Encyclopaedia Britannica > Volume 7, ETM-GOA

GEM [ 605 ] G E M
To the above we fhall add the following receipts and
proceffes, contained in a Memoir by M. Fontanieu of
the Royal Academy of Sciences at Pans, and faid to
have met with much approbation.
I. Of the Bafes. Although the different calces of lead
are all adapted to produce the fame effe& in vitrifica¬
tion ; yet M. Fontanier prefers lead in fcales, and
next to that minium, as being the moft conftantly
pure. It is neceffary to lift through a filk fieve the
preparations of lead one wiflies to make ufe of in the
vitrification, in order to feparate the groffer parts,
as alfo the lead found in a metallic ftate when white
lead in fcales is employed.
The bafe of fadlitious gems is calx of lead and rock-
cryftal, or any other ftone vitrifiable by the calces al¬
ready mentioned. Pure fand, flint, and the tranfpa-
rent pebbles of rivers, are fubftances equally fit to
make glafs : but as it is firft neceflary to break the
mafles of cryftal, ftones, or pebbles, into fmaller parts;
fo by this operation particles of iron or copper are fre¬
quently introduced, and to thefe dull or greafy matters
are alfo apt to adhere. Our author therefore begins by
putting the pounded cryftal or pebbles into a crucible,
which he places in a degree of heat capable of making
the mafs red-hot; he then pours it into a wooden bowl
filled with very clear water; and fhaking the bowl
from time to time, the fmall portions of coals furniftied
by the extraneous bodies fwim on the furface of the
water, and the vitrifiable earth, with the iron, &c.
refts on the bottom. He then decants the water; and
having dried the mafs, he pounds it, fifts the powder
through the fineft filk fieve : he then digefts the powder
during four or five hours with marine acid, ftraking the
mixture every hour. After having decanted the ma¬
rine acid from the vitrifiable earth, he wafhes the lat¬
ter until the water no longer reddens the tindture of
tournfol. The faid earth being dried, is palled through
a filk fieve, and is then fit for ufe. Nitre, fait of tar¬
tar, and borax, are the three fpecies of falls that enter
with quartz and the feveral calces of lead into M. Fon-
tanieu’s vitrifications.
Much of the fuccefs in the art of making coloured
ftones depends on the accurate proportion of the fub¬
ftances made ufe of to form the cryftal which ferves as
a bafe to the fadlitious ftones. After having tried a
great variety of receipts, our author found they might
be reduced to the following.
1. Take two parts and a half of lead in fcales, one
part and a half of rock-cryftal or prepared flints, half a
part of nitre, as much borax, and a quarter part of
glafs of arfenic. Thefe being well pulverized and
mixed together, are to be put into a Heflian crucible,
and fubmitted to the fire. When the mixture is well
melted, pour it into cold water: then melt it again a
fecond and a third time ; taking care, after each melt¬
ing, to throw it into frefti cold water, and to feparate
from it the lead that may be revived. The fame cru¬
cible fnould not be ufed a fecond time, becaufe the
glafs of lead is apt to penetrate it in fuch a manner as
to run the rifle of lofing the contents. One muft alfo
be careful to cover the crucible well, to prevent any
coals getting into it, which would reduce the calx of
lead, and fpoil the compofition.
z. Take two parts and a half of white cerufs, one
part of prepared flints, half a part of fait of tartar,
and a quarter part of calcined borax: melt the mixture
in a Heflian crucible, and then pour it into cold water;
it is then to be melted again, and wafhed a fecond
and a third time, the fame precautions being obferved
as for the firft bafe.
3. Take two parts minium, one part lock-cryftal,
half a part of nitre, and as much fait of tartar: this
mixture being melted, muft be treated as the former.
4. Take three parts of calcined borax, one part of
prepared rock- cryftal, and one part of fait of tartar; thefe
being well mixed and melted together, muft be poured
into warm water: the water being decanted and the
mafs dried, an equal quantity of minium muft be added
to it; it is then to be melted and waffled feveral times
as direfted above.
5. That called by our author the Mayence bafe, and
w'hich he confiders as one of the fineft cryftalline com-
pofitions hitherto known, is thus compofed : Take
three parts of fixed alkali of tartar, one part of rock-
cryftal or flint pulverized : the mixture to be well
baked together, and then left to cool. It is afterwards
poured into a crucible of hot water to diflblve the
fritt; the folution of the fritt is then received into a
ftone-ware pan, and aquafortis added gradually to the
folution till it no longer effervefees : this water being
decanted, the fritt muft be waffled in warm water till it
has no longer any tafte : the fritt is then dried, and
mixed with one part and a half of fine cerufs or white
lead in fcales; and this mixture muft be well levigated
with a little diftilled water. To one part and a half
of this powder dried add an ounce of calcined borax :
let the whole be well mixed in a marble mortar, then
melted and poured into cold water as the other bafes al¬
ready deferibed. Thefe fufions and lotions having been
repeated, and the mixture dried and powdered, a
12th part of nitre muft be added to it, and then melt¬
ed for the laft time; when a very fine cryftal will be
found in the crucible.
6. As a compofition for furnifhing very fine white
ftones : Take eight ounces of cerufs, three ounces of
rock cryftal pulverized, two ounces of borax finely
powdered, and half a grain of manganefe; having
melted and waffled this mixture in the manner diredted
above, it will produce a very fine white cryftal.
II. Of the Colour*. The calces of metals, as already
obferved, are the fubftances employed to colour fadii-
tious gems; and on the preparation of thefe calces
depends the vividnefs of their colours.
- a. From Cold.'] To obtain the mineral purple
known by the name of precipitate of Cafms, M. Fonta¬
nieu employs the following different proceffes.
1. Diffolve fome pure gold in aqua regia, prepared
with three parts of precipitated nitrous acid and one
part of marine acid ; and to haften the diflblution, the
matrafs fhould be placed in a fand-bath. Into this
folution pour a folution of tin in aqua regia. The
mixture becomes turbid, and the gold is precipitated
with a portion of the tin, in the form of a reddifh pow¬
der ; which, after being wafhed and dried, is called
precipitate of Cafius.—The aqua regia employed to
diflblve the tin is compofed of five parts nitrous
acid and one part of marine acid : to eight oun¬
ces of this aqua regia are added fixteen ounces of
diftilled