used.
The first is hollow; it describes circles with the utmost facility, and
serves, when required, to perforate hard stones. The second is a disk
quite blunt at the edge. The third is a sharp-edged disk of very
frequent use, serving as a saw. The fourth is a rod, terminated by a
little sphere, and is very frequently used. Fig. 124 shows all the
tools used by the engraver.
As
precious stones suitable for engraving have always a considerable and
sometimes a very high value, it is important to be able to make use of
every portion of them. They are therefore sawn instead of being ground
down, so that the portion removed may also be made use of. This
operation may be performed in different ways. The most ancient and
simple method consists in fixing the stone to the extremity of a
support, and cutting it by the friction of a bow, strung with two iron
wires twisted together, and impregnated with diamond-dust. This method,
however, is both tedious and irregular, and hence instruments,
infinitely more rapid and precise, have been substituted for the bow.
Fig.
125 represents the mill of the lapidary, with the polishing disk
replaced by a steel disk with a cutting edge, against which the workman
applies the stone with his left hand, while he sets the apparatus in
motion with his right. The disk is sprinkled with diamond-dust, which
the workman collects
