CHAP. VII. ON THE COMPOSITION OF ARTIFICIAL GEMS. GREAT INTEREST FORMERLY ATTACHED TO THIS SUBJECT. DIFFERENT COMPOSITIONS FOR ARTIFICIAL GEMS. MODE OF PREPARATION. ROCK CRYSTAL FORMERLY EMPLOYED. NOT SUPERIOR TO SAND. DIAMOND PASTES. SELECTION OF VARIOUS PASTES FOR IMITATING DIFFERENT GEMS. REASONS FOR SUCH SELECTION. A VERY considerable portion of every treatise on glassmaking, which was in existence a century ago, and which comprises nearly the whole of what has ever been published on the subject, was devoted to the art of composing factitious gems. A great deal of mystery would seem to have been affected upon this subject on the part of the manufacturers, each one of whom was, or pretended to be, possessed of some secret recipe, which he thought superior to all others for the composition of these orna ments. A corresponding anxiety to acquire a knowledge of these mysteries being evinced on the part of the public, the authors above alluded to, so far acquiesced in this feeling as to load their writings with one receipt after another, in almost endless succession, and in following which the artist was assured, that he might successfully rival nature in the production of these much-admired objects. The greater part of the compositions thus recommended, if indeed they were ever used, have long since passed into neglect; and it will not be necessary, in the present day, to insert more than a very few directions on the subject, which are given upon the authority of M. Fontanieu, as being well qualified, with the addition of various colouring matters, for counterfeiting precious stones. No. 1. is composed of 20 parts of litharge, 12 of silex, 4 of nitre, 4 of borax, and 2 parts of white arsenic. These ingredients should be fritted together in a crucible, and afterwards melted, in which state the whole must be poured suddenly into cold water. Any portion of lead which may have been revived in the metallic state will then be apparent and must be separated. The glass may then be remelted for use. No. 2. For this composition, mix together 20 parts of ceruse, 8 of silex in powder, 4 of carbonate of potash, and 2 of borax. When these are perfectly melted, the whole should be poured into water, and then remelted in a clean crucible, in the same manner as No. 1. No. 3. consists of 16 parts of minium, 8 of rock crystal in powder, 4 of nitre, and 4 of carbonate of potash. These ingredients must be melted and remelted in the manner already described as necessary with the preceding mixtures. It No. 4. differs essentially from the three foregoing combinations in being without any portion of lead. is made with 24 parts of borax, 8 parts of rock crystal, and 8 of carbonate of potash. The rock crystal, previous to its use for this purpose, must be reduced to a state of great purity, by fusing it with an excess of alkali, and then precipitating it by an excess of acid, in the form of an impalpable powder. No. 5. The processes necessary for the production of this species of glass are much more complex than the preceding. In the first place, 3 parts of alkali are to be fritted with 1 part of rock crystal, which mixture must then be dissolved in water and saturated with dilute nitric acid. The silex which is precipitated by this means must then be edulcorated and dried, when it will appear in the form of a very fine impalpable powder. Two parts of this must be melted in a crucible with 3 parts, by weight, of the best ceruse, and the glass which results must be poured into water. Break this down and remelt it with one twelfth of its weight of borax, and pour it again into water. If this last product is once more melted with one twelfth of its weight of nitre, the result will be a very fine hard glass, having an extremely beautiful lustre. The length of time required for fusing hard glasses or pastes is at the least twenty-four hours. The process herein directed, of pouring the melted glass into water, and then remelting, is found to be of considerable use in thoroughly and intimately mixing the ingredients together. Of the foregoing compositions, No. 1. will be found extremely fusible, on account of its considerable proportion of fluxing materials. It calls for the employment of the very best description of crucibles, in order to withstand, for the requisite time, the corroding effects of the mixture. If any kind of glass into the composition of which lead has not entered, is applied to and melted on the interior surface of the crucible, so as to line it with a perfect glaze previous to use, the evil just mentioned will be materially remedied. In order to make a perfect glass, which at the same time shall be sufficiently workable, 2 parts of silex require from 3 to 4 parts, by weight, of oxide of lead; but a somewhat smaller quantity of the latter may be used, if the deficiency is made up by the addition of some other fluxing material: the glass in this case will prove both hard and brilliant; and, when properly set, will exhibit a much nearer imitation of the diamond than most other vitreous compositions. It was formerly imagined by artists who wrought these artificial gems, that if the glass employed by them had for its basis rock crystal, rather than sand, flint, or any other mineral of the like character, the result was a much harder glass than ordinary. This idea, is, however, wholly without foundation; for when the crystal has once been fused through the admixture of any kind of flux, the hardness of the mineral will be irrecoverably lost, as this quality depends altogether upon its natural aggregation, which, in such case, is necessarily destroyed. Rock crystal is, perhaps, somewhat purer than most other siliceous substances, some of which contain minute traces of iron, and which may possibly impair the beauty of some colours which are imparted to glass. The same means as are used to render flint friable, are employed for that purpose with rock crystal: this should on no account be ground in metallic vessels. Some artists have succeeded, to a certain extent, in producing a very fine, hard, brilliant, and colourless glass paste, in imitation of the diamond, and have even given to this a very considerable play of light, or, as it is technically termed, water; but it has not been found practicable to compound any vitreous substance which could for a moment deceive the eye of any person accustomed to witness the superior brilliancy of real gems. The best of these mock diamonds require, indeed, the aid of artifice in the mode of their setting, to render them in any great degree ornamental. M. Fontanieu recommends his glass, No. 1., described in this Chapter, as being better qualified than any other for making artificial diamonds. To bring this glass to such a degree of brilliancy and clearness as will prove at all satisfactory, it must be retained in a state of perfect fusion for a considerable space of time. Loysel recommends, for the same purpose, the employment of a different composition, the result of which will be a glass, having the same specific gravity as the white oriental diamond, and for this reason better imitating that resplendent substance in its refractive and dispersive powers. His recipe is as follows: White sand purified by being washed Calcined borax Oxide of arsenic 100 parts. 150 30 to 35 10 1. This composition is easily fusible at a moderate heat; but like that proposed by Fontanieu, requires to be kept in a melted state for two or three days, to perfect the refining, and to cause the dissipation of the superabundant alkali. The same author has furnished the following receipts for the formation of pastes, qualified, upon the addition of appropriate colouring materials, for the imitation of various gems. The remarks already made as to the length of time required for the due preparation of the diamond paste equally apply to these compositions : White sand, purified in the manner ceipt Red oxide of lead Calcined potash, and nitre, of each 100 parts. 200 20 to 25. The specific gravity of this glass, water being 1, will This will have a greater specific gravity varying from 4 to 4.5. In making his selection between one or other of these pastes, the artist should be guided by their various specific gravities, choosing preferably that glass which is nearest in this respect to the particular gem which he is desirous of imitating; and this, not with the view of providing himself with an additional means of deception, but because, the refractive and dispersive powers of different transparent bodies being determined by their comparative weights, the resemblance will, by such a |