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Diamonds For Mechanical Purposes

( Originally Published 1911 )



IN addition to carbonado or carbon, there are other forms of diamond which are used largely for mechanical purposes. Of these the principal is called " bort." This is crystallized diamond not sufficiently transparent or clear to cut as jewels. A large part of the product of the diamond fields is composed of this material. It is estimated that one-quarter of the yield of the Brazilian diamond fields and about forty-five per cent. of the African mines, consists of bort. Some of the African mines yield a larger proportion than others, notably, the Premier and the Kimberley.

Usually it is too brittle for drill purposes, the cryptocrystalline carbon being harder and better able to resist pressure. Nevertheless, bort, in sizes from one to three carats, is used to a certain extent in drills which are not forced to any great depth, or through very refractory strata.

Crystals weighing one-half to one carat each are used extensively as teeth in stone-saws for sawing marble and stone for building purposes. Revolving saws up to 75 inches in diameter and sometimes over, carry up to 100 diamonds weighing in the larger sizes 25 or 30 carats worth $20 per carat. These saws with a rim speed of 10,000 to 12,000 feet per minute, cut into limestone over 7 inches per minute. Large quantities are used in electrical machines as jewels for meters, etc. It is recorded that Solomon used diamonds to cut the stones for the temple at Jerusalem, so that the enterprise of our modern machinists may be but the resurrection of very ancient methods, and it is not improbable that the Jewish King obtained his diamonds from the same or neighboring sources, for there is evidence of ancient mining in Rhodesia, in a section where late discoveries of diamonds have been made.

Small pieces of bort, and crystals which are full of inclusions and fractures, are crushed to a powder for use as an abrasive.

Unlike carbon, which is found with the gem diamond stones in one district only in Brazil and to a very limited extent in Borneo, bort is found in all diamond diggings the world over.

As there has been no sustained effort, by controlling the output, to maintain prices, the price of bort has varied considerably since the discovery of the African diamond mines. It has depended largely on the demand created by the use of it for mechanical purposes. In 1875 the price for unassorted lots at the mines was about 50 cents per carat, but as it was found useful for more purposes, and the number grew of those who knew its value for their uses, the price rose gradually, until by 1883 the mines were getting an average of about $2 per carat. These are much less than the market prices for assorted goods during the same period. Since then there has been considerable fluctuation, but the tendency has been upward, especially since the rough was marketed in Lon-don, though the opening of the Premier with its big output of bort, broke the upward movement. Present New York quotations are as follows :

1 t0 3 carat crystals for drills $8.00 to $15.00 per ct.

1/2 t0 1 carat crystal for saws, etc 3.00 to 4.00 per ct.

16 to 20 to the ct. crystals for meters, etc 3.00 to 3.50 per ct.

Small and poor crystals for crushing 75

In the early years of the African mines, dealers made large profits. The market price in 1875 was about $4 per carat for fragments, and $10 for crystals. It fell steadily to 40 cents to $1.50 in 1892. It then rose to $4 to $8 in 1901, since which it has declined steadily again. Small diamonds, or corners of crystals, having an edge suitable for glass cutting and called " glaziers' diamonds," have a wide range of price, selling from $6 to $50 per carat.

" Flats " are thin crystals or parts of crystals into which holes are bored so that they can be used as dies for drawing wire. In many of the fine and delicate adjustments required now, in electrical machinery especially, it is necessary that wire shall be drawn to a gauge infinitesimally exact. Constant drawing of wire through metal dies, even of the hardest, soon enlarges the hole, and consequently the size of the wire also, but with a diamond die, enormous lengths can be drawn without any appreciable difference. These tiny plates of diamonds have therefore become valuable assistants in the progress of machinery and its adaptation to applied science. They are sold now for $3.50 to $8.00 per carat. The dies for which diamonds are used are for drawing fine wires. The holes range usually from 0.001 inch to 0.064 inch, though they can be made accurate to 0.000i inch. The wear of metals on diamonds increases in the following order, it is said: Gold, silver, copper, brass, bronze, platinum, nickel, iron, crucible steel.

" Splints " are sharp-pointed splinters of diamond crystal. They are obtained from the refuse of the cleaving and cutting establishments and also, since the use of the grease-table, from the mines, with the unbroken crystals. As noted elsewhere, the matrix of the African mines contains many fractured crystals, and the grease-tables hold small splintered pieces which formerly escaped attention when hand picking was the custom. They are used for small drills, for turning jewels for watches, electrical machinery and similar purposes, and at present bring from $3 to $10 per carat.

In the use of diamond in any form for mechanical purposes, care must be taken to avoid crushing or over-heating. The hard fragments of a broken diamond involved in machinery turning rapidly, do serious damage almost instantaneously, and overheated, the crystal loses consistency and carbonizes the soft iron of the setting, turning it at once into hard steel. This applies particularly to carbon when used for deep borings. A hard blow will often crush carbon to fragments, and heat injures the quality. A stoppage of the supply of water to the borer has been known to change the hard car-bons of the drill to a mass resembling black glass which yielded to a file, while the soft iron of the bit was at the same time turned to steel. Great skill and care is necessary also in the setting of the carbons in a drill for deep boring. If one gets loose, it quickly tears itself and the bit to pieces, and fishing for a loose carbon through a small tube several thousand feet down in the earth is wearisome and expensive.

Carbonado, or carbon, is the most important form of diamond for mechanical purposes, as it is used in the larger operations of deep boring.

The colors of carbons vary from light brown to jet black. Usually they are lighter on the inside, but with long exposure after splitting, the pieces grow darker. There is a wide difference in the quality, and the toughness can never be determined by the color, and not always by the appearance. The specific gravity test is safest. Usually those of a dense, close texture, are hard, but sometimes porous, or open texture pieces, are very hard, and close grained ones, soft. Formerly carbons were all split in Europe, but 25 or 30 years ago Mr. I. C. Yawger built a machine here for that purpose, and much of it has since been done in this country. To split a carbon, it is placed between hard chisels or cutters and subjected to heavy pressure or a blow.

In diamond drills, pieces of carbon, usually 8 pieces, are set in circular rims of soft steel or iron, 4 on the inner side of the center of the rim, and 4 on the outer, placed alternately. The metal is burnished well up on to the carbons to withstand the strain and hold them. These bits are attached to tubes in sections, and borings have been made in this way to a depth of over 6,000 feet. Only carbon can stand such a strain and pressure; bort is too brittle. Owing to the increasing demand and consequent advance in the price of carbon, these bits are expensive necessities in mining explorations.

A bit for deep borings will require 8 carbons of not less than 3/ carats each, or 28 carats for the bit, which would bring the cost of the carbons alone, for one bit, at the present price of $85 per carat, to $2380.

The carbons are bought for cash, at first hands, in unassorted lots of all sizes and qualities, running from 300 to 1,500 carats. As with all expensive material, frauds are perpetrated on the unwary by some unscrupulous dealers. Poor diamonds are fixed up to look like carbons, bogus carbons are mixed with the genuine, and sometimes unadulterated frauds are palmed off for genuine.

In the natural state, carbons usually show no regular form of crystallization, though octahedrons, and cubes have been found. Under the microscope, however, they appear to be formed of minute diamond crystals, and carbon powder is composed of bright brown half transparent diamond octahedrons, frequently with opaque enclosures. Carbon therefore appears to be a mass of infinitesimal diamond crystals. To one out-side the trade, the stones have no appearance of value whatever. They are light in weight and therefore do not impress one as the heavier metallic ores do. Ir-regular in shape, of a dull grayish-black, brownish, some-times greenish, color, there is nothing about them t0 suggest value, yet half a dozen of them as large as hickory nuts would be worth several thousand dollars. Close examination under a loup will discover a porous-looking surface covered with angular indentations having a lace-like appearance and a wave-like arrangement. In and about the crevices are numerous infinitesimal glistening specks like the faces of small crystals. Some pieces have a vitreous sheen like a piece of molten glass., This characterizes many of the carbons from the Morro district.

There are certain risks attached to the breaking up of the large stones which make them highly speculative. There are sometimes vicissitudes of price in the journey from the cascalho to the machine maker, and they lose not less than ten per cent. of the weight in breaking. For the large carbon of 1895 the finder got about $16,000; the owner of the claim receiving one-fourth of the amount. It went through several hands and was sold in Bahia city for 121,000 milreis, equal at that time to about $25,400. The London buyer paid about $32,000 for it, and after breaking it up, got nearly $36,000 for it. The smaller one of 1901 brought the finder $17,380, or about five times as much comparatively, so much had the price advanced in the six years. The rapid development of electrical and other machinery is indicated by the rapid rise in the price of carbons. In 1884, $4 to $4.50 per carat was paid to miners in the fields for unassorted lots of good material. In 1898 the price was up to $11 and over. In 1902 it was reported in London that £8.10 to £9 per carat was paid in Brazil for fine quality carbons of the desirable sizes, though consular reports quoted $24 as the price paid in the field for unassorted stones over three-quarters of a carat; $7.20 for half to three-quarters of a carat stones, and $2.75 for smaller ones mixed with imperfect pieces and refuse diamonds.

The limits of prices given by Bahia firms to their field buyers to be paid in the spring of 1906 was given by former Vice-consul Rowe as follows :

1st quality, 6 to 120 graos 31 milreis per grao = $42.45 per ct.
120 graos upward 30 milreis per grao = 41.07 per ct.
2nd quality (porosis) 15 milreis per grao = 20.54 per ct.
Crystallino 10 milreis per grao = 13.69 per ct.
Ballas, broken pieces 6 graos up 10 milreis per grao = 13.69 per ct.
Ballas, broken pieces 4 graos up 20 milreis per grao = 27.38 per ct.
Ballas, broken pieces,3 graos up 12 milreis per grao = 16.44 per ct.
Fundos 2 milreis per grao = 2.74 per ct.

Ballas or Borts

1st quality white, 6 graos up 30 milreis per grao = $41.07 per ct. Colored, 6 graos up 25 milreis per grao = 34.25 per ct. A grao is about 1/4 carat ; 72 graos =1 oitava = 17 1/2 carats.

It is difficult to tabulate prices exactly, as they vary according to conditions and the average quality of the lots. Though in a general way prices at the fields follow the market, they do not adjust themselves as quickly to the immediate demand throughout the scattered diggings in the interior wilds, as at Bahia city. Nevertheless, as there is competition among the field-buyers, and they are kept well informed by the houses they rep-resent, the diggers receive on an average, a good share of the market value, though naturally they do not benefit as fully from a sharp advance of price.

As in Brazil, London sells chiefly in unassorted lots, but Germany has established a profitable business in carbons, by assorting and selling separately, according to individual requirements.

Though the source of supply is comparatively near New York and a large quantity of carbons is used in the United States, our supplies come chiefly via Europe. There is a monthly steamer plying between Bahia and New York, but several steamers leave Bahia each week for British ports. At one time firms exporting up to $150,000 per annum paid a tax of $1,500; if the exports exceeded that amount, $3,000 per annum. There was an export tax of seven per cent. ad valorum, but this was abolished for a tax on individual shippers calculated to bring the amount up to what it would be at seven per cent. if all the diamonds shipped were declared. Many dealers met this by combining to ship as one firm. These taxes prevent the beginning on a small scale of export in a new direction, though it is probable that more goes to New York direct than official reports show.

The price of carbons in New York at present (1909) is quoted as follows by Mr. J. S. Rose :

Carbons for mining drills, 3 to 6 carats $60.00 t0 $85.00 per ct.
Carbons for mining drills, 1 1/2 to 2 1/2 carats 45.00 to 55.00 per ct.
Carbons, 1 carat 35.00 to 40.00 per ct.
Carbons, 1/2 to 3/4 carat 30.00 per ct.
Carbons, 1/4 carat 15.00 per ct.
Carbons, smaller 8.00 per ct.

Since which the price of fine carbon rose rapidly to $95 two or three years ago, dropping back to the present price of $85 per carat. Stones down to one carat in weight are occasionally used in mining drills for some purposes. The smaller ones are used for emery-wheel dressers, turning hard stones and hard rubber, drilling semi-precious stones, eyeglasses, etc.

In 1904—6 there was an enormous consumption of carbon in the United States, owing to the great development of the machine industry. Sales amounted to nearly $3,000,000 annually; considerably more than the amount declared in the exports of Brazil. Prime car-bon brought as high as $95 per carat. At present the price at the fields is about 10 lbs. to £Io. 10s. per carat.

Among other things, diamonds are used for points, lens, drills, dental drills, pivot jewels, glaziers' tools, glass cutters' sparks, etc. Bort is used for stone saws, prospecting drills, emery-wheel dressers, wire dies, electrical jewels, small tools and to crush for powder.

Misunderstanding regarding prices arises from a habit of quoting prices in trade journals without stating where those prices rule. The price of carbonado in the Transvaal has been quoted at $60 when it was selling for much less in New York, and there are great differences between prices at the mines and in the various markets of the world, owing to the wide range of quality in the unassorted lots at the mines and the varied assortments made to suit the demand of different countries and also to the fact that the character of the material is speculative. It is also necessary to ascertain the exchange value of the milreis at the time, when quotations are made in Brazilian money, as there have been great variations in the value of the milreis.



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