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PRECIOUS STONES.
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61
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in
a steel mortar furnished with a well-fitting cylindrical pestle of
steel, the dust being used in the same way as that from bruting. The
impure variety of Diamond, called Bort, is also extensively used and
also the finely granular opaque variety, Carbonado. Bort is harder than
the pure crystalÂlised variety, and Carbonado is as hard or harder, and
is also less brittle than the pure Diamond.
The
abrasive agent is mixed with a little olive oil in a capsule and a
small quantity of the mixture applied to the upper surface of the lap
or skief. The clamp with the dop in position is then lowered so that
the gem rests on the lap. The position must, of course, be accurately
adjusted to grind the stone in the desired plane; usually one operator
manipulates four dops, placed equally distant round the lap so as to
distribute the pressure evenly. As before menÂtioned, the clamps are
weighted with lead weights to give sufficient pressure, without which
the work would progress much more slowly. With a given weight it is
obvious that the pressure on a given area will be less with a large
facet than with a small, since the same total force is applied over a
larger area in the former case than in the latter; hence to maintain an
even pressure the weights must be altered. The weights applied vary
from 2 to 30 lbs.
As
an instance of the importance of having a proper speed, it may be
recalled that in the cutting of the Koh-i-noor the work was being done
with the wheel at 2,400 revolutions per minute. When the cutters came
to one part, however, no progress seemed to be made; greater pressure
was applied, with the result that particles of the iron disc, mixed
with oil and diamond-powder, became ignited, and then the solder began
to melt. At one
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