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]

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1 cm

martis prepared with vinegar, eight grains ; let the
whole be finely pulverized and fifted ; put this toge-
ther in a crucible, leaving one inch empty ; lute it
well, and put it into a potter's furnace, where they
make their earthen-ware, and let it ftand there as
long as they do their pots ; when cold, break the cru-
cible, and you will find a matter of a fine emerald
colour, which, after it is cut and fet in gold, will fur-
pafs, in beauty, an oriental emerald; if you find that
your matter is not refined, or purified enough, put it
again, the fecond time, into the fame furnace, and in
lifting off the cover you will fee the matter mining ;
you may then break the crucible, but not before, for
if you fhould put the matter into another crucible,
the pafte would be cloudy and full of bliffers; if you
cannot come to a potter's furnace, you may build one
youvfelf with a fmall expence, in which you may
put twenty crucibles at once, each with a different
colour, and one baking will produce a great variety
of gems : heat your furnace with hard and dry wood
and keep your matter in fufion twenty-four hours,
which time it will require to be thoroughly purified,
and if you let it ftand four or fix hours longer, it will
not be the worfe for it.

To make pafte for imitating an oriental topaz :—The
colour of this ftone is like water tinged with faffron,
or rhubarb ; to imitate it, take of prepared natural
cryflal one ounce, of red lead feven ounces, finely
powdered and fearced ; mix the whole together,
and put it into a crucible, not quite full by an inch,
leaft the matter fhould run over, or flick to the cover
of the crucible in rifmg, then proceed as directed

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