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62
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PRECIOUS STONES.
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point
after six hours' work no effect seemed to have resulted, but on
increasing the speed to 3,000 revolutions per minute the grinding
proceeded satisfactorily, though the total time spent in cutting it was
456 hours ; it was found, too, in this case that they were cutting
across the angle of meeting of two cleavage planes, or " across the
grain " as it is expressed. This is then also a good instance of the
importance of grinding in the right direction, for the hardness varies.
It is found that the directions of least hardness are in lines between
the centres of opposite octahedral faces—that is, the cutting must not
take the direction of the crystallographic axis in the Diamond.
The
abrading material quickly works its way into the pores of the cast-iron
disc, hence the disc does not become worn away so quickly as one might
expect.
When
one facet has become ground down to its proper shape and size and has
received as high a polish as possible, the dop is altered in the holder
so as to bring the stone into position for another facet to be ground.
When as many of the required planes have been formed as the position of
the stone in the solder will allow, the dop is returned to the solderer
to have the stone re-set at another angle. Frequently the dops are so
hot that they cannot be conveniently handled, so wooden holders are
provided; they are of a shape similar to a dumbbell with a hollow down
the handle, into which the peg of the dop is placed.
To keep the stone from becoming overheated the dop is from time to time placed in water to cool.
More recently clamps have been used fitted with divided arcs, so that the stone may be placed on the lap at exactly
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