in natural history, a common name for all precious stones; of which there are two classes, the pellucid and semipellucid.
The bodies composing the class of pellucid gems are bright, elegant, and beautiful fossils, naturally and essentially compound, ever found in small detached masses, extremely hard, and of great lustre.
The bodies composing the class of semipellucid gems, are stones naturally and essentially compound, not inflammable nor soluble in water, found in detached masses, and composed of crystalline matter debauched by earth; however, they are but slightly debased, and are of great beauty and brightness, of a moderate degree of transparency, and are usually found in small masses.
The knowledge of gems depends principally on observing their hardness and colour. Their hardness is commonly allowed to stand in the following order: The diamond the hardest of all; then the ruby, sapphire, jacinth, emerald, amethyst, garnet, carnelian, chalcedony, onyx, jasper, agate, porphyry, and marble. This difference, however, is not regular and constant, but frequently varies. Good crystals may be allowed to succeed the onyx; but the whole family of metallic glairy flours seem to be still softer.—In point of colour, the diamond is valued for its transparency, the ruby for its purple, the sapphire for its blue, the emerald for its green, the jacinth for its orange, the amethyst carnelian for its carnation, the onyx for its tawny, the jasper, agate, and porphyry, for their vermillion, green, and variegated colours, and the garnet for its transparent blood-red.
All these gems are sometimes found coloured and spotted, and sometimes quite limpid and colourless. In this case the diamond-cutter or polisher knows how to distinguish their different species by their different degrees of hardness upon the mill. For the cutting or polishing of gems, the fine powder of the fragments of those that are next in degree of hardness is always required to grind away the softer; but as none of them are harder than the diamond, this can only be polished by its own powder. Cronstedt observes of gems in general, that the colour of the ruby and emerald are said to remain in the fire, while that of the topaz flies off: hence it is usual to burn the topaz, and thence substitute it for the diamond. "Their colours (says our author) are commonly supposed to depend upon metallic vapours; but may they not more justly be supposed to arise from a phlogiston united with a metallic or some other earth? because we find that metallic earths which are perfectly well calcined give no colour to any glass; and that the manganese, on the other hand, gives more colour than can be ascribed to the small quantity of metal which is to be extracted from it." M. Magellan is of opinion, that their colour is owing chiefly to the mixture of iron which enters their composition; but approves the sentiment of Cronstedt, that phlogiston has a share in their production, it being well known that the calces of iron when dephlogisticated produce the red and yellow colours of marble, and when phlogisticated to a certain degree produce the blue or green colours.
With regard to the texture of gems, M. Magellan observes, that all of them are foliated or laminated, and of various degrees of hardness. Whenever the edges of these laminae are sensible to the eye, they have a fibrous appearance, and reflect various shades of colour, which change successively according to their angular position to the eye. These are called by the French chatonnerie; and what is a blemish in their transparency, often enhances their value on account of their scarcity. But when the substance of a gem is composed of a broken texture, consisting of various sets of laminae differently inclined to each other, it emits at the same time various irradiations of different colours, which succeed one another according to their angle of position. This kind of gems has obtained the name of opals, and are valued in proportion to the brilliancy, beauty, and variety of their colours. Their crystallization, no doubt, depends on the same cause which produces that of salts, earths, and metals, which is treated of under the article Crystallization: but as to the particular configuration of each species of gems, we can hardly depend upon any individual form as a criterion to ascertain each kind; and when we have attended with the utmost care to all that has been written on the subject, we are at last obliged to appeal to chemical analysis, because it very often assumes various forms. The following table shows the component parts of gems according to the analysis of Bergman and M. Achard; the letter B prefixed to each denoting Bergman's analysis, and A that of Achard.
| Gem | Argil. | Silic. | Calc. | Iron | |----------------------|--------|--------|-------|------| | Red oriental ruby | B | 40 | 39 | 9 | | Ditto | A | 37.5 | 42.5 | 9 | | Blue oriental sapphire | B | 58 | 35 | 5 | | Ditto | A | 58 | 33 | 6 | | Yellow topaz from Saxony | B | 46 | 39 | 8 | | Green oriental emerald | B | 60 | 24 | 8 | | Ditto | A | 60 | 23 | 10 | | Yellow-brown orient hyacinth | B | 40 | 25 | 20 | | Ditto | A | 42 | 22 | 20 | | Tourmalin from Ceylon | B | 39 | 37 | 15 | | Ditto from Brazil | B | 50 | 34 | 11 | | Ditto from Tyrol | B | 42 | 40 | 12 | | Garnet from Bohemia | A | 30 | 48 | 11 |
The chrysopele from Koscinitz in Silesia was likewise analysed by M. Achard; who found that it contained 456 grams of siliceous earth, 13 of calcareous, 6 of magnesia, 3 of copper, and 2 of iron. "This (says M. Magellan) seems to be the only gem that contains no argillaceous earth."
Imitation or Counterfeiting of Gems in Glass. The art of imitating gems in glass is too considerable to be passed without notice: some of the leading compositions therein we shall mention upon the authority of Neri and others.
These gems are made of pastes; and are noway inferior to the native stones, when carefully made and well polished, in brightness or transparency, but want their hardness.
The general rules to be observed in making the pastes are these: 1. That all the vessels in which they are made be firmly luted, and the lute left to dry before they are put into the fire. 2. That such vessels be chosen for the work as will bear the fire well. 3. That the powders be prepared on a porphyry stone; not in a metal mortar, which would communicate a tinge to them. 4. That the just proportion in the quantity of the several ingredients be nicely observed. 5. That the materials be all well mixed; and, if not sufficiently baked the first time, to be committed to the fire again, without breaking the pot; for if this be not observed, they will be full of blisters and air-bladders. 6. That a small vacancy be always left at the top of the pot, to give room to the swelling of the ingredients.
To make paste of extreme hardness, and capable of all the colours of the gems, with great lustre and beauty.—Take of prepared crystal, ten pounds; salt of pumice, six pounds; sulphur of lead, two pounds; mix all these well together into a fine powder; make the whole with common water into a hard paste; and make this paste into small cakes of about three ounces weight each, with a hole made in their middle; dry them in the sun, and afterwards calcine them in the straitest part of a potter's furnace. After this, powder them, and levigate them to a perfect fineness on a porphyry-stone, and set this powder in pots in a glas-furnace to purify for three days: then cast the whole into water, and afterwards return it into the furnace, where let it stand 15 days, in which time all foulness and blisters will disappear, and the paste will greatly resemble the natural jewels. To give this the colour of the emerald, add to it bras thrice calcined; for a sea-green, bras firmly calcined to a redness; for a sapphire, add zaffer, with manganese; and for a topaz, manganese and tartar. All the gems are thus imitated in this, by the same way of working as the making of coloured glasses; and this is so hard, that they very much approach the natural gems.
The colour of all the counterfeit gems made of the several pastes, may be made deeper or lighter, according to the work for which the stones are designed; and it is a necessary general rule, that small stones for rings, &c. require a deeper colour, and large ones a paler. Besides the colours made from manganese, verdigris, and zaffer, which are the ingredients commonly used, there are other very fine ones which care and skill may may prepare. Very fine red may be made from gold, and one not much inferior to that from iron; a very fine green from brass or copper; a sky-colour from silver, and a much finer one from the granates of Bohemia.
A very singular and excellent way of making the paste to imitate the coloured gems is this: Take a quantity of saccharum saturni, or sugar of lead, made with vinegar in the common way; set it in sand, in a glass body well luted from the neck downwards; leave the mouth of the glass open, and continue the fire 24 hours; then take out the salt, and if it be not red but yellowish, powder it fine, and return it into the vessel, and keep it in the sand-heat 24 hours more, till it becomes as red as cinnabar. The fire must not be made so strong as to melt it, for then all the process is spoiled. Pour distilled vinegar on this calcined salt, and separate the solution from the dregs; let the decanted liquor stand six days in an earthen vessel, to give time for the finer sediment to subside; filter this liquor, and evaporate it in a glass body, and there will remain a most pure salt of lead; dry this well, then dissolve it in fair water; let the solution stand six days in a glazed pan; let it subside, then filter the clear solution, and evaporate it to a yet more pure white and sweet salt; repeat this operation three times; put the now perfectly pure salt into a glass vessel, set it in a sand-heat for several days, and it will be calcined to a fine impalpable powder, of a lively red. This is called the sulphur of lead.
Take all the ingredients as in the common composition of the pastes of the several colours, only instead of red lead, use this powder; and the produce will well reward the trouble of the operation, as experience has ten proved.
A paste proper for receiving colours may be readily made by well-pounding and mixing five pounds of white sand cleansed, three pounds of red lead, two pounds of purified pearl-ashes, and one pound of nitre. A softer paste may be made in the same manner, of five pounds of white sand cleansed; red lead, and purified pearl-ashes, of each three pounds; one pound of nitre, half a pound of borax, and three ounces of arsenic. For common use a pound of common salt may be substituted for the borax. This glass will be very soft, and will not bear much wear if employed for rings, buckles, or such imitations of stones as are exposed to much rubbing; but for ear-rings, ornaments worn on the breast, and those little used, it may last a considerable time.
In order to give paste different colours, the process is as follows. For
Amethyst. Take ten pounds of either of the compositions described under Colouring of Glass, one ounce and a half of manganese, and one dram of zaffer; powder and fuse them together.
Black. Take ten pounds of either of the compositions just referred to, one ounce of zaffer, five drams of manganese, and five drams of iron, highly calcined; and proceed as before.
Blue. Take of the same composition ten pounds; of zaffer fix drams; and of manganese two drams; and proceed as with the foregoing.
Chrysolite. Take of either of the compositions for paste above described, prepared without saltpetre, ten pounds, and of calcined iron five drams; and pursue the same process as with the rest.
Red Cornelian. Take of the compositions mentioned under Colouring of Glass two pounds; of glaas of antimony one pound; of the calcined vitriol called scarlet oker two ounces; and of manganese one dram. Fuse the glaas of antimony and manganese with the composition; then powder them, and mix them with the other, by grinding them together, and fuse them with a gentle heat.
White Cornelian. Take of the composition just referred to two pounds; and of yellow oker well washed two drams; and of calcined bones one ounce. Mix them, and fuse them with a gentle heat.
Diamond. Take of the white sand six pounds; of red-lead four pounds; of pearl ashes purified three pounds; of nitre two pounds; of arsenic five ounces; and of manganese one scruple. Powder and fuse them.
Eagle-marine. Take ten pounds of the composition under Glass; three ounces of copper highly calcined with sulphur; and one scruple of zaffer. Proceed as before.
Emerald. Take of the same composition with the last nine pounds; three ounces of copper precipitated from aquafortis; and two drams of precipitated iron. See Emerald.
Garnet. Take two pounds of the composition under Glass; two pounds of the glaas of antimony, and two drams of manganese. For vinegar garnet, take of the composition for paste, described in this article, two pounds; one pound of glaas of antimony, and half an ounce of iron, highly calcined; mix the iron with the uncoloured paste, and fuse them; then add the glaas of antimony powdered, and continue them in the heat till the whole is incorporated.
Gold or full yellow. Take of the composition for paste ten pounds, and one ounce and a half of iron strongly calcined; proceeding as with the others.
Deep purple. Take of either of the compositions for paste ten pounds; of manganese one ounce; and of zaffer half an ounce.
Ruby. Take one pound of either of the compositions for paste, and two drams of calx Caflis, or precipitation of gold by tin; powder the paste, and grind the calx of gold with it in a glaas, flint, or agate mortar, and then fuse them together. A cheaper ruby paste may be made with half a pound of either of the above compositions, half a pound of glaas of antimony, and one dram and a half of the calx of gold; proceeding as before.
Sapphire. Take of the composition for paste ten pounds; of zaffer three drams and one scruple; and of the calx Caflis one dram. Powder and fuse them. Or the same may be done, by mixing with the paste one-eighth of its weight of smalt.
Topaz. Take of the compositions under Glass ten pounds, omitting the saltpetre; and an equal quantity of the Gold-coloured hard Glass. Powder and fuse them. See Topaz.
Turquoise. Take of the composition for blue paste already described, ten pounds; of calcined bone, horn, or ivory, half a pound. Powder and fuse them.
Opake white. Take of the composition for paste ten pounds; and one pound of calcined horn, ivory, or bone; and proceed as before.
Semitransparent white, like opal. See Opal. To the above we shall add the following receipts and processes, contained in a Memoir by M. Fontanius of the Royal Academy of Sciences at Paris, and said to have met with much approbation.
I. Of the Bases. Although the different calces of lead are all adapted to produce the same effect in vitrification; yet M. Fontanier prefers lead in scales, and next to that minium, as being the most constantly pure. It is necessary to sift through a silk sieve the preparations of lead one wishes to make use of in the vitrification, in order to separate the grosser parts, as also the lead found in a metallic state when white lead in scales is employed.
The base of fictitious gems is calx of lead and rock-crystal, or any other stone vitrifiable by the calces already mentioned. Pure sand, flint, and the transparent pebbles of rivers, are substances equally fit to make glas; but as it is first necessary to break the masses of crystal, stones, or pebbles, into smaller parts; so by this operation particles of iron or copper are frequently introduced, and to these dust or greasy matters are also apt to adhere. Our author therefore begins by putting the pounded crystal or pebbles into a crucible, which he places in a degree of heat capable of making the mass red-hot; he then pours it into a wooden bowl filled with very clear water; and shaking the bowl from time to time, the small portions of coals furnished by the extraneous bodies swim on the surface of the water, and the vitrifiable earth, with the iron, &c., rests on the bottom. He then decants the water; and having dried the mass, he pounds it, sifts the powder through the finest silk sieve; he then digests the powder during four or five hours with marine acid, shaking the mixture every hour. After having decanted the marine acid from the vitrifiable earth, he washes the latter until the water no longer reddens the tincture of tourniol. The said earth being dried, is passed through a silk sieve, and is then fit for use. Nitre, salt of tartar, and borax, are the three species of salts that enter with quartz and the several calces of lead into M. Fontanius's vitrifications.
Much of the success in the art of making coloured stones depends on the accurate proportion of the substances made use of to form the crystal which serves as a base to the fictitious stones. After having tried a great variety of receipts, our author found they might be reduced to the following:
1. Take two parts and a half of lead in scales, one part and a half of rock-crystal or prepared flints, half a part of niter, as much borax, and a quarter part of glas of arsenic. These being well pulverized and mixed together, are to be put into a Hessian crucible, and submitted to the fire. When the mixture is well melted, pour it into cold water; then melt it again a second and a third time; taking care, after each melting, to throw it into fresh cold water, and to separate from it the lead that may be revived. The same crucible should not be used a second time, because the glas of lead is apt to penetrate it in such a manner as to run the risk of losing the contents. One must also be careful to cover the crucible well, to prevent any coals getting into it, which would reduce the calx of lead, and spoil the composition.
2. Take two parts and a half of white cerus, one part of prepared flints, half a part of salt of tartar, and a quarter part of calcined borax; melt the mixture in a Hessian crucible, and then pour it into cold water; it is then to be melted again, and washed a second and a third time, the same precautions being observed as for the first base.
3. Take two parts minium, one part rock-crystal, half a part of niter, and as much salt of tartar; this mixture being melted, must be treated as the former.
4. Take three parts of calcined borax, one part of prepared rock-crystal, and one part of salt of tartar; these being well mixed and melted together, must be poured into warm water; the water being decanted and the mass dried, an equal quantity of minium must be added to it; it is then to be melted and washed several times as directed above.
5. That called by our author the Mayence base, and which he considers as one of the finest crystalline compositions hitherto known, is thus composed: Take three parts of fixed alkali of tartar, one part of rock-crystal or flint pulverized; the mixture to be well baked together, and then left to cool. It is afterwards poured into a crucible of hot water to dissolve the frit; the solution of the frit is then received into a stone-ware pan, and aquafortis added gradually to the solution till it no longer effervesces; this water being decanted, the frit must be washed in warm water till it has no longer any taste; the frit is then dried, and mixed with one part and a half of fine cerus or white lead in scales; and this mixture must be well levigated with a little distilled water. To one part and a half of this powder dried add an ounce of calcined borax; let the whole be well mixed in a marble mortar, then melted and poured into cold water as the other bases already described. These fusions and lotions having been repeated, and the mixture dried and powdered, a 12th part of nitre must be added to it, and then melted for the last time; when a very fine crystal will be found in the crucible.
6. As a composition for furnishing very fine white stones: Take eight ounces of cerus, three ounces of rock-crystal pulverized, two ounces of borax finely powdered, and half a grain of manganese; having melted and washed this mixture in the manner directed above, it will produce a very fine white crystal.
II. Of the Colours. The calces of metals, as already observed, are the substances employed to colour fictitious gems; and on the preparation of these calces depends the vividness of their colours.
a. From Gold.] To obtain the mineral purple known by the name of precipitate of Cassius, M. Fontanius employs the following different processes.
1. Dissolve some pure gold in aqua regia, prepared with three parts of precipitated nitrous acid and one part of marine acid; and to hasten the dissolution, the matsais should be placed in a sand-bath. Into this solution pour a solution of tin in aqua regia. The mixture becomes turbid, and the gold is precipitated with a portion of the tin, in the form of a reddish powder; which, after being washed and dried, is called precipitate of Cassius.—The aqua regia employed to dissolve the tin is composed of five parts nitrous acid and one part of marine acid; to eight ounces of this aqua regia are added sixteen ounces of distilled distilled water. Some leaves of Malaca tin, about the size and thickness of a hump, is then put into this diluted aqua regia, till it will dissolve no more of them; which operation, our author observes, requires commonly twelve or fourteen days; though it might probably be hastened by beating the tin still thinner, and then rolling it into the form of a hollow cylinder, or turning it round into spiral convolutions, and thus exposing a greater extent of surface to the action of the menstruum. In order to prepare more readily the precipitate of Caffius, M. Fontanieu puts into a large jug eight ounces of solution of tin, to which he adds four pints of distilled water; he afterwards pours into this metallic lye some solution of gold, drop by drop, taking care to stir the whole with a glass tube; when the mixture becomes of a deep purple colour, he ceases dropping the solution of gold; and in order to hasten the precipitation of the mineral purple, pours into the mixture a pint of fresh urine. Six or seven hours after, the precipitate is collected at the bottom of the vessel; the fluid is then decanted; and the precipitate, washed once or twice, is dried till it becomes a brown powder.
2. Pour into a vessel of fine tin with a thick bottom four ounces of the solution of gold; three minutes after add two pints of distilled water. Let this mixture stand in the tin vessel during seven hours, taking care to stir it every hour with a glass tube; afterwards pour it into a conical glass jug, and add to it a pint of new urine; the mineral purple is soon precipitated, and then is to be washed and dried.
3. Distil in a glass cornute placed in a bath of ashes, some gold dissolved in aqua regia, made with three parts nitrous and one part marine acid; when the acid is passed over and the gold contained in the cornute appears dry, leave the vessel to cool, then pour into it some new aqua regia, and proceed to distil as before. Replace the aqua regia twice upon the gold and distil the same. After these four operations, pour by little and little into the cornute some oil of tartar per deliquium, which will occasion a brisk effervescence; when this ceases, distil the mixture till it becomes dry, and then put some warm water into the cornute. Shake the whole, and pour it into a cucurbit, when a precipitate is deposited the colour of which is sometimes brown and sometimes yellow; After having washed this precipitate, dry it. Our author says, this mineral purple was much superior to the foregoing, since two grains of it only were sufficient to an ounce of the base, whilst it required of the other two a tenth part of the base. And he adds, that he found a means of exalting the colour of the precipitate of Caffius, by putting to it a fifth part of its weight of glass of antimony finely powdered, and of nitre in the proportion of a dram to eight ounces of the base.
4. From Silver.] The calx of silver, being vitrified, produces a yellowish grey colour. This calx enters only into the composition of the yellow artificial diamond and the opal. M. Fontanieu introduces it into the base in the form of luna cornuta.
In order to prepare it, he directs to dissolve the silver in precipitated nitrous acid, and afterwards to pour into it a solution of sea-salt: a white precipitate is obtained; which, being washed and dried, melts very readily in the fire, and is soon volatilized if not mixed with vitriifiable matters. To make the yellow diamond, 25 grains of this luna cornuta are put to an ounce of the fourth base: the dose of silver may be diminished according to the shade of yellow that one wishes to procure.
c, From Copper.] The calx of copper imparts to white glas the finest green colour; but if this metal be not exactly in a state of calx, it produces a brownish red colour. Mountain blue, verdigris, and the residue of its distillation, are the different preparations of copper which our author employs to make the artificial emeralds.
d, From Iron.] Although it has been asserted, that the calces of iron introduce a very fine transparent red colour into white glass, M. Fontanieu could only obtain from it a pale red a little opake. The calx of iron that he employed was in the proportion of the tenth part of the base.
There are several ways of preparing the calx of iron called crocus Martis, or saffron of Mars. In general, it is necessary that this metal be so far deprived of its phlogiston, that the magnet ceases to attract it; thus one may use the calces of iron found upon the bars of the furnaces, which serve to distil aquafortis. By digesting filings of steel with distilled vinegar, then evaporating and replacing the vinegar 10 or 12 times upon these filings and drying them alternately, a calx of iron is obtained, which must be sifted through a silk sieve, and then calcined. The calx of iron thus obtained by the vinegar, our author says, only introduced into his bases a green colour inclining to a yellow.
By the following process a saffron of Mars of the finest red colour is obtained: Let an ounce of iron filings be dissolved in nitrous acid in a glass cornute, and distilled over a sand-bath to dryness. After having replaced the acid or the dry calx, and re-distilled it a second and a third time, it is then educorated with spirits of wine, and afterwards washed with distilled water.
e, From the Magnet.] It is necessary to calcine the magnet before it be introduced into the vitrifications; Having therefore torridised the magnet during two hours, it must be washed and dried. It is only employed in the composition of the opal.
f, From Cobalt.] The calx of cobalt is only proper to introduce a blue colour into glass; but this metal is rarely found free from iron and bismuth, and therefore it is first necessary to separate them from it. This is done by calcining the ore of cobalt in order to disengage the arsenic; afterwards the calx must be distilled in a cornute with sal ammoniac, and the iron and the bismuth are found sublimed with this salt. The distillation must be repeated with the sal ammoniac till this salt is no longer coloured yellow. The cobalt which remains in the cornute is then calcined in a potherd, and becomes a very pure calx; which being introduced into the base, in the proportion of a good part, gives it a very fine blue colour, the intensity of which may be increased at discretion by the addition of calx of cobalt. In order to prepare black enamel resembling that which is called black agate of Iceland; melt together a pound and a half of one of the bases, two ounces of the calx of cobalt, two ounces of prepared with vinegar, and two ounces of manganese.
g. From Tin.] The calx of tin is not vitrifiable alone; and when deprived of phlogiston is of a white colour; it renders opake the glas with which it is melted, and forms white enamel. For this purpose, calcine the putty of tin; then wash and dry it, and sift it through a silk sieve. Take five pounds of the second base, the same quantity of the calcined putty of tin, and 48 grains of manganese.
h. From Antimony.] Antimony is only susceptible of vitrification when its calx contains phlogiston, and then it produces a reddish or hyacinth coloured glas; but if the antimony be in a state of absolute calx, such as the diaphoretic antimony, then it is no longer vitrifiable, and may be substituted for calx of tin to make white enamel. M. Fontanieu introduces the glas of antimony in the composition of artificial topazes. For the oriental topaz, he takes 24 ounces of the first base and five drachms of the glas of antimony. To imitate the topaz of Saxony, he adds to each ounce of the base five grains of the glas of antimony. For the topaz of Brazil, he takes 24 ounces of the first base, one ounce 24 grains of glas of antimony, and 8 grains of the precipitate of Calcius.
i. From Manganese.] This mineral, employed in a small quantity, renders the glas whiter; a larger quantity produces a very fine violet colour, and a still larger dose of it renders the glas black and opake.
There are two ways of preparing manganese, 1. The most simple consists in exposing it to a red heat, and then quenching it with distilled vinegar; it is afterwards dried and powdered, in order to pass it through a silk sieve. 2. Haudiquier de Blancour describes the second manner of preparing the manganese, proper to furnish a red colour, and names it fusible manganese. Take of manganese of Piedmont one pound; torrefy and pulverize it; then mix it with a pound of nitre, and calcine the mixture during 24 hours; afterwards wash it repeatedly in warm water, till the water of the lyes has no longer any taste; dry the manganese, and mix with it an equal weight of sal ammoniac; levigate this mixture on a slab of porphyry with oil of vitriol diluted with water to the strength of vinegar. Dry the mixture, and introduce it into a cornute; distil by a graduated fire; and when the sal ammoniac is sublimed, weigh it, and add to the mixture an equal quantity. Then distil and sublime as before, and repeat the operation six times, being careful at each time to mix the sal ammoniac and the manganese upon the porphyry with diluted oil of vitriol.
At Tournhault in Bohemia, there is sold a fusible glas of a yellow colour, very like that of the topaz of Brazil, which, when exposed to a degree of fire in a cupel sufficient to redden it, becomes of a very fine ruby colour, more or less deep according to the degree of fire to which it has been exposed. Our author assayed this glas, and found it to contain a great deal of lead, but was not able to discover any gold in it.
III. Of the different degrees of Fire necessary for Fabulous Gems. Our author observes, that there are three degrees of heat very different in their energy. The fire kept up in the wind-furnaces in the laboratories of chemists, is less active than that whose effect is accelerated by the means of bellows; and a fire supported by wood, and kept up during 60 hours without interruption, produces singular effects in vitrification, and renders the glas finer and less alterable.
When recourse is had to the forge, in order to operate a vitrification, it is necessary to turn about the crucible from time to time, that the mass may melt equally. Some coal also should be replaced, in proportion as it consumes towards the nozzle of the bellows; for without this precaution, we should run the risk of cooling the crucible opposite to the flame, and probably of cracking it, when all the melted mass running among the coals would be totally lost. Though this is the readiest way of melting, it should not be employed out of choice; for the crucible often breaks, or coals get into it, and reduce the calx of lead to a metallic state.
The wind-furnace is either square or round. A small cake of baked clay or brick, of the thickness of an inch, is placed upon the grate; and upon this cake is placed the crucible, surrounded with coals. The degree of heat produced by this furnace is much less than that of the forge; but in order to succeed in the vitrification, M. Fontanieu recommends the use of a furnace described by Kunckel, which, with some necessary alterations, is represented on Plate CCXX. The interior part is so disposed, that we may place crucibles at three different heights; and the name of chambers is given to those steps upon which the crucibles are placed. Fig. 4. is a plan of the kiln at the first chamber, and fig. 5. a plan of the kiln where the fire is placed. Fig. 6. exhibits the elevation; A the ash-pit; B the door to put in the wood; C the door of the first chamber; D the door of the second chamber; E the third chamber; F the flue or chimney; GG iron-hoops which surround the kiln to strengthen it. Fig. 7. is a section of the kiln; H the ash-pit with its air-hole; I the chamber for the fire with an air-hole; K the first chamber for the crucibles; L the second chamber; M the dome; N the chimney; OO air-holes.
It is obvious, that the degree of heat cannot be equal in the said three chambers. The chamber K is that where the heat is greatest, afterwards in that of L, and lastly, in that of M. We should begin by placing the crucibles according to their size, in these different chambers; by which means the best effect in vitrification is produced.
In order to conduct the fire well, only three billets of white wood should be put into the furnace at a time for the first 20 hours, four billets at a time for the next 20 hours, and five billets for the last 20 hours; in all 60 hours. The furnace is then left to cool, care being taken to stop the air-holes with some lute; and, in about 48 hours after, when the kiln is quite cold, the crucible is to be withdrawn.
IV. The Compositions. 1. For the white diamond: Take the base of Mayence. This crystal is very pure, and has no colours.
2. For the yellow diamond: to an ounce of the fourth base, add for colour 25 grains of luna cornea or 10 grains of glas of antimony.
3. For the emerald: 1. To 15 ounces of either of the bases, add for colour one dram of mountain-blue and six grains. grains of glaas of antimony; or, 2. To an ounce of the second base, add for colour 20 grains of glaas of antimony and three grains of calx of cobalt.
4. For the sapphire: To 24 ounces of the Mayence base, add for colour two drams 46 grains of the calx of cobalt.
5. For the amethyst: To 24 ounces of the Mayence base, add for colour four drams of prepared manganese and four grains of precipitate of Cassius.
6. For the beril: To 24 ounces of the third base, add for colour 96 grains of glaas of antimony and four grains of calx of cobalt.
7. For the black agate: To 24 ounces of either of the bases, add two ounces of the mixture directed above in par. f.
8. For the opal: To an ounce of the third base, add for colour 10 grains of luna cornea, two grains of magnet, and 26 grains of absorbent earth.
9. For the oriental topaz: To 24 ounces of the first or third base, add for colour five drams of glaas of antimony.
10. For the topaz of Saxony: To 24 of the same base, add for colour six drams of the glaas of antimony.
11. For the topaz of Brazil: To 24 ounces of the second or third base, add for colour one ounce 24 grains of the glaas of antimony and eight grains of precipitate of Cassius.
12. For the hyacinth: To 24 ounces of the base made with rock-crystal, add for colour two drams 48 grains of glaas of antimony.
13. For the oriental ruby: 1. To 16 ounces of the Mayence base, add for colour a mixture of two drams 48 grains of the precipitate of Cassius, the same quantity of crocus Martius prepared in aquafortis, the same of golden sulphur of antimony and of fusible manganese, with the addition of two ounces of mineral crystal; or, 2. To 20 ounces of the base made with flints, add half an ounce of fusible manganese and two ounces of mineral crystal.
14. For the balas ruby: 1. To 16 ounces of the Mayence base, add the above colouring powder, but diminished a fourth part; or, 2. To 20 ounces of the base made with flints, add the same colouring powder, but with a fourth less of the manganese.
The fictitious gems are easily distinguished from the natural, by their softness and fusibility; by their solubility in acids; by their caulking only a single refraction of the rays of light; and, in many cases, by their specific gravity, which exceeds 2.76 in all precious gems of the first order, as the diamond, ruby, sapphire, &c.
Imitation of Antique Gems. There has been at different times a method practised by particular persons of taking the impressions and figures of antique gems, with their engravings, in glaas of the colour of the original gem. This has always been esteemed a very valuable method, and greatly preferable to the more ordinary ones of doing it on sealing-wax or brimstone: but, to the misfortune of the world, this art being a secret only in the hands of some particular persons who got their bread by it, died with them, and every new artist was obliged to re-invent the method; till at length Mr Homberg, having found it in great perfection, gave the whole process to the world to be no more forgotten or lost; and since that time it has been very commonly practised in France, and sometimes in other places.
Mr Homberg was favoured in his attempts with all the engraved gems of the king's cabinet; and took such elegant impressions, and made such exact resemblances of the originals, and that in glaases so artfully tinged to the colour of the gems themselves, that the nicest judges were deceived in them, and often took them for the true antique stones. These counterfeit gems also serve, as well as the original ones, to make more copies from afterwards; so that there is no end of the numbers that may be made from one; and there is this farther advantage, that the copy may be easily made perfect, though the original should not be so, but should have sustained some damage from a blow or otherwise.
The great care in the operation is to take the impression of the gem in a very fine earth, and to press down upon this a piece of proper glaas, softened or half melted at the fire, so that the figures of the impression made in the earth may be nicely and perfectly expressed upon the glaas. In general, the whole process much resembles that of the common founders. But when it is brought to the trial, there is found a number of difficulties which were not to be foreseen, and which would not at all affect the common works of the founder. For his purpose, every earth will serve that is fine enough to receive the impressions, and tough enough not to crack in the drying: these all serve for their use, because the metals which they cast are of a nature incapable of mixing with earth, or receiving it into them, even if both are melted together, so that the metal always easily and perfectly separates itself from the mould; but it is very difficult in these casts of glaas. They are composed of a matter which differs in nothing from that of the mould, but that it has been run into this form by the force of fire, and the other has not yet been so run, but is on any occasion ready to be so run, and will mix itself inseparably with the glaas in a large fire: consequently, if there be not great care used, as well in the choice of the glaas as in the manner of using it, when the whole is finished there will be found great difficulty in the separating the glaas from the mould, and often this cannot be done without wholly destroying the impression.
All earths run more or less easily in the fire as they are more or less mixed with saline particles in their natural formation. As all salts make earths run into glaas, and as it is necessary to use an earth on this occasion for the making a mould, it being also necessary to the perfection of the experiment that this earth should not melt or run, it is our business to search out for this purpose some earth which naturally contains very little salt. Of all the species of earth which Mr Homberg examined on this occasion, none proved so much divested of salts, or so fit for the purpose, as the common tripela, or Tripoli, used to polish glaas and stones. Of this earth there are two common kinds; the one reddish, and composed of several flake or strata; the other yellowish, and of a simple structure. These are both to be had in the shops. The latter kind is from the Levant; the former is found in England, France, and many other places. This tripela must be chosen soft and smooth to the touch, and not mixed with with sandy or other extraneous matter. The yellowish near the furnace by degrees, and gradually heated till it cannot be touched without burning the fingers; then it is to be placed in the furnace under a muffle, surrounded with charcoal. Several of these small crucibles may be placed under one muffle; and when they are properly disposed, the aperture of the muffle should have a large piece of burning charcoal put to it, and then the operator is to watch the process, and see when the glass begins to look bright: this is the signal of its being fit to receive the impression. The crucible is then to be taken out of the fire; and the hot glass must be pressed down upon the mould with an iron instrument, to make it receive the regular impression: as soon as this is done, the crucible is to be set at the side of the furnace out of the way of the wind, that it may cool gradually without breaking. When it is cold, the glass is to be taken out, and its edges should be grated round with pincers, which will prevent its flying afterwards, which is an accident that sometimes happens when this caution has been omitted, especially when the glass is naturally tender. The different coloured glasses are of different degrees of hardness, according to their composition; but the hardest to melt are always the best for this purpose, and this is known by a few trials.
If it be desired to copy a stone in relief which is naturally in creux, or to take one in creux which is naturally in relief; there needs no more than to take an impression first in wax or sulphur, and to mould that upon the paste of tripela instead of the stone itself: then proceeding in the manner before directed, the process will have the desired success.
A more simple and easy method than the above, is by taking the casts in gypsum, or Plaster of Paris as it is commonly called. For this purpose, the gypsum must be finely pulverised, and then mixed with clear water to the consistence of thick cream. This is poured upon the face of the gem or feal of which the impression is wanted, and which must be previously moistened with oil to facilitate the separation of the cast; and in order to confine the liquid plaster, it is only necessary to pin a slip of oiled paper round the sides of the feal by way of a cap or rim. When the plaster is dry, it is to be taken off, and set before the mouth of the furnace, in order to free it entirely from moisture; when it is fit to be used as a matrix in the same way as that formed with the tripoli earths. Only no crucible or other receptacle is at all necessary; the casts being formed like to many small cakes half an inch thick, and thus put into the furnace with the bits of glass upon them. The glass, after coming to a proper heat, is pressed down upon the mould with an iron spatula to receive the desired impression, the pressure requisite being more or less according to the size of the stone. This method has been long practised very successfully, and with no small emolument, by that ingenious feal-engraver Mr Deuchar of Edinburgh. The only respect in which it is inferior to the other more expensive and expensive methods, consists in the chance of air-bubbles arising in pouring on the plaster; which chance, however, is less in proportion to the fineness of the gypsum employed. When air-bubbles do occur, the casts may be laid aside, as it is so easy to replace them.
The application of pastes to multiply and preserve the impressions of caméaux and intaglios, is an object very interesting to artists and to antiquaries, as well as to men of learning and taste in the fine arts. This art, though only lately restored in any degree of perfection, is of very considerable antiquity. The great prices which the ancients paid for the elegant gems engraved by the celebrated Greek artists, could not but early suggest to them the idea of multiplying their numbers, by taking off their impressions in wax, in sulphur, in plaster, or in clay; but more particularly in coloured glaas, or that vitrified substance commonly called pastes.
As the impressions on pastes are durable, and imitate the colours and brilliancy of the original stones, they serve the same purposes as the gems themselves. This art was therefore practised not only by the Greeks, but by all the nations who cultivated Grecian taste.
Many of the finest gems of antiquity are now lost, and their impressions are to be found only on ancient pastes. Great therefore is the value of these pastes. Numerous collections of them have been formed by the curious. Instances of this are found in the Florentine Museum, in Stosch's work on ancient gems with inscriptions, in Winckelmann's description of Stosch's cabinet, and in the noble collection of Mr Charles Townley in London.
The art of taking impressions of gems seems not to have been altogether lost even in the Gothic ages; for Heraclius, who probably lived in the ninth century, and wrote a book De coloribus et artibus Romanorum, teaches in very plain though not elegant terms how to make them. Indeed, some of the few persons who then possessed this art, taking advantage of the ignorance of the times, sold pastes for original gems. Thus the famous emerald of the abbey of Reichenau near Constance, although a present made by Charlemagne, is now found to be a piece of glass. And thus the celebrated emerald vase in the cathedral of Genoa is likewise found to be a paste (a). The Genoese got this vase at the taking of Cesarea in the year 1101 as an equivalent for a large sum of money; nor was any imposition then suspected, for in the year 1319 they pawned it for 1200 marcs of gold.
But this ingenious art, revived indeed in Italy in the time of Laurence of Medici and Pope Leo X. was not cultivated in an extensive manner till the beginning of the present century, when M. Homberg restored it, as already mentioned. In this he is said to have been greatly assisted and encouraged by the then duke of Orleans regent of France, who used to amuse himself with that celebrated chemist in taking off impressions in pastes from the king of France's, from his own, and other collections of gems.
According to the French Encyclopedists, M. Clachant the elder, an engraver of some note, who died at Paris in 1781, learned this art from his royal highness, to whose household his father or he seems to have belonged. Mademoiselle Feloix next cultivated this art, and it is believed still carries it on. She had been taught by her father, who in quality of garçon de chambre to the regent had often assisted in the laboratory of his master, where he acquired this knowledge. Her collection consists of 1800 articles.
Baron Stosch, a Prussian, who travelled over Europe in quest of original engraved stones and impressions of ancient gems, for the elegant work which he published and Picart engraved (b), was well acquainted with this art. He had taught it to his servant Christian Dehn, who settled at Rome, where he made and sold his well-known sulphur impressions and pastes. He had collected 2500 articles. Dolce has arranged them in a scientific order, and given a descriptive catalogue of them.
It was chiefly from Dehn's collection that the taste for sulphurs and pastes has become so universal. They are great objects of study, and often require much learning to explain them. They have unquestionably served to extend and improve the art of engraving on stones; and have been of infinite use to painters, to statuaries, and to other artists, as well as to men of classical learning and fine taste.
It is very difficult to take off impressions, and perfectly to imitate various coloured cameos. It cannot be properly done in wax, sulphur, plaster, or glas of one colour only. The difficulties arising from their size and form, and from the various nature of the different sorts of glas, which do not well unite into different strata, are very numerous: nor could the complete success in this chemical and mechanical branch of the art produce a tolerable cameo. Impressions or imitations, if unassisted by the tool of the engraver, do not succeed because the undercutting and deep work of most of the originals require to be filled up with clay or wax, that the moulds may come off safe without injuring them. Hence the impressions from these moulds come off hard and destitute of delicacy, sharpness, and precision of outline, till the underworking of the moulder is cut away. But Mr Reiffenstein at Rome, by his genius, perseverance, and the assistance of able artists, has overcome these difficulties; and has had the satisfaction of succeeding, and producing variegated cameos which can hardly be distinguished from the originals.
Mr Lippart of Dresden, an ingenious glazier, and an enthusiast in the fine arts, practised this branch not unsuccessfully; but not finding sufficient encouragement for his pastes of coloured glas, or perhaps from local difficulties in making them well and cheap, he abandoned this art. He substituted in its place impressions of fine white alabaster or selenite plaster. Such impressions, when carefully soaked in a solution of white Castile soap, then dried, and rubbed over with a soft brush, take a very agreeable polish. They show the work perhaps to better advantage than red or white sulphurs do; but they are not so durable, and are liable to be defaced by rubbing.
Of these impressions Mr Lippart published three different collections, each of them containing 1000 articles; and to the merit of having increased the number of Mademoiselle Feloix and Christian Dehn's collections, which are all inserted in his, he added that of employing two learned Germans to arrange and describe them. The first thousand were arranged and described by the late professor Christ at Leipzig, and the second and third thousand by professor Heine.
(a) See M. de la Condamine's Diff. in Memoir. de l'Acad. Roy. de Paris, 1757. (b) Gemmae antiquae coloratae, sculptor nominibus insignitae, re incitae, per Bernardum Picart. Amsterdam. 1724, folio. Heine at Goettingen. Nor did Mr Lippart stop here; but to make the study of antiquity more easy and acceptable to artists, he selected out of the whole collection of 3000, a smaller one of 2000 of the best and most instructive subjects, of which he himself drew up and published a description in German.
But of all the artists and ingenious men who have taken impressions of engraved gems in sulphur and in pastes, no one seems to have carried that art to such perfection as Mr James Tassie, a native of Glasgow but who has resided in London since the year 1766. His knowledge in various branches of the fine arts, particularly in that of drawing, naturally led him to it. The elegant portraits which he models in wax, and afterwards moulds and casts in paste, which entirely resemble cameos, are well known to the public.
Mr Tassie, profiting of all the former publications of this sort, and by expense, industry, and access to many cabinets in England and other kingdoms to which former artists had not obtained admission, has now increased his collection of impressions of ancient and modern gems to the number of above 15,000 articles. It is the greatest collection of this kind that ever existed; and serves for all the purposes of artists, antiquaries, scholars, men of taste, and even philosophers. The great demand for his pastes was perhaps owing in the beginning to the London jewellers, who introduced them into fashion by setting them in rings, seals, bracelets, necklaces, and other trinkets.
The reputation of this collection having reached the emperors of Russia, they were pleased to order a complete set; which being accordingly executed in the best and most durable manner, were arranged in elegant cabinets, and are now placed in the noble apartments of her imperial majesty's superb palace at Czarsk Zelo.
Mr Tassie, in executing this commission, availed himself of all the advantages which the improved state of chemistry, the various ornamental arts, and the knowledge of the age, seemed to afford. The impressions were taken in a beautiful white enamel composition, which is not subject to shrink or form air-bladders; which emits fire when struck with steel, and takes a fine polish; and which shows every stroke and touch of the artist in higher perfection than any other substance. When the colours, mixed colours, and nature of the reflective originals, could be ascertained, they were imitated as completely as art can imitate them; inasmuch that many of the paste intaglios and cameos in this collection are such faithful imitations, that artists themselves have owned they could hardly be distinguished from the originals. And when the colour and nature of the gems could not be authenticated, the pastes were executed in agreeable, and chiefly transparent, colours; constant attention being bestowed to preserve the outlines, extremities, attributes, and inscriptions.
It was the learned Mr Raspe (from whom this account is taken) who arranged this great collection, and made out the descriptive catalogue. His arrangement is nearly the same with that of the late Abbé Winkelmann, in his description of the gems which belonged to Baron Stosch. But as modern works were inserted in this collection, he found it necessary to make a few alterations, and added some divisions to those of M. Winkelmann, as will appear from the following prospectus, with which we shall conclude this detail.
I. Ancient Art and Engravings. A. Egyptian. Hieroglyphics, sacred animals, divinities, priests. B. Baalidian, Gnostic, and other talismans, &c. C. Oriental and barbarous ancient and modern engravings. D. Greek and Roman originals, copies, and imitations (the Etruscan are clasped with the Greek works.) E. Mythology or fabulous age. Gods, inferior divinities, religious ceremonies. F. Heroic age before the siege of Troy. G. Siege of Troy. H. Historic age. Of Carthage, Greece, Rome, subjects unknown. I. Fabulous animals and chimeras. J. Vases and urns.
II. Modern Art and Engravings. A. Religious subjects. B. Portraits of kings and sovereigns. C. Portraits of illustrious men in alphabetical order. D. Portraits unknown. E. Devices and emblems. F. Cyphers, arms, supporters, and medley of modern history.