By far the simplest in composition of all the precious stones is diamond, which is pure crystallized carbon; but its manufacture is attended by well-nigh insuperable difficulties. If carbon be heated in air, it burns at a temperature well below its melting point; moreover, unless an enormously high pressure is simultaneously applied, the product is the other form of crystallized carbon, namely, the comparatively worthless graphite. Moissan’s interesting course of experiments were in some degree successful, but the tiny diamonds were worthless as jewels, and the expense involved in their manufacture was out of all proportion to any possible commercial value they might have.
Next to diamond the simplest substances among precious stones are quartz (crystallized silica) and corundum (crystallized alumina). The crystallization of silica has been effected in several ways, but the value in jewellery of quartz, even of the violet variety, amethyst, is not such as to warrant its manufacture on a commercial scale. Corundum, on the other hand, is held in high esteem; rubies and sapphires, of good colour and free from flaws, have always commanded good prices. The question of their production by artificial means has therefore more than academic interest.
Ever since the year 1837, when Gaudin produced a few tiny flakes, French experimenters have steadily prosecuted their researches in the crystallization of corundum. Frémy and Feil, in 1877, were the first to meet with much success. A portion of one of their crucibles lined with glistening ruby flakes is exhibited in the British Museum (Natural History).
Fig. 53.—Verneuil’s
Inverted Blowpipe.
In 1885 the jewellery market was completely taken by surprise by the appearance of red stones, emanating, so it is alleged, from Geneva; having the physical characters of genuine rubies, they were accepted as, and commanded the prices of, the natural stones. It was eventually discovered that they had resulted from the fusion of a number of fragments of natural rubies in the oxy-hydrogen flame. The original colour was driven off at that high temperature, but was revived by the previous addition of a little bichromate of potassium. Owing to the inequalities of growth, the cracks due to rapid cooling, the inclusion of air-bubbles, often so numerous as to cause a cloudy appearance, and, above all, the unnatural colour, these reconstructed stones, as they are termed, were far from satisfactory, but yet they marked such an advance on anything that had been accomplished before that for some time no suspicion was aroused as to their being other than natural stones.
A notable advance in the synthesis of corundum, particularly of ruby, was made in 1904, when Verneuil, who had served his apprenticeship to science under the guidance of Frémy, invented his ingenious inverted form of blowpipe (Fig. 53), which enabled him to overcome the difficulties that had baffled earlier investigators, and to manufacture rubies vying in appearance after cutting with the best of nature’s productions. The blowpipe consisted of two tubes, of which the upper, E, wide above, was constricted below, and passing down the centre of the lower, F, terminated just above the orifice of the latter in a fine nozzle. Oxygen was admitted at C through the plate covering the upper end of the tube, E. A rod, which passed through a rubber collar in the same plate, supported inside the tube, E, a vessel, D, and at the upper end terminated in a small plate, on which was fixed a disc, B. The hammer, A, when lifted by the action of an electromagnet and released, fell by gravity and struck the disc. The latter could be turned about a horizontal axis placed eccentrically, so that the height through which the hammer fell and the consequent force of the blow could be regulated. The rubber collar, which was perfectly gas-tight, held the rod securely, but allowed the shocks to be transmitted to the vessel, D, an arrangement of guides maintaining the slight motion of the vessel strictly vertical. This vessel, which carried the alumina powder used in the manufacture of the stone, had as its base a cylindrical sieve of fine mesh. The succession of rapid taps of the hammer caused a regular feed of powder down the tube, the amount being regulated by varying the height through which the hammer fell. Hydrogen or coal-gas was admitted at G into the outer tube, F, and in the usual way met the oxygen just above the orifice, L. To exclude irregular draughts, the flame was surrounded by a screen, M, which was provided with a mica window, and a water-jacket, K, protected the upper part of the apparatus from excessive heating.
Fig. 54.—‘Boule,’
or Pear-shaped
Drop.
The alumina was precipitated from a solution of pure ammonia—alum, (NH4)2SO4.Al2(SO4)3.24H2O, in distilled water by the addition of pure ammonia, sufficient chrome-alum also being dissolved with the ammonia-alum to furnish about 2½ per cent. of chromic oxide in the resulting stone. The powder, carefully prepared and purified, was placed, as has been stated above, in the vessel, D, and on reaching the flame at the orifice it melted, and fell as a liquid drop, N, upon the pedestal, P, which was formed of previously fused alumina. This pedestal was attached by a platinum sleeve to an iron rod, Q, which was provided with the necessary screw adjustments, R and S, for centring and lowering it as the drop grew in size. Great care was exercised to free the powder from any trace of potassium, which, if present, imparted a brownish tinge to the stone. The pressure of the oxygen, low initially both to prevent the pedestal from melting, and to keep the area of the drop in contact with the pedestal as small as possible, because otherwise flaws tended to start on cooling, was gradually increased until the flame reached the critical temperature which kept the top of the drop melted, but not boiling. The supply of powder was at the same time carefully proportioned to the pressure. The pedestal, P, was from time to time lowered, and the drop grew in the shape of a pear (Fig. 54), the apex of which was downwards and adhered to the pedestal by a narrow stalk. As soon as the drop reached the maximum size possible with the size of the flame, the gases were sharply and simultaneously cut off. After ten minutes or so the drop was lowered from the chamber, M, by the screw, S, and when quite cold was removed from the pedestal.