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Vitrum Annulare, Pastes

Vitrum Annulare, Pastes Page of 384 Vitrum Annulare, Pastes Text size:minus plus Restore normal size   Mail page  Print this page
324                 NATURAL HISTORY OF GEMS.
means of an iron spatula, coated with French chalk in order to prevent adhesion. It is then removed from the furnace, and annealed or suffered to cool gradually at its mouth, when the glass, after being cleaned from the tripoli, will be found to have taken a wonderfully sharp impression of the stamp, but in reverse, whether the prototype be in relievo or incavo. When a cameo is the model, all the undercutting must be stopped up with wax before taking the cast, otherwise it tears away the soft matrix when withdrawn ; and on this account camei in paste are never so satisfactory as intagli.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 he 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 terra­cotta, on which point more will bo said in its due place), for antique pastes have a much rougher surface than the modern, and are full of air-bubbles. One singular property, however, distinguishes the ancient manufacture : 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 white kind. This was due to their 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 (1600) 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
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