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Pickle and Flux


#1
 Pickle used by jewellers after heating their work is always
acid, and Sparex is a mild acid.  It's use is mainly to remove
flux which is borax based and is an alkali. Flux would
otherwise need abrading to remove it.  The pickle also cleans
up other stains, but NOT the dreaded firestain.  Only removal
by nitric acid, cyanide de-plating or abrasion will remove
that.

Hi John,

When I am too lazy to pickle my pieces at an intermediate stage
(I have to heat a copper ladle with pickle with a torch where
I’m employed), I just use the hot ultrasonic. 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, as this (oxidized
from the soldering heat) is dissolved in the pickle (gets
coppersulfate with sulphuric acid). This results in a pale
colour with a slightly green tint, as what you see is the colour
of a gold-silver alloy without any copper. To restore the
original colour, yes, you have to mechanical abrade the surface
layer, electrostrip, or use nitric (to dissolve copper and
silver).

What fluxes do you use? Mine are all neutral (I tested), being
salts of boric acid, some with fluoride and phosphates, the
traditional being borax, Na2B4O7.

For melting I use a mixture of 2 parts sodium carbonate, 2 parts
potassium carbonate and 1 part borax.

regards, Markus


#2

Firestain ( Firescale, or whatever one calls it . . .) usually
appears on Sterling Silver . . . it looks like grey bloches . . .
one can polish, polish and polish and these bloches will still
not vanish! It’s a pain!!! I usually file, sand and then polish
to try and avoid it.


#3
   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
    / / \ \
   / (___) \
  (_________)

#4

Firestain ( Firescale, or whatever one calls it . . .) usually
appears on Sterling Silver . . . it looks like grey bloches . . .
one can polish, polish and polish and these bloches will still
not vanish! It’s a pain!!!

Hi,

That most probably is subsurface oxidation. Silver can dissolve
quite a lot of oxigen when heated, so does copper. This way the
oxigen is able to oxidize the copper deep inside the workpiece,
and there is no way to get rid of this oxidation, pickle will
only act on the surface.

To prevent, it is best to cover the workpiece with a white paste
flux, boric acid is not very good, as it has too high a working
range for silver solders or easy gold solders. With boric acid
which turns into borium trioxide at 500 C, metal oxides are
changed to metaborates, and these will not move away from the
place they form, to give access for new boriumtrioxide, below
900 C. You’d have to replace the boric acid with flux at the
joint.

Silversmiths who make vessels use borax junk to make a thick
paste.

regards, Markus


#5

What is Sparex. I usually use Sulphuric acid as a pickle. It
usually goes green after a while with the copper from the
silver. By the way only add Sulphuric acid to water not the
other way round. Very dangerous. Richard W UK


#6

Just a few thoughts regarding firestain. It is quite common in
silver. Most people use 92.5% pure silver which contains some
copper. The purer grades are much mre expensive and often too
soft to use. As John said firestain is caused by copper in the
silver and creates a grey film over the surface of the siver. It
can usually be polished out with Tripoli compound and rouge. The
rouge used with a swansdown mop will show up any firestain that
is left. You can electroplate the silver to cover the firestain.
You can also remove it with a reverse electroplating process. I
have always found Johnson Matthey fluxes to be the best. They
are quite expensive, butdo the job very well. They also do
something to prevent firestain. I don’t recall the name, but by
the time I get your reply I should have found the name. Anyone
else know it? Richard W UK


#7
 What is Sparex. I usually use Sulphuric acid as a pickle. It
usually goes green after a while with the copper from the
silver. By the way only add Sulphuric acid to water not the
other way round. Very dangerous.

Sparex is a BRAND name . . . I believe the crystals are the same
chemical (Sulphuric acid.)


#8
Sparex is a BRAND name . . . I believe the crystals are the same
chemical (Sulphuric acid.)

Sorry to butt in but, the active ingredient is Sodium Bisulfate.
And it is cheaper to buy the dry acid for adjusting your
swimming pool or hot tub, it is the same chemical. Check the
labels.

Joe B.


#9
Sorry to butt in but, the active ingredient is Sodium
Bisulfate. And it is cheaper to buy the dry acid for adjusting
your swimming pool or hot tub, it is the same chemical.  Check
the labels.

I knew that, but didn’t know what the technical name for that is
(the hot tub/swimming pool stuff!)