ivory
about five or six inches long, and this holder is steadied against a
rest fixed near the right-hand side of the lap ; the wheel is in some
cases rotated by hand, but more often by power, as in Diamond grinding.
The stone is first roughly ground to shape and then the facets are
ground on; a close watch has to be kept for the appearance of any small
cracks in the work, which if not carefully dealt with are apt to extend
and spoil the stone.
In
grinding the relatively softer gems, the abrading material, as in
grinding the Diamond, presses its way into the metal of the lap.
The
stone is now handed to another worker to be polished. This process
consists of carefully following the previous work and rendering the
facets as smooth and bright as possible. The lap used is similar to the
grinding lap in type, but is usually of softer material, in the case of
softer stones often of wood, covered with leather or paper, on which
the polishing material is smeared. When laps of copper, lead, or pewter
are used the surface is finely scratched with a piece of sandstone, so
as to give minute grooves, arranged similarly to the large grooves on a
millstone; this has the effect of evenly distributing the polishing
material. Several substances are used in polishing: rouge and
putty-powder (respectively oxides of iron and tin), rotten stone, fine
pumice, tripolite and bole being amongst these. Tripolite is mixed with
sulphuric acid for use, the others are mixed with water. All these
substances must be in the form of the very finest powder, or the work
will not acquire the high polish desired.
When a large portion of rough material has to be removed before grinding can be commenced, it is usually effected by
