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72
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PRECIOUS STONES.
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abundance
of sulphurous acid in the air. It is also hastened very considerably by
the amount of lead present ; some pastes containing over 50 per cent.
The following percentage composition of a colourless sample will give
some idea of the constituents :—
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There
are many formulas, all much the same, varying only in the proportions
used of the above ingredients. Great care has to be taken in the fusion
to prevent frothing ; the mixture is fired for some time at such a
temperature that caking, or as it is technically called, "fritting,"
only takes place, after which the temperature is raised until fusion is
complete. The contents of the crucible are then slowly cooled,
precautions being taken to prevent vibration or any disturbance in the
mass likely to introduce air bubbles ; for this reason the glass is
kept molten for some time after complete fusion. This clear glass is
used for imitation diamonds.
Coloured
stones, such as Eubies, Amethysts, etc., are imitated by adding to the
above glass traces of metallic oxides or other suitable colouring
mediums.
The imitation ruby is made by fusing together—
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