63 PRECIOUS STONES.
little
unscrupulous work is done in passing off imitations as genuine, such
imitations being usually sold for what they are worth, and. nothing
more.
One
of the most celebrated producers of imitations of engraved gems—in
modern times—James Tassie, a native of Pollokshaws, near Glasgow,
settled in London towards the end of the eighteenth century, and there
copied no less than 1,500 rare engraved gems. He not only made copies,
but also original cameo portraits of his contemporaries, some of the
best examples of this class of work that we have.
The
method of making " paste " copies is comparatively simple. If the gem
to be copied is a cameo, a mould of the raised portion is first taken
in rotten stone or other fine coherent powder; a piece of glass is then
put into the mould and melted. After cooling slowly, the cast in glass
has simply to be trimmed, the back ground perfectly flat, and cemented
on to a suitable background of real stone. The process is slightly
different for copying intaglios, these gems having the subject cut or
sunk into the stone; therefore, to produce a copy, an impression has
to be taken first. Imitation intaglios or seals are, however, rare.
The glass, or, as it is technically known, " strass " or "paste" (Italian, pasta, dough)
used in the above work is, generally speaking, some opaque form, maybe
translucent, rarely transparent. The white opaque paste is made by
adding oxide of tin or bone-ash (phosphate of lime) to the ordinary
clear glass used for this class of work. These opaque slags being much
like Onyx, Agate, and other varieties of the so-called " Scotch
Pebble," great quantities
