held
there ; the other end of the clamp is formed into two vertical legs or
supports of such a height that when resting on the table, with the bar
of the clamp horizontal and the dop fixed, the gem mounted on the dop
is about an inch above the table. The clamps are made of iron, and when
in use are further weighted, as the need occurs, with lead weights. In
the case of the Diamond, grinding and polishing occur simultaneously,
and are performed on a rapidly rotating disc of iron called a " skief"
or "lap." This lap is a wheel of about twelve inches diameter and one
inch thick, mounted on a steel shaft running on pivot bearings ; the
wheel is of porous cast-iron, and it is mounted so as to rotate in a
horizontal plane an inch or so above the level of the bench. The rate
of rotation is always high, and the harder the stone under treatment
the greater the speed necessary to obtain a cutting effect. When the
speed is high enough, a gem may be abraded by a substance of its own
degree of hardness, or even one of a lower degree, though, of course,
the harder the abrading agent in comparison to the gem, the more rapid
the progress with the work. The usual speed may be taken as 2,000 to
3,000 revolutions per minute, which gives a peripheral cutting velocity
of, roughly, 100 to 150 feet per second, or say 75 to 100 miles per
hour. Power is applied by steam, gas, water, or from an electromotor,
and the spindle is driven by a small pulley running below the bench.
The abrading material in the case of Diamond is always the powder of
the same mineral, as no harder substance is available ; this is
naturally expensive, hence the precaution taken to save all dust from
the process of bruling. In the cleaving, too, such fragments as are too
small to be cut for gem use are saved, and are later crushed
