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G'day; I wouldn't be too keen on fluxing and heating any stone. In
the very far and distant days when chemical analyses were done with
very little instrumentation, a series of strictly ordered tests were
performed on each unknown sample. Firstly the sample had to be got
into solution, and one method of doing this with insoluble 'unknowns'
was to heat with borax. One of the tests for certain metals was to
heat a tiny portion of the 'unknown' with borax in a loop of
platinum wire using oxidising and also reducing flames. The colour
of the borax bead was then carefully examined. For instance, small
amounts of a cobalt compound gave a deep blue. Manganese gave purple;
copper beads could be blue-green or red or black depending on an
oxidising or reducing flame being used. Iron; yellow-brown or
olive, and so on.
This of course indicates that some of the material has literally
dissolved in the borax. So is it not a danger that borax or
boron-containing compounds might etch the surface of gemstones? Very
hot and therefore liquid borax is a powerful solvent. Fluxes
containing fluorides will certainly etch quartz stones like agates,
amethysts and so on. It will also attack the corundums.
Another possibility is that a strongly adherent coating of a flux
might have a different coefficient of expansion to that of the
gemstone, and thus, when cooling might cause some crazing or cracking
of the gem..
I would be most reluctant to flux gemstones.
Just another possible side to the story. -- Cheers for now,
John Burgess; johnb AT ts.co.nz of Mapua Nelson NZ