What do you call firestain? If it is the copper oxides on
the surface of gold alloys from heating, these are very well
removed by the pickle, only the surface then becomes poor of
copper,...
G’day Markus; Firestain is indeed copper oxides, and I agree that
most is removed by pickle, but sometimes things don’t go right
(often, with me!) The job stays at (dull) red heat for too long and
the copper oxide penetration is deep. The surface looks OK after the
pickle, but polishing removes the thin layer of no-copper metal, and
the firestain is disclosed, and naughty words do follow. #@%*
I have little experience with gold, having worked it only about 4 or
5 times, and admittedly it was 9ct, but I managed to cope with the
stain OK.
I think I mentioned long ago that I am one of the (often despised)
hobbyists, though have been doing it for about 25 years, almost
entirely in sterling, (It’s cheap!) which is a real nasty for
firestain of course. I also mentioned that I don’t like to sell -
which is why I don’t do much with gold.
ME> What fluxes do you use? Mine are all neutral (I tested),
being
ME> salts of boric acid, some with fluoride and phosphates, the
ME> traditional being borax, Na2B4O7.
You bring up a difficult point, Mark because to a chemist the
words ‘basic’ or ‘acidic’ have slightly different meanings to
what most people understand. I used the word ‘alkaline’ instead
of basic - which is what I should have said. I don’t want to get
too tied up with technicalities here, but the element boron, B is
amphoteric which it means it can act as an acid or a
base, depending on the circumstances, and so is silicon, a
similar element (Si). Silica (SiO2) (sand) is technically an
acid, yet your tests will show neutrality; but it behaves like an
acid when very hot. Borax solutions are alkaline, and solid borax
is a base, but boric acid is very poorly ionised in solution and
only special tests show that it is acidic. But when the solid is
very hot it acts as a base. Thus a mixture of borax and boric
acid actually disolve many metallic oxides. In the days when I
did chemical analyses one of the tests for certain metals was to
mix a tiny bit of the unknown with borax, and heat it strongly in
a bunsen flame. The borax bead so made (on a platinum wire)
became coloured; thus copper could be red or green, manganese was
purple, nickel apple-green, and so on. Flux? I use a commercial
flux called ‘Easyflo’. Don’t know the composition, but it does
contain boric acid, borax, sodium fluoride, and other things.
(They don’t tell you!) I find it far superior to plain borax. I
also use Pripps Flux (make it up myself) to coat work before
heating as a preventative of firestain, which it does, (to a
certain extent) but as a straightforward flux I prefer Easiflo.
By the way, I use 10% sulphuric acid as a pickle as it doesn’t
need heating. And no, I don’t have lacy clothing as a result!
Sorry to be long-winded, but I’ve said before I tend to be a bit
pedagogic! Cheers, –
/\
/ / John Burgess,
/ /
/ //\ @John_Burgess2
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