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Crystallus, Rock-crystal

Crystallus, Rock-crystal Page of 384 Crystallus, Rock-crystal Text size:minus plus Restore normal size   Mail page  Print this page
CRYSTALLUS.
113
comes an imitation of the Emerald. Crystal may also be coloured by the dry method, by placing the pieces upon orpiment and arsenic mixed together in a crucible, and so exposing them to heat. The cold method consists in steeping the Crystal in oil of turpentine saturated with verdigris, or spirits of wine holding dragon's blood or other coloured resins in solution, the depth of tint produced being proportioned to the time of steeping. Such false Eubies are still known to the French lapidaries according to Barbot by the name of Rubaces. In London they figure as " Rubies of Ancona." But the substance of the Crystal is not really coloured ; it is merely the dye, which, in­sinuating itself into the innumerable minute fissures pro­duced by the sudden cooling, that apparently stains the entire mass ; and the great art in the process is so to regulate the heat that these fissures be not too apparent upon the surface of the imitative gem.
It is, however, not unlikely that the ancients employed also the more simple method now so much in use, and which produces most of the Carbuncles seen in the London shops, viz., after cutting the Crystal into shape, then to paint the reverse with the required colour, and so to set it with a backing. The fact that anciently gems were for the most part set in such a manner would greatly favour the execution of this fraud, to baffle which Pliny expressly notices that the better· kinds of Jaspis and Chrysolithus were mounted à jour, " funda perspicua." Although the Roman jewellers made up the false Sardonyx of three layers by cementing together as many slices of different coloured stones, yet they do not seem to have been acquainted with Doublets, that favourite device with the modern trade, in which a thin slice of the true gem, improved by cementing a paste of the proper colour under­neath, assumes the appearance of a first-class stone in its
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