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Vitrum Annulare, Pastes

Vitrum Annulare, Pastes Page of 453 Vitrum Annulare, Pastes Text size:minus plus Restore normal size   Mail page  Print this page
VITRUM ANNULARE.
345
be stopped up with wax before taking the cast, otherwise it tears away the soft matrix when withdrawn ; and on this account camei in paste are never so satisfactory as intagli.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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