Hamish- I was taught to never put boric on corundum when heating.
It will fuse to the stone and come off in the pickle leaving an
orange peel finish on the corundum. I saw it happen in a busy trade
shop in the past where there was a newbie who didn't know about
corundum and boric acid and the rest of us were too busy to catch
him before he did it.
Think about it this way: Flux, including boric acid and borax, work
to prevent metal from oxidizing, and to dissolve metal oxides that
do form. Corundum is aluminum oxide. The flux wants to dissolve that
metal oxide, so it is able to etch into the corundum when hot.
Corundum, already being an oxide, is happiest in an oxidizing
atmosphere. A reducing flame, without flux coating, can reduce the
metal (aluminum) oxide, leaving an irridescent looking surface film,
while the flux coatings that protect metals and diamonds from
oxidation, can damage the corundum.
Considerations in dealing with this are:
At lower temps, just barely enough to melt the boric acid fire coat,
If time at temp is kept to a minimum, the potential for damage is
also minimal. But still, don't do this to valuable stones. Slight
etching may not be visible as such, but might still affect the look
of the stone. Boric acid firecoat is not as active in damaging the
corundum as are soldering fluxes themselves (batterns, or white paste
fluxes) You can often work fairly safely with just the fire coat,
but actual soldering flux is much riskier. In general, this would be
in doing repair work on commercial quality (inexpensive) stones.
Often, these stones may already have some slight damage from paper
wear or normal wear and tear, so a little slight etching of surfaces
might be completely invisible. But recognize that in the end, you're
damaging, or risking damaging, a customers stones.
If you can work with a neutral to slightly oxidizing flame and
without fire coat or flux on the stone, then that will avoid etching
or damaging the corundum. But then you have to more carefully address
oxidiation of the metal, solder flow, etc, which will require careful
placement of fire coat (boric acid, etc.) and soldering flux on only
the metal where needed. Note, I don't know whether Firescoff will
also etch corundum. It might be that their formula doesn't cause this
risk, but I don't know. That factor might easily make that pricey
product worth keeping around...
And none of the above addresses the other risks to corundum when
heating: Heat treated stones in particular, but others too, can
change color from heating.
Inclusions in the stones can cause damage. Liquid filled inclusions
in particular are disasters waiting to happen, and are not always so
easy to see.
These can cause catastrophic breakage of a stone at temperatures far
lower than any etching from flux. The single costliest stone I ever
broke was such a sapphire. Nice large (around 8 carats) heart shaped
Ceylon sapphire. I didn't specifically heat the stone, just tried to
flow a little solder on part of the undergallery where a drill had
nicked a wire. Tiny flame, not pointed anywhere near the sapphire.
The boric acid protecting the diamonds around the sapphire never even
melted. But the sapphire got warm enough that the large, but hard to
see liquid filled inclusion caused the stone to split in half. My
boss was not happy. I've not taken that sort of risk again since.