Next he models the subject in wax, as delicately as possi ble, which he means to represent, in order that he may constantly avail himself of the resemblance, or he has some other subject, which he is to imitate, before him. His tools are few and simple, consisting of a lathe nearly on the same principle as the turning lathe, into the end of the spindle of which he can insert points, knobs, or circles. These project a certain distance, and receive a rapid mo tion from a wheel below. The gem is fixed with mastic to the end of a small piece of wood as a handle, and the artist sketches the subject upon it with a copper point or a dia mond. It is then applied to the end of one of the tools in motion, which, being wet with diamond dust and olive oil, quickly grinds off the surface ; and, by long perseverance, or frequent repetition, produces the outline of the figure, and then the hollow or relief. Frequent impressions must be taken in the course of the work, to shew where there is any excess or defect ; for the slightest error in one part, may occasion the alteration of the whole. Such impres sions, however, are necessary only in intaglios,* not in ca meos, because both the prominences and depressions are there obvious to the eye ; and experienced artists consider it more easy to work on a convex surface, from the distance interposed between the tool and the hand. Although the most elegant curves may be produced with skill and atten tion, much difficulty is experienced in cutting angles ; for the tool being the segment of a circle, can never be closely applied; and hence all the letters on the ancient gems, even of the first masters, are imperfect.
The tools employed are either of soft iron or copper ; and in defect of diamond powder, that of the ruby, or other hard stones, may be substituted. In cutting the amethyst, the Bohemian emerald, or crystal, water is used to wet the diamond dust instead of oil. Tools of still softer metal, along with tripoli, or rotten stone, and lastly brushes, are used in giving gems their final polish ; but it has been ob served, that in this respect, the moderns have never been able to equal the ancients. We describe this process in few words, but it is a slow and tedious operation, and requires a penetrating eye, and a delicate hand. The artist must be master of drawing and modelling, and he must be a suf ficient naturalist to be intimately acquainted with the pro perties and imperfections of the various finer stones.