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172 A POPULAR TREATISE ON GEMS
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The jewellers' wax used for mounting gems is made of three parts rosin, one part beeswax, and four parts fine brickdust.
CLEANING THE GEMS.
The following composition I have found to be the best for thoroughly cleaning gems, particularly when set: Take one part flowers of sulphur and two parts of rotten-stone or bone-ashes, which, when mixed, is used by rubbing it on a piece of buckskin, and with that and a stiff hair-brush, alternately rubbing the gems, finishing with a softer skin or cloth to remove the dust.
IMITATIONS OF GEMS.
Pliny mentions the imitation of jewels by glass fluxes, and it is sufficiently proved that the ancients were far advanced in this art. The Egyptian mummies were provided with glass buttons of green and blue color, and during the reign of the Roman empire, colored glass was very general; and we find antique cameos carved in various colored glass, representing the onyx; likewise colored glass cemented with real onyx; but they never attained such perfection in their art as to set at defiance the skill of the artist and jeweller to distinguish between the genuine and spurious ones. The imitation of gems may be divided into three classes:
A. The Pastes. The basis of these imitations is a fine, pure, and white glass composition, called strass, after its inventor, Strass of Strasburgh, in the seventeenth century, who first conceived the importance of imitating the real gems as respects their hardness, specific gravity, and re-
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