The Artist's Assistant, In the Study and Practice of Mechanical Sciences: Calculated for the Improvement of Genius. Illustrated with Copper-Plates (Birmingham, [ca. 1785])

276 THE ARTIST's ASSISTANT.

colour, which was done in about five or fix hours:
if then the blue colour was not quite gone, we began
our operation afrefh until it was white and clear.
It is to be obferved, that the heat of the fire, in the
beginning of the operation, muft increafe by flow de-
grees, and alfo in the fame manner decreafe ; for if
the ftone comes either too fuddenly into the heat, or
from the heat into the cold, it is apt to tarn dark, or
fly to pieces.

In like manner all other precious ftones lofe their
colour, fome fooner than others, according as they
are either harder or fofter. The amethift is very
light, and requires but a flow fire, for if it has too
much heat, it becomes dark, or turns into chalk.

This is the art whereby inferior precious ftones
are changed into diamonds; they are afterwards cut
in the middle, and a colour given them : and from
Jience comes the fecond fort of falfe diamonds, or
doublets,

A plain direction concerning the polifhing of
thefe counterfeits, and alfo of natural gems: - It is
to be obferved that all glafs, or artificial ftones, may
be cut and polifhed after one method ; namely, by
ftrewing fine powdered emery upon a leaden plate
with water, holding the ftone firm, and grinding it
in what form or ihapc one plevifcs.

If you fling ground tripoli, mixed with water,
upon a pewter plate, and add a little copper afhes
amongft it, it will have the fame erFeft.

Pulverized antimony ftrewed upon a fmooth plate
of lead, with the tripoli and vinegar, polifhes not
only glafs, cryftal, garnets, caicedons, agates and
amethifts, but all other natural ftones, except the
diamond. The diamond is only cut with the

diamond