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OSTRACIAS,
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pharmacopoeia
as caustics. Of these the most ponderous, the "Placitis," was divided
into the "Onychitis," "externally azure, but spotted like the Onyx
inside ; " and the " Ostracias," " entirely black," and the most
powerful of all. It may therefore have been an agent in gem-engraving,
like vitriol at present.
Cadmia
(or Cadmea) is supposed to mean also Calamine-stone, from which one may
suspect that the Ostracias of the Greek gem-engravers was the
Marcasite, or Iron Pyrites, which, fractured, much resembles
Calamine-stone ; when roasted, turns to a rusty led (testacea), and
which has been used from time immemorial in Persia for polishing the
harder gems. This last is indeed the strongest of my reasons for their
identification. Ben Mansur states, "the Laal (Spinel) takes a polish
with difficulty, and for a long time they were unable to polish it,
until at last they brought it about by means of the Gold Marcasite
called Ebrendsche." De Laet, in 1647, states that the Spinel
and Balais could only be polished by means of the Pyrites. Modem
lapidaries use vitriol for the same purpose, the principle of both
processes being identical, vitriol being merely the extract of the Iron
Pyrites.
It
is indubitable that the Greeks derived all their processes of the
Glyptic art from Persia, whence by the way of the Ionian cities the use
of intagli had been introduced amongst them ; and whence, from Sardis
in particular, most of the gems they possessed were exported.
But
the use of Pyrites amongst the Romans in this art is not a mere matter
of conjecture. Heraclius, in his all but unintelligible doggrel, ' De
Artibus Romanoram,' written in the 7th century, actually speaks of
engraving upon glass with " the hard stone known by the name of Pyrites."
Nevertheless my conjecture that actual copper-slag was
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