Good advice. One question though, are you not covering rubies and sapphires with any thing at all? I've heard so many mixed things here. It's a consensus that soldering flux will etch, but if boric acid will as well, then what do you cover small corundums with? >Thanks, Andy
Boric Acid is less active than the usual mixed up fluxes, so it
etches corrundum only slowly. for quick work, a little boric
probably won’t hurt the stones. But be quick, and get the stones no
hotter than you need to. The basic chemistry here is that corrundum
is a metallic oxide, which is what fluxes dissolve happily. Since
it’s already an oxide, an oxidizing atmosphere doesn’t easily hurt
it. A reducing atmosphere, however, can cause the surface of the
corrundum to degrade, as it tries to reduce the oxide to the metal.
Leaves a sort of irridescent look which is not especially desirable.
Better than actual etching of the stone, though, as it’s a thin
layer, more easily polished off. (The aluminum oxide based platinum
rouge compounds will take off such a layer, with only slight
softening of facet edges. Not the best solution, but with small
cheapie sapphires, practical enough) If you keep the flux only on
the metal, and simply don’t use boric acid otherwise, and use a
neutral to very slightly oxidizing flame for the soldering operation,
the stone will come out fine, at least as regards this subject. Other
forms of heat damage, of course, are another issue, not affected by
the presence or absence of flux, etc. The metal, of course, will
oxidize. But that’s what pickle is for… Easier to buff off a bit
of firescale on your gold, than to have to recut the sapphires.
Peter Rowe