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VITRUM ANNULARE.
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345
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be
stopped up with wax before taking the cast, otherwise it tears away the
soft matrix when withdrawn ; and on this account camei in paste are
never so satisfactory as intagli. If it be wished to imitate a gem full
of internal flaws, like the Carbuncle or the Emerald, the effect is
produced by omitting the annealing, and throwing the paste, still hot,
into cold water.
The
process followed by the ancients was doubtless in principle the same,
except that it is evident their moulds were taken in a much coarser
material (probably in terra-cotta, on which point more will be said in
another chapter), for antique pastes have a much rougher surface than
the modern, and are full of air-bubbles. One singular property,
however, distinguishes the ancient : they are much harder than
window-glass, and will scratch it as readily as does a splinter of
flint ; whereas all modern glass, if coloured, is softer than the
transparent kind. This is due to its different composition, for at
present the German glass, made entirely with soda, is much harder (even
resisting the file) than the English, into the composition of which
enters a large proportion of lead. De Boot tells us that in his time
(1609) rock-crystal pounded was used in the celebrated glass-houses of
Venice in making their best articles ; and also generally by the
Italians in the manufacture of false gems (an art they were then famous
for), for which he, sapient old alchemist as he was, gives many curious
and valuable recipes. In the present day the trade is transferred to
Paris ; and it will be seen by a reference to Barbot's article on Strass, how
important an ingredient crystal is still regarded. Crystal in fact
being the purest form of silex, is very superior to the silicious sand
now employed for the sake of economy in the manufacture. The use of
this component can be traced back to the earliest times, for according
to Pliny (xxxvi. 66), " some assert that in India glass is made
out of pounded crystal, and in consequence no other sort is comparable
to the Indian." The same fact seems to be implied in the vague
expressions of Theophrastus (49) : " For if glass be made, as is
reported, out of the hyalites, this substance also is the product of condensation."
Besides this superior hardness, another supposed criterion of an antique paste is the beautiful iridescence, often coating its
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