bon 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
toughÂness 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.
