Gems. 
—_——— 
Practical 
art of ma- 
king ficti- 
sious gems. 
~ ous branches of the fine arts, he confined hims 
128 
was born in Scotland, and practised modelling and sculp- 
ture in Dublin during three years preceding 1766. He 
then repaired to London,where, amidst the study of vari- 
princi- 
pally to the composition of coloured pastes and artificial 
gems. The former were first brought into repute by 
the London jewellers, employing them for ornamental. 
purposes ; the latter consisted ofa beautiful hard white 
enamel, which struck fire with steel, and was not affect- 
ed by heat... Tassie carried on an extensive traffic in 
these fictitious gems, which he sold from eighteen pence 
to two guineas, according ‘to the quality ; and the late 
empress of Russia, a liberal patron-of the arts, commis- 
sioned:a:complete set of his whole collection. This he 
executed ina masterly manner, imitating original gems 
where circumstances admitted of it, *where ‘other- 
wise the pastes were chiefly transparent; and cabinets 
containing them are now im the imperial -palace of 
Czarsko Celo. As public fashions are fluctuating, pastes 
began to ‘lose their value, and fictitious engraved gems 
are at present in little repute in Britain, though imita- 
tions of precious stones are still worn as personal orna-~ 
ments, 
The. practical part of the art of making fictitious 
gems, in so. far as relates to colour and consistence, is 
amply explained by M. Fontanieu, from whose work 
we:shall quote a few observations, as his principles have. 
received general approbation. M. Fontanieu’s primary 
object was the discovery of a colourless base, constitu- 
ting the elements of all gems, which, by the addi- 
tion of metallic oxides, would imitate their ive 
tinge ; and he found it might be obtained from diffe- 
rent siliceous substances, pure sand, flint, pebbles, or 
rock crystal. Any of these, such as crystal or pebbles, 
being pounded, are put into a crucibleand heated red 
hot ; the contents are emptied into a wooden bow! fill- 
ed with clean cold water, and being shaken from time 
to time, the lighter particles of extraneous matter es- 
cape and rise to the surface, while the rest, together 
with any metallic particles, remain below. The water 
is.then decanted, and the mass being dried and pound- 
ed, is sifted through a sieve of the finest silk, after 
which the powder is digested four or five hours in the 
muriatic acid, shaking the mixture frequently. Having 
poured off the acid, a pure vitrifiable earth remains, 
which, being frequently washed, is again dried and 
sifted when it becomes fit for use. From the earth 
now obtained, M. Fontanieu formed six different bases, 
of which the fifth seems to be that which, in respect of 
quality, is preferred by himself; and his proportions are 
all calculated in parts of eight ounces each. The first 
base is formed by 20 ounces of lead in scales, 12 ounces 
of prepared rock crystal or flint, 4 ounces of nitre, 4 
ounces of borax, and 2 ounces of arsenic, which being 
well pulverized: and mixed, are melted in a Hessian 
crucible, and poured into cold water. The mass is melt- 
ed again a second and a third time, always in a new 
erucible, -and after each melting poured into cold wa- 
ter as at first, taking care to separate the lead that may 
be revived. This forms the first base. The second is 
obtained from a mixture of 20 ounces of white ceruse, 8 
ounces of prepared flint, 4 ounces of salt of tartar, and 
2 ounces of calcined borax, all melted ina Hessian cru- 
cible, and poured into cold water. The melting must 
be repeated, and the mass washed a second and third 
tinie with the same precautions as before. A com- 
pound of 16 ounces of minium, 8 ounces of erystal, 4 
ounces of nitre, and 4 ounces of salt of tartar, consti- 
stutes the third base, being treated as in the precedin, 
examples, The fourth is formed by 8 ounces of call 
GEMS. 
crystal, 24 ounces of calcined borax, 8 ounces salt of 
tartar mixed and melted together, and poured inte 
warm water. The mass being dried,:an equal quanti- 
ty of minium is to be added; and the whole repeatedly 
melted and washed as before. Fifthly, a base judged by 
the author as one of the finest ine compositions, ° 
and called by him the Mayence base, is procured in the’ 
following manner: Eight ounces of rock crystal, or: 
flint pulverised, is baked with 24 ounces of salt 
of tartar, and the mixture left to cool. It is afterwards: 
poured into a crucible of hot water to dissolve the frit, 
and the solution is received into a stone ware and 
the nitrous acid added until: it no longer effervesces. 
The water being decanted, the frit must be washed im 
warm water until it ceases to have any taste ; and the 
frit being then dried and mixed with 12 ounces:of fine 
ceruse, or white lead in scales, the mixture is to be well’ 
levigated with a little distilled water. An ounce.of cal+ 
cined borax is now -to be added to twelve ounces of: 
this powder when dried, the whole well mixed in a 
marble mortar, then melted and poured into cold water, 
in the same manner as in the rest of the bases a’ 
described. After repeating these fusions and lotions,: 
and the mixture being dried and. pulverised,. five 
drachms of nitre are to be added, and the whole Son 
melted for the last time, a mass of crystal will be found - 
in the crucible with a beautiful lustre. | Lastly, a very 
fine white crystal may be obtained from eight ounces) 
of ceruse, two ounces of borax finely powdered, half.a. 
grain of manganese, and three ounces of rock crystal 
treated as above. vessanl er 
The colour of artificial gems is obtained from me-’ 
tallic oxides, a fact that must have been almost coeval 
with the discovery of vitrification; but the opinion of. 
chemists has differed as to their proportions and,com- 
binations. The diamond being colourless, is. imitated 
simply by the Mayence base ; and M. Fontanieu gives» 
ig =r for making all other fictitious gems, of aie 
the following are examples. | The oriental topaz —— 
pared by adding five drachms of antimony to ur 
24 ounces of the first and third base: the amethyst, 
by taking 24 ounces of the Mayence base, to which are. 
added for colour, four drachms of manganese, he 
ed by being exposed to a red heat, and quen in 
distilled vinegar ; then dried and powdered, and passed 
through a sill sieve, and also four grains of precipitate ~ 
of cassias. The hyacinth is obtained from a base. of. 
prepared rock crystal, and two drachms 48 grains ad-’ 
ded for colour. ‘T’o imitate the beryl, 96 grains:of an-" 
timony;, and four grains of the. calx of cobalt, are ad«, 
ded to 24 ounces of the third base. Precautions, how=" 
ever, are necessary in preparing the oxides; and these 
the author has detailed at length in his original work, 
of which there is an abstract in the Journal de Physique, 
1778. A fictitious turquoise, from a composition by 
M. Sauviac, has pd appeared in France, which is a 
perfect imitation of the original, and the colours pene- 
trate the substance of the mass. It is now ascertained 
that the genuine turquoise is not what may be called an 
original product of nature, but the petrified bone of 
an animal. The same artist, M. Sauviac, has likewise 4 
made fictitious malachite. onmgta 
The quality of artificial gems consists in their abso=. 
late uniformity of texture, , compactness, hardness,and: * 
lustre ; and it is surprising to. what perfection repeated: 
practice has brought the art. Fictitious gems may be: 
detected from their softness, which always yields to the _ 
file, and from their small specific gravity. a Pe 
The artist having chosen his natural gem, it is put. 
into the hands of the lapidary to give it’ surface and. 
* 
