The Flame-Fusion Process
The Verneuil oven, which made possible the synthesis of large, fine crystals of ruby and sapphire, consists essentially of a type of blowpipe that is inverted in such a manner that the flame is directed downward. The French term for this kind of an oven, or blowpipe, "chalumeau" is still used to some extent for the Verneuil apparatus. In a sense, this piece of equipment is reminiscent of the inventions of the famed artist, Rube Goldberg, in that it utilizes an electromagnet or an eccentric cam to actuate a hammer that taps a container with a silver like base to supply the alumina powder. It is essentially a stationary oxyhydrogen blowpipe that melts the finely powdered, purified aluminum-oxide powder as it falls through the flame. That maintain an optimum flow of powder through the flame, the container, which is comparable to a salt shaker, is tapped at regular intervals from above. The distance the hammer drops, and hence the strength of the blow, is regulated by a disc on top of the container; this varies from the beginning to the end of the boule formation. (Note: The word boule, which is pronounced BOOL, is the name used for the more or less carrot-shaped mass of corundum produced in the furnace. It comes from the French meaning "ball". Occasionally, the German term "birne" is used.). The growing boule and the orifice of the blowpipe are surrounded by a heat resistant, circular walled area, which usually has a window also of a heat resistant material such as mica through which the operator, wearing smoked glasses, can observe the boule as it grows. Figure a shows one type of a Verneuil furnace, and Figure b is a photograph of a bank of furnaces in a modern manufacturing plant.
When the torch is lighted, either the cam-or electromagnet operated mechanism raises the hammer and drops it on the container to start powder flowing through the flame. The powder falls on a heat-resistant fire-clay rod within the furnace chamber and forms a small pyramid of alumina. The heat is increased and the tip of the tiny pyramid becomes a molten mass. The heat is then increased gradually as the powder continues to fall and the pinhead starts to enlarge. The temperature range at which the blowpipe is operated varies from about 1900°C. to 2400°C. The melting point of corundum is approximately 2050°C. As powder drops through the flame and hits the top of the pinhead, it melts and flows slightly towards the edges before it forms a crystalline solid. As the process continues, the top enlarges until it is approximately two-thirds to three-fourths of an inch in diameter, which is the usual size. Boules vary in weight from about 150 carats in some colors to several hundreds of carats for others. Ruby and white-sapphire boules often attain a weight of 400 carats, and sometimes as much as 750 carats. Figure c illustrates the growth stages of a boule and Figure d is a dose-up view of a completed boule in the furnace charmer.
U.S. manufacturers usually rotate the boule as it grows, to cause any gas bubbles that may form to rise to the surface and escape. This, however, is not always done; indeed, few European manufacturers engage in this practice, with the result that most synthetic corundum produced abroad contains bubbles. The temperature, of the tip of the boule is controlled by maintaining the proper distance from the flame. In order to avoid excessive bubble formation, the tip should be above the melting but below the boiling point. Since these temperatures are only about 100°C apart for corundum, the regulation of distance is critical.
The aluminum-oxide powder is produced by the direct ignition of an exceedingly pure alum (ammonium alum, expressed by the formula NH4 Al(S04)2{12H20} in a fused-quartz dish in a suitable furnace. It is then powdered and screened through a very fine mesh. Since it is imperative that the material be very pure, to ensure proper growth of the boule, the manufacturer often prepares the alum starting with pure metallic aluminum and chemically pure sulphuric acid and ammonia.
Linde Air Products Company, the major U. S. manufacturer of synthetics who no longer produce gem materials, and several European firms controlled the crystallographic orientation of the boule by mounting a seed crystal (i.e., a small piece from an earlier boule) on top of the fire-clay rod. Orientation in a finished boule may be determined exactly with a polariscope, but there are several ways of ascertaining the position of the optic axis that do not require equipment. It is usually in the plane defined by the length of the boule and by the normal to the boule's fitted side or sides. If a seed crystal has been used, the optic axis will not be parallel to the length of the boule but at an angle (usually about 30°), for if the optic axis corresponds to the boule's long axis, the strain will be increased and the necessary splitting will be less likely to result in a clean, flat break. The preferred orientation is with the long axis of the boule at right angles to one of the directions of rhombohedral parting. When the tip (small end) of the boule is tapped lightly with a hammer, while the opposite end rests on a flat surface, it usually falls apart cleanly. The optic axis lies in the plane of the break.
If colorless sapphire is to be the product, nothing is added to the pure alumina. lf a color is desired, it is necessary to add the appropriate oxide or oxides at the time the power is calcined. From one to two percent of chromium oxide is necessary to impart the red color to ruby. Pink is produced by using smaller quantities of chromium oxide. Dark brownish red, resembling garnet or Siamese ruby, is produced by adding small amounts of iron oxide. Blue is produced by adding about two percent of iron oxide and one percent of titanium oxide; this tends to be concentrated and the iron is virtually lost during the process, seemingly being confined to the edges of the boule. The titanium in the final product is also considerably less than that added to the powder. Light blue is produced by using smaller quantities of these same oxides. Purple and violet in various tones result from a combination of iron, chromium and titanium oxides. Yellow in all color variations of the natural stone can be induced by using iron, nickel, uranium, titanium and thallium. Orange is produced by combining the coloring agents for yellow and red. Synthetic alexandrite like sapphire, which has a color change from bluish gray to a near amethyst color, is produced by adding about three percent of vanadium oxide. Dark tourmaline green results when cobalt, magnesium, zinc and vanadium oxides are used; yellow-green, reminiscent of peridot, is obtained by incorporating lesser amounts of the same coloring agents. Other greens, particularly those made in an attempt to duplicate the color of emerald, are produced by the addition of nickel, Iron and titanium oxides. Although these colors are more attractive than the peridot or tourmaline greens, they never approach a true emerald color.
Recent modifications of the furnace have permitted the manufacture of a wide variety of forms for industrial purposes. Thin rods (Figure e) permit instrument and watch jewels to be roughed out merely by slicing the rod. This is in contrast to the material Produced in Europe, where the usual carrot-shaped boule must be split, sliced, and sawn into small squares; the squares then have to be rounded up before the regular jewel fabrication can be undertaken. The Swiss industry operates at such a low cost that, even though boules are produced in rod shapes in this country, thus saving so many steps in the manufacturing process, it is still impossible for American jewel manufacturers to compete with the Swiss factories. Rods measuring one-sixteenth or one-eighth inch in diameter average six inches in length, but they are available in lengths up to thirty inches. Stiff springs and "pig-tail" guide are other uses for the rods. Disc like forms one inch or more in diameter and fairly thick are widely used. A conical shape is made by permitting accumulation to take place as the sides of a disc are drawn back; this form finds application as missile nose cones. Button shapes are made also. Some of the products are of direct interest to jewelers; for instance, balls of synthetic ruby as small as one half millimeter in diameter are used for the tips of ballpoint pens. Larger balls, measuring up to three-fourths of an inch in diameter, are used in a variety of ways, including check valves for corrosive liquids. Phonograph needles are another important application of this versatile material. Because of the resistance offered to wear by corundum, it is popular for making wire drawing dies and nozzles for extruding liquids. The bearing surfaces of very fine balances, including some diamond balances, are made of synthetic corundum, it is popular for making wire-drawing dies and nozzles for extruding liquids. The bearing surfaces of very fine balances, including some diamond balances, are made of synthetic corundum, instead of the traditional agate. Finally, transparent colorless synthetic sapphire has been used successfully as a component in lens systems.
Developments by American manufacturers, particularly Linde Air Products Co., have been outstanding in this field. The European makers added colors and increased boule size, but from the time the Linde scientists started to make synthetics in 1942 until the present, their contributions to basic technology have been great. Not only were they first to produce the rods and other shapes commercially, but they were first to introduce an annealed boule in a blue color that did not require splitting length wise to prevent total loss through cracking. In 1947, the Linde firm introduced synthetic star sapphires and rubies. Subsequently, it Was shown that the method of manufacture consisted of adding titanium oxide to the basic formula and inducing it to crystallize out as minute rutile needles by heat treating the finished boule. This is done at a temperature of 1900°C for approximately twenty-four hours, followed by a reduction in temperature for an additional period ranging up to forty-eight hours. In additions to allowing the rutile needles to develop, the annealing process also frees the boule from strain. Since the needles orient themselves according to the structure of the corundum, a pattern of three sets of needles oriented at 60°to each other in planes perpendicular to the prism faces of the crystal develops; this is the same pattern that silk assumes in natural corundum. Because of the even distribution and minute size of the needles, the stones have an outstanding star. (In the first year or two of manufacture, the rutile inclusions were confined to a band across the top of the cabochon. As a result, most of the cabochon appeared transparent and one pair of rays did not extend to the edge of the stone.) This achievement could be likened to the synthetic of diamond as a monument to scientific ingenuity.
Following Linde's success, at least one attempt in the United States was made to infringe on their patent. A former employee started production, principally of synthetic star rubies, but was forced to suspend manufacture. Later, a firm near Munich, West Germany, began to make star sapphires and export them.
After a custom's court hearing, an agreement reached whereby the German firm could import their stones into the United States with royalty payments to Linde. The German stones have certain differences that occasionally may be used to distinguish them from the Linde stones. The color of the imported stones tends to be more natural, but the star is not as sharp; in addition, they are apt to be more translucent than first-quality American stones. When observed on the back, the German stones almost always show several concentric rings and a "bull's-eye" of darker color, whereas the Linde stones rarely do. The implication here is that the German boules are made so that the boule axis and the optic axis are the same; furthermore, they are small, so that only a single stone can be cut from each one. Many stones can be cut from the relatively large Linde boules.
Another recent entry into the synthetic star marked is the Westropa Import Company, Inc. of New York, who will distribute the "I" STAR sapphire. It will be manufactured in Israel using equipment acquired from the Union Carbide Corporation.
The Hydrothermal Process
In 1957, Bell Laboratories, Murray hill, New Jersey, announced the production of synthetic ruby, as well as colorless and green sapphire. ln this process, colorless seed crystals in the form of oriented wafers of ordinary synthetic corundum are suspended in a solution within a silver-lined hydrothermal "bomb", or autoclave, built to withstand pressures of 30,000 pounds per square inch, high temperatures and corrosive solutions. The raw materials used to produce ruby consist of one of the hydrated alumina compounds in a sodium-carbonate solution, to which is added one-tenth of a gram of sodium chromate to a liter of the solution. The selected autoclave is heated from below, which causes some of the raw material to go into solution. The seed crystals are strung along the center of the vessel above the heat source, and thus are in an area slightly cooler than that below. The solution becomes saturated with alumina and is carried by convection currents to the cooler upper areas. At the lower temperature the solution is supersaturated; therefore, the excess alumina is deposited on the seed - crystal wafers as corundum. The process is a variation of the one used by this company for synthesizing quartz the equipment for which is shown in Figure f. This growth process probably comes closest to that by which ruby is formed in nature. However the main problem appears to be heavy veiling and cracking which may originate from too rapid a growth.
Flux-Grown Ruby
In this process, a mixture of lead oxide and boron oxide is used as the solvent. The components of ruby, (Al2O3 + Cr2O3) are dissolved in the solvent at 1200 to 1300°C in a platinurn crucible, and on slow cooling the solubility of ruby is exceeded, producing crystallization. When growth is essentially completed, the remaining solvent is poured off: Alternatively, the contents may be cooled in the furnace and the flux dissolved away from the ruby crystals by boiling in large quantities of acid over a period of many days.
Growth striations are not seen in flux-grown ruby, if the temperature control is good. Three types of inclusions are seen in this material: massive flux inclusions appearing yellow to pink, a coarse type of finger print inclusion, plus a very fine finger-print inclusion. The very fine finger-print type could be quite misleading, since the same type of inclusions are seen in natural rubies. The growth process is quite expensive and large quantities of this type of synthetic ruby are not expected to be available in the near future. The strong fluorescence and absence of other types of inclusions should help distinguish these synthetic rubies from natural stones. Chatham Created Gems, Inc and Kashan Laboratories Inc are currently manufacturing flux-grown rubies.
Recently, Chatham has created a flux-grown blue sapphire which is sold only as crystal dusters. Its properties are similar to the natural, is usually color zones, contains typical flux inclusions and fluoresces a yellowish green under short-wave ultraviolet.
