To
obtain the maximum portion of the gem-stone the diamond is divided into
smaller ones by sawing it through the middle, which gives two stones of
equal size. If it is sawed above the middle, two stones of unequal size
are produced. The shape of the stones and the flaws or imperfections
form the guide in the sawing. With large stones, both cleaving and
sawing may be required. The smaller portions cut go into the 'melee'.
The sawing is done with the aid of a thin phosphor bronze disc 2" or 3"
in diameter
revolving
at a speed of 2,500-4,000 revolutions per minute. The diamond to be
sawed is embedded in a metal cup-like holder with a mixture of plaster
of Paris and glue. The portion to be removed is exposed, and is held
mechanically against the edge of the bronze disc, which has been
impregnated with diamond dust and oil. Sawing a one carat stone takes
about eight hours and larger stones may take several days, but one man
could supervise the work of about twenty sawing machines.
Diamond
dust could be used to saw or cut diamond in certain directions, e.g. in
faces parallel to crystallographic axes, i.e. faces of the cube. It is
found impossible to saw the diamond, if the direction of cutting
differs by a few degrees from that of a cubic face.
(4) Cutting
By
cutting, the stone receives the rough shape or outline. It is now done
mechanically and more rapidly than in former days. The stone to be cut
or shaped is mounted in a brass holder called the 'dop' with a hard
cement. The dop is then screwed to the chuck of a motor-driven lathe. A
second stone, similarly fixed to a dop, is screwed to the end of a
specially designed stick, and held against the one revolving on the
lathe. The stone mounted on the stick serves as a cutting tool.
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