Can you please tell me how synthetic opals are made?
I’ll quote fro Kurt Nassau’s “Gems Made by Man”, 1980.
(paragraphs slightly trimmed as appropriate…
"Three steps are required to produce synthetic opal. First, it
is necessary to form silica spheres, all of exactly the same
size. Second, these spheres must be orderd into a close-packed
arrangement. In the last and most difficult step, it necessary
to fill the spaces between the spheres and compact the opal so
that it becomes hard enough to be usable.
"The first step is the fairly well known preparation of
momdisperse silica sheres starting with some organic silicon
compound such as tetraethyl orthosilicate. This may be dispersed
in fine droplets in an alcohol-water mixture and converted by the
addition of ammonia or some other mild alkali into silica
spheres, which will then contain a little water…
"The second step is the easiest one, because it merely requires
permitting the silica spheres to settle from a dispersion in
water of controlled acidity. This step may take over a year…
"The product of the second step is fragile… … Heating at
not too high a temperature can produce some sintering. Pressure
may also be used, but no in a normal hydraulic press, which would
merely destroy the ordiring of the spheres. In hydrostatic
pressing, the soft opal might be… [compressed] uniformly
without distorting it. It is possible that some additional
silica is added at this stage t fill up the pores between the
spheres, possibly in the form of a silica gel…
From the above you get a general idea of how it’s done. It’s
obvious as well that certain parts of the process are
proprietary, since Dr. Nassau is offering possibilites, not known
and observed methods, especially for the third step… At the
time this book was written, Gilson synthetic opal was the only
such product on the market.