alumina,
which crystallises out as one of the ahove three varieties according to
the colouring medium added; this may be a trace of potassium dichromate
or chromium fluoride, the colour produced depending upon the amount
used, a mere trace of the latter giving the Ruby, a little more the
Sapphire, and still more producing green Corundum.
J.
Morozewiez employed somewhat similar methods, and obtained Spinels
(oxide of magnesium and aluminium) and crystals of Corundum 1*5 m.m. in
diameter, the colour, however, not being due to chromium, but iron.
For good Sapphire blues the writer has found a trace of cobaltic nitrate to be the best.
From
a commercial point of view, the most successful experiments ever
carried out were by Fremy and Verneuil in 1878, and later in 1890, when
they produced Rubies of such good quality and size that they were sold
for watchmakers' purposes. They fused in a clay crucible, at a
temperature of about 1,500° C. a mixture of alumina and barium fluoride
containing a trace of potassium di-chromate. The fusion was kept molten
for a week, and then cooled down very slowly, small crystals of
Ruby—and, if cobalt oxide was substituted for the di-chromate—Sapphires
separating out in the mass.
In
1885 Rubies were put on the market of sufficient size and quality to be
mounted, cut or uncut, for jewellery. These stones, upon examination
with a lens, were seen to have been artificially made, as they
contained many minute bubbles and other signs of strain. They were
produced in Geneva, and received the name of "Geneva Rubies," but the
method of their production was a trade secret, and, as far as the
writer is aware, is still so.
