Artificial Diamond

Scientific American
(June 7, 1913), p. 515

"A New Way of Making Artificial Diamonds"
    A Paris engineer, M.E. de Boismenu, claims to have produced small diamonds by a new electric furnace method. It will be remembered that the late Prof. Moissan succeeded in obtaining very small diamonds (of microscopic size) in the electric furnace, but the process required special skill, and in any case the results were merely of scientific interest. M. de Boismenu employs a new principle, which has the advantage of being very easy to carry out in practice by a skilled operator. Moreover, the process will undoubtedly be further improved so as to secure larger specimens than those so far produced, which range up to 2-1/2 millimeters in diameter.
    The inventor occupies a prominent position as director of an electric carbide furnace plant in France and conceived the idea that the diamond could be produced by electrolysis of a bath of molten carbide between the usual carbon electrodes.
    The furnace used is built of refractory brick and has two carbon electrodes 6-1/2 inches in diameter, one of which can be adjusted by hand. The bed of the furnace is first packed with a mixture of powdered lime and carbon, which serves to hold a trough-shaped receptacle made of fused calcium carbide, as this is found best to hold the molten bath within the furnace. The carbons work within this trough, and are packed around with rather large fragments of carbide. By leaving the current on the bath of molten carbide for a number of hours, an electrolytic action takes place by which the carbide is decomposed and the negative pole becomes surrounded by a black carbonaceous mass, in which are found embedded small crystals. These crystals answer all the tests for the diamond.
    The first conclusive operation was made on April 13, 1908, in the inventor’s experimental laboratory in the suburbs of Paris, using direct current from a small dynamo plant therein installed. After heating up the electrodes, they were drawn one inch apart, and calcium carbide was gradually fed in in small lumps, so as to produce a molten bath. The carbons were then gradually separated until finally they were 10 inches apart. The heat commenced at 11 a.m. and ended at 5 p.m. with a continuous run of 6 hours. The current used was 800 amperes at 34 volts. There were 8 pounds of melted carbide in the bath. At 3 o’clock a pile of carbide fragments were heaped upon the bath, and the whole was covered with a mixture of equal parts of lime and carbon so as to stop up the interstices, and finally the furnace was covered with two refractory slabs. The furnace ran in this way up to the end of the test, when the current was stopped and the furnace allowed to cool off overnight. The scoriaceous mass resulting from this operation, weighing from 600 to 700 grams, was placed in a vessel of water and allowed to remain overnight. The residue was examined the next morning. During the night it had disaggregated in the water and formed a black mud, which was decanted and then slowly dried over an alcohol lamp. At once M. Boismenu’s attention was attracted by small brilliant points standing out against the black background. He was able to pick these particles out by forceps and thus separated about a dozen of them. They appeared as small transparent crystals of somewhat irregular shape whose size varied from ½ to 1-1/2 millimeters. Under the microscope they showed the characteristic appearance of diamonds. The specimens will scratch a plate of glass under very slight pressure, and the scratches are deep and remarkably clear; steel can also be scratched by them.
    From April 20th to June 5th the furnace made fifteen runs, of which eleven were very successful. The last two of these, the furnace ran for 12 hours with 700 to 800 amperes at 24 to 25 volts, and some of the crystals reached one-tenth of an inch in diameter. This seemed to be as far as one could go with the present small plant, and a new one will be required for further work. The specimens were submitted to two jewelers of Paris, who were unable to distinguish them from natural diamonds. One of the largest specimens could even be cut, and the author sent it to Amsterdam for the purpose. It was returned cut with 32 facets with remarkable dexterity.
    M. de Boismenu hopes to be able to continue his experiments in the near future, provided that funds are forthcoming for installing an electric furnace upon a larger scale. In closing, we should mention that the process has been patented by the author.

French Patent # 531, 091
Processs and Apparatus for the Fabrication of Diamonds in the Electric Furnace

   It has been found that the metallic carbides originating by combination of carbon with the alkaline metals and alkaline-earths can be decomposed by electrolysis at very high temperatures, to give on one hand the metal and on the other hand the carbon that can collected in the neighborhood of the anode in the form of a diamond crystals, in the operating conditions of a properly established device.

    When one electrolyzes an amenable metallic carbide in a molten bath in an electric furnace it must be protected from oxidizing conditions. It is difficult to contain this bath, and one must use materials that are more refractory, or it will react with the molten mass that is produced.

    The present invention has for its object a process and devices allowing collecting of diamonds, using the same principle, more easily and with a higher yield than it has been possible to do so far.

    To this effect, one has recourse to a furnace of a crucible form appropriately established in graphite or in agglomerated coal, and presenting the following particulars being able to exist separately or in combination:

    1. The crucible and the electrodes, except for the end in the interior of the crucible, are nested in a mixture well coal cup and powder lime filling the envelope of the electric furnace.

    2. The lower part of the crucible is surrounded by entouree by an annular refractory enclosure designed to receive the excess bath of fusion, and communicates with the crucible by suitable  openings.

    3. The electrodes are isolated and protected, having their entry in the crucible, by very refractory pipes or tubes, for example in magnesia.

    The description which follows, when compared to the additional drawings and examples, will explain the nature and the advantages of the invention.

    Figure 1 shows in cross section an electric furnace for the manufacture of diamond, established in accordance with the first mode of realization of the invention.

    Figures 2 and 3 show respectively furnaces established in accordance with the second and third modes of realization of the invention.

    To avoid the disadvantages mentioned above, the first procedure consists of making the electrolyzed calcium carbide in a crucible itself made out of calcium carbide and surrounded by a garnish constituted preferably by a mixture of lime powder 80% and powder coal 20%. During effective experiments, applicant has noted that it forms around this crucible, during the operation, a hard hull and impermeable has the air, constituted by lime having undergone a beginning of fusion and having crystallized. It is in the interior of this protective hull that the phenomena are carried out and the reactions form the synthetic diamond.

    To regulate the electrolytic actions and to contain the fusion bath, it is advantageous to simply take a crucible out of coal graphite, or same in agglomerated carbon, by having recourse simultaneously to the provisions which will be indicated hereafter.

    If one takes, for example, the ordinary furnace represented in Figure 1, the crucible B in the form of a cup, made of coal graphite or agglomated coal of very good quality; this crucible, which has advantageously a thickness of 3 centimeters, intended to receive the bath C of metallic carbide in fusion. The edges of this crucible are bored with two holes of suitable diameter to insert both electrodes, which are isolated and protected, while passing in the crucible, by very refractory tubes D, advantageously made of magnesia.

    Around the lower part of the crucible an enclosure is formed for example annular channel  E to receive the surplus of the bath of fusion, and the finished reactions which one wants to carry out. This channel preferably is constituted by refractory plates and and tubes provided with some vents, and it is in relation to the lower part of the crucible by openings F, example with two, and receiving a diameter of 2 to 3 cm.

    The whole of the crucible, the enclosure and the electrodes, is nested in a mass G constituted by a mixture of coal and powder lime filling all the interior of the electric furnace.

    Another provision, represented in Figure 2, consists in inclining the electrodes from 30 to 60 degrees. The shape of the crucible is slightly modified as it is seen Figure 2, and the other provisions of the operation remain the same.

    One can finally, as Figure 3 shows it, lay out the electrodes vertically. In this case, the negative lower electrode H butts in a crucible B, made of coal graphite like previously, and the shape of chalice, and which is to joined to it by a threading K. The other provisions (enclosure, openings, and pulvurent mass) are the same ones that those described above.

    During experiments which it have been effected, it is advantageous to leave with a tension of 30 to 35 volts, in order to determine the size of the arc; then, as soon as the fusion bath is well formed, there is no more arcing. The furnace has a function in resistance and it is necessary to decrease the voltage, and to bring it for example around 25 volts. The intensity can be determined by choosing for example a current density of 3.5 amps by cq. o prevent excessive corrosion of the electrodes and parts of the furnace. In particular, the applicant has established and obtained resultants with a furnace provided with cylindrical coals having 11 to 12 cm of diameter and 1.20 to 1.25 meter in length, with a tension of 20 to 25 volts, an intensity of 500 to 600 amps, and a power of 12 kw.

    It is clearly understood that the numerical provisions and information which are indicated above are examples and by no means restrictive and which can be modified without leaving the framework of the invention.

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