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GEM, ARTIFICIAL
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See also:GEM, ARTIFICIAL . The See also:term " Artificial Gems " does not mean imitations of real gems, but the actual formation by artificial means of the real See also:precious See also: S. See also:Marsden in 1881; but their results have not been verified by others who have tried to repeat them, and the See also:probability is that what was then thought to be diamond was in reality See also:carborundum or See also:carbide of See also:silicon.
Attempts have been made by two methods to make carbon diamonds—carbonado, in fact—and a small quantity of trans-crystallize in the transparent form. One is to crystallize it slowly
from a See also:solution in which it has been dissolved. The difficulty is to find a solvent. Many organic and some inorganic bodies hold carbon so loosely combined that it can be separated out under the See also:influence of chemical See also:action, See also:heat or See also:electricity, but invariably the carbon assumes the black amorphous form. The other method is to try to fuse the carbon by fierce heat, when from See also:analogy it is argued that on cooling it will solidify to a clear limpid crystal. The progress of See also:science in other directions has now made it See also:pretty certain that the true mode of making diamond artificially is by a See also:combination of these two methods. Until recently it was assumed that carbon was non-volatile at any attainable temperature, but it is now known that at a temperature of about 3600 C. it volatilizes readily, passing without liquefyig directly from the solid to the gaseous See also:state. Very few bodies See also:act in this manner, the See also:great See also:majority when heated at atmospheric pressure to a sufficient temperature passing through the intermediate See also:condition of liquidity. Some few, however; which when heated at atmospheric pressure do not liquefy, when heated at higher pressures in closed vessels obey the common See also:rule and first become liquid and then volatilize. See also:Sir See also: Enormous as such pressures and temperatures may appear to be, they have been exceeded in some of Sir See also:Andrew See also:Noble's and Sir F. See also:Abel's re-searches; in their investigations on the gases from See also:gunpowder and See also:cordite fired in closed See also:steel See also:chambers, these chemists obtained pressures as great as 95 tons to the sq. in., and temperatures as high as 4000° C. Here then, if the observations are correct, we have sufficient temperature and enough pressure to liquefy carbon; and, were there only sufficient time for these to act on the carbon, there is little doubt that the artificial formation of diamonds would soon pass from the microscopic See also:Stage to a See also:scale more likely to satisfy the requirements of science, if not those of See also:personal adornment. It has See also:long been known that the See also:metal See also:iron in a molten state dissolves carbon and deposits it on cooling as black opaque See also:graphite. Moissan carried out a laborious and systematic See also:series of experiments on the solubility of carbon in iron and other metals, and came to the conclusion that whereas at See also:ordinary pressures the carbon separates from the solidifying iron in the form of graphite, if the pressure be greatly increased the carbon on separation will form liquid drops, which on solidifying will assume the crystalline shape and become true diamond. Many other metals dissolve carbon, but molten iron has been found to be the best solvent. The quantity entering into solution increases with the temperature of the metal. But temperature alone is not enough; pressure must be superadded. Here Moissan ingeniously made use of a See also:property which molten iron possesses in common with some few other liquids—water, for instance—of increasing in See also:volume in the act of passing from the liquid to the solid state. Pure iron is mixed with carbon obtained from the calcination of See also:sugar, and the whole is rapidly heated in a carbon crucible in an electric See also:furnace, using a current of 700 amperes and 40 volts. The iron melts like See also:wax and saturates itself with carbon. After a few minutes' See also:heating to a temperature above 4000° C.—a temperature at which the See also:lime furnace begins to melt and the iron volatilizes in clouds—the dazzling, fiery crucible is lifted out and plunged beneath the See also:surface of See also:cold See also:water, where it is held till it sinks below a red heat. The sudden cooling solidifies the See also:outer skin of molten metal and holds the inner liquid See also:mass in an iron grip. The expansion of the inner liquid on solidifying produces enormous pressure, and under this stress the dissolved carbon separates out in a hard, transparent, dense form—in fact, as diamond. The succeeding operations are long and tedious. The metallic See also:ingot is attacked with hot aqua regia till no iron is See also:left undissolved. The bulky See also:residue consists chiefly of graphite, together with translucent flakes of See also:chestnut-coloured carbon, hard black opaque carbon of a See also:density of from 3.0 to 3.5, black See also:parent colourless diamonds showing crystalline structure. Besides these there may be See also:corundum and carbide of silicon, arising from impurities in the materials employed. Heating with strong sulphuric See also:acid, with hydrofluoric acid, with nitric acid and See also:potassium chlorate, and fusing with potassium fluoride—operations repeated over and over again—at last eliminate the graphite. and impurities and leave the true diamond untouched. The precious residue on microscopic examination shows many pieces of black diamond, and other colourless transparent pieces, some amorphous, others crystalline. Although many fragments of crystals are seen, the writer has scarcely ever met with a See also:complete crystal. All appear broken up, as if, on being liberated from the intense pressure under which they were formed, they burst asunder. See also:Direct See also:evidence of this phenomenon has been seen. A very See also:fine piece of diamond, prepared in the way just described and carefully mounted on a microscopic slide, exploded during the See also:night and covered the slide with fragments. This bursting See also:paroxysm is not unknown at the Kimberley mines.
Sir See also: Diamonds so made See also:burn in the See also:air when heated to a high temperature, with formation of carbonic acid; and in lustre, crystalline form, See also:optical properties, density and hardness, they are identical with the natural stone. It having been shown that diamond is formed by the separation of carbon from molten iron under pressure, it became of See also:interest to see if in some large metallurgical operations similar conditions might not prevail. A See also:special form of steel is made at some large establishments by cooling the molten metal under intense See also:hydraulic pressure. In some samples of the steel so made See also:Professor Rosel, of the university of See also:Bern, has found microscopic diamonds. The higher the temperature at which the steel has been melted the more diamonds it contains, and it has even been suggested that the hardness of steel in some measure may be due to the carbon distributed throughout its mass being in this adamantine form. The largest artificial diamond yet formed was found in a See also:block of steel and slag from a furnace in Luxembourg; it is clear and crystalline, and See also:measures about one-fiftieth of an inch across. A striking See also:confirmation of the theory that natural diamonds have been produced from their solution in masses of molten iron, the metal from which has gradually oxidized and been washed away under cycles of atmospheric influences, is afforded by the occurrence of diamonds in a See also:meteorite. On a broad open See also:plain in See also:Arizona, over an See also:area of about 5 M. in See also:diameter,,See also:lie scattered thousands of masses of metallic iron, the fragments varying in See also:weight from half a ton to a fraction of an See also:ounce. There is little doubt that these fragments formed See also:part of a meteoric shower, although no See also:record exists as to when the fall took place. Near the centre, where most of the fragments have been found, is a See also:crater with raised edges, three-quarters of a mile in diameter and 600 ft. deep, bearing just the See also:appearance which would be produced had a mighty mass of iron—a falling star—struck the ground, scattered it in all directions, and buried itself deeply under the surface, fragments eroded from the surface forming the pieces now met with. Altogether ten tons of this iron have been collected, and specimens of the See also:Canyon Diablo meteorite are in most collectors' cabinets. Dr A. E. See also:Foote, a mineralogist, when cutting a See also:section of this meteorite, found the tools injured by something vastly harder than metallic iron, and an See also:emery See also:wheel used for grinding it was ruined. He attacked the specimen chemically, and soon afterwards announced to the scientific See also:world that the Canyon Diablo meteorite contained diamonds, both black and transparent. This startling See also:discovery was subsequently verified by Professors C. See also:Friedel and H. Moissan, and also by Sir W. Crookes. The See also:Ruby.—It is evident that of the other precious stones only the most prized are See also:worth producing artificially. Apart from their inferior hardness and See also:colour, the demand for what are known as " semi-precious stones " would not pay for the necessarily great expenses of the factory. Moreover, were it to be known that they were being produced artificially the demand—never very great—would almost cease. The only other gems, therefore, which need be mentioned in connexion with their artificial formation are those of the corundum or See also:sapphire class, which include all the most highly prized gems, rivalling, and sometimes exceeding, the diamond in value. Here a remarkable and little-known fact deserves See also:notice. Excepting the diamond and sapphire, each of the precious stones—the See also:emerald, the See also:topaz and amethyst—possesses a more noble, a harder, and more highly-prized counterpart of itself, alike in colour, but See also:superior in brilliancy and hardness; still more See also:strange, the precious stone to which its special name is usually attached is the variety the least prized. The ruby itself might almost be included in the same See also:category. The true ruby consists of the See also:earth alumina, in a clear, crystalline form, having a minute quantity of the See also:element See also:chromium as the colouring See also:matter. It is often called the " See also:Oriental Ruby," or red sapphire, and when of a paler colour, the " See also:Pink Sapphire." But the ruby as met with in jewellers' shops of inferior See also:standing is usually no true ruby, but a "See also:spinel ruby " or " balas ruby," sometimes very beautiful in colour, but softer than the Oriental ruby, and different in chemical composition, consisting essentially of alumina and See also:magnesia and a little See also:silica, with the colouring matter chromium. The colourless basis of the true Oriental precious stones being taken as crystallized alumina or See also: A. Gaudin made true rubies, of microscopic size, by fusing See also:alum in a carbon crucible at a very high temperature, and adding a little chromium as colouring matter. In 1847 J. J. Ebelmen produced the white sapphire and See also:rose-coloured spinel by fusing the constituents at a high temperature in boracic acid. Shortly afterwards he produced the ruby by employing See also:borax as the solvent. The boracic acid was found to be too volatile to allow the alumina to crystallize,but the use of borax made the necessary difference. But it was not till about the See also:year 1877 that E. See also:Fremy and C. See also:Fell first published a method whereby it was possible to produce a crystallized alumina from which small stones could be cut. They first formed See also:lead aluminate by the See also:fusion together of lead See also:oxide and alumina. This was kept in a state of fusion in a fireclay crucible (in the composition of which silica enters largely). Under the influence of the high temperature the silica of the crucible gradually decomposes the lead aluminate, forming lead silicate, which remains in the liquid state, and alumina, which crystallizes as white sapphire. By the admixture of 2 or 3 of a chromium See also:compound with See also:original materials the resulting white sapphire became ruby. More recently Edmond Fremy and A. See also:Verneuil obtained artificial rubies by reacting at a red heat with See also:barium fluoride on amorphous alumina containing a small quantity of chromium. The rubies obtained in this manner are thus described by Fremy and Verneuil: " Their crystalline form is See also:regular; their lustre is adamantine; they See also:present the beautiful colour of the ruby; they are perfectly transparent, have the hardness of the ruby, and easily scratch topaz. They resemble the natural ruby in becoming dark when heated, resuming their rose-colour on cooling." See also:Des Cloizeaux says of them that " under the See also:microscope some of the crystals show bubbles. In converging polarized See also:light the coloured rings and the negative black See also:cross are of a remarkable regularity." Other experimentalists have attacked the problem in other directions. Besides those already mentioned, L. Elsner, H. H. De See also:Senarmont, Sainte-Claire Deville, and H Caron and H. Debray have succeeded with more or less success in producing rubies. The See also:general See also:plan adopted has been to form a mixture of salts fusible at a red heat, forming a liquid in which alumina will dissolve. Alumina is now added till the fused mass will take up no more, and the crucible is left in the furnace for a long time, sometimes extending over See also:weeks. The solvent slowly volatilizes, and the alumina is deposited in crystals, coloured by whatever colouring oxide has been added. Mention has been made above of a stone frequently substituted for the true ruby, called the " spinel " or " balas " ruby. The spinel and ruby occur together in nature, stones from See also:Burma being as often spinel as true Oriental ruby. In the artificial See also:production of the ruby it sometimes happens that spinel crystallizes out when true Oriental ruby is expected. The fusion See also:bath is so arranged that only red-coloured alumina shall crystallize out, but it is difficult to have all the materials of such purity as to ensure the complete absence of silica and magnesia. In this case, when these impurities have accumulated to a certain point they unite with the alumina, and spinel then separates, as it crystallizes more easily than ruby. When all the magnesia and silica have been eliminated in this way the bath resumes its deposition of crystalline ruby. Rubies of fine colour and of considerable size have been shown in See also:London, made on the See also:Continent by a See also:secret process. The writer has seen several cut stones so made weighing over a See also:carat each, the uncut crystals measuring half an inch along a crystal edge, and weighing over 7o grains, and a clear See also:plate of ruby cut from a single crystal weighing over 10 grains. Ruby has been made by Sir W. See also:Roberts-See also:Austen as a by-product in the production of metallic chromium. Oxide of chromium and See also:aluminium See also:powder are intimately mixed together in a refractory crucible, and the mixture is ignited at the upper part. The aluminium and chromium oxide react with See also:evolution of so much heat that the reduced chromium is melted. Such is the intensity of the reaction that the resulting alumina is also completely fused, floating as a liquid on the molten chromium. Sometimes the alumina takes tip the right amount of chromium to enable it to assume the ruby colour. On cooling the melted alumina crystallizes in large flakes, which on examination by transmitted light are seen to be true ruby. The development of the red colour is said by C. Greville-See also:Williams only to take place at a white heat. It is not due to the presence of chromic acid, but to a reaction between alumina and chromic oxide, which requires an elevated temperature. Artificially made but real rubies have been put on the See also:market, certain conditions have to be fulfilled in See also:order to get the alumina in a transparent form. The temperature must not be higher than is absolutely necessary for fusion. The melted product must always be in the same part of the oxyhydrogen See also:flame, and the point of contact between the melted product and the support should be reduced to as small an area as possible. M. Verneuil uses a See also:vertical See also:blowpipe flame directed on a support capable of See also:movement up and down by means of a See also:screw, so that the fused product may be removed from the See also:zone of fusion as it gets higher by addition of fresh material. The material employed is either composed of small, valueless rubies, or alumina coloured with the right amount of chromium. It is very finely powdered and fed in through the blowpipe orifice, whence it is blown in a highly heated condition into the zone of fusion. The support is a small cylinder of alumina placed in the See also:axis of the blowpipe. As the operation proceeds the fine grains of powder driven on to the support in the zone of fusion form a See also:cone which gradually rises and broadens out until it becomes of sufficient size to be used for cutting. Rubies prepared in this way have the same specific gravity and hardness as the natural ruby, and they are also dichroic, and in the vacuum See also:tube under the influence of the See also:cathode stream they phosphoresce with a discontinuous spectrum showing the strong alumina See also:line in the red. When properly cut and mounted it is almost impossible to distinguish them from natural stones. The Sapphire.—Auguste See also:Daubree has shown that when a full quantity of chromium is added to the bath from which white sapphire crystallizes the colour is that of ruby, but when much less chromium is added the colour is blue, forming the true Oriental sapphire. The real colouring matter of the Oriental sapphire is not definitely known, some chemists considering it to be chromium and others See also:cobalt. Artificial sapphires have been made of a See also:fair size and perfectly transparent by the addition of cobalt to the igneous bath of alumina, but the writer does not consider them equal in colour to true Oriental sapphire. The Oriental Emerald.—The stone known as emerald consists chemically of silica, alumina and glucina. Like the ruby, it owes its colour to chromium, but in a different state of oxidation. As already mentioned, there is another stone which consists of crystallized alumina coloured with chromium, but holding the chromium in a different state of oxidation. This is called the Oriental emerald, and, owing to its beauty of colour, its hardness and rarity, it is more highly prized than the emerald itself and commands higher prices. The Oriental emerald has been produced artificially in the same way as the ruby, by adding a larger amount of chromium to the alumina bath and regulating the temperature. The Oriental Amethyst.—The amethyst is See also:rock crystal (See also:quartz) of a bluish-violet colour. It is one of the least valuable of the precious stones. The sapphire, however, is found occasionally of a beautiful violet colour; it is then called the Oriental amethyst, and, on account of its beauty and rarity, is of great value. It is evident that if to the igneous bath of alumina some colouring matter, such as See also:manganese, is added capable of communicating a violet colour to the crystals of alumina, the Oriental amethyst will be the result. Oriental amethyst has been so formed artificially, but the stone being known only as a curiosity to mineralogists and experts in precious stones, and the public not being able to discriminate between the violet sapphire and amethystine quartz, there is no demand for the artificial stone. The Oriental Topaz.—The topaz is what is called a semi-precious stone. It occurs of many See also:colours, from clear white to pink, orange, yellow and See also:pale green. The usual colour is from See also:straw-yellow to See also:sherry colour. The exact composition of the colouring matter is not known; it is not entirely of See also:mineral origin, as it changes colour and sometimes fades altogether on exposure to light. Chemically the topaz consists of alumina, silica and See also:fluorine. It is not so hard as the sapphire. There is also a yellow variety of quartz, which is sometimes called " false topaz." The Oriental topaz, on the other See also:hand, is a precious stone of great value. It consists of clear crystalline sapphire GEMINIANI coloured with a small quantity of ferric oxide. It has been produced artificially by adding iron instead of chromium to the See also:matrix from which the white sapphire crystallizes. The See also:Zircon.—The zircon is a very beautiful stone, varying in colour, like the topaz, from red and yellow to green and blue. It is sometimes met with colourless, and such are its refractive See also:powers and brilliancy that it has been mistaken for diamond. It is a compound of silica and zirconia. H. Sainte-Claire Deville formed the zircon artificially by passing silicon fluoride at a red heat over the oxide zirconia in a See also:porcelain tube. Octahedral crystals of zircon are then produced, which have the same crystalline form, appearance and optical qualities as the natural zircon. Additional information and CommentsThere are no comments yet for this article.
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