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Pink tint after soldering 14k gold


#1

I searched the archives for this one, and it appears to be a shared
problem. I also asked around some industry people here, and there
seems to be a general degree of confusion about it.

Here’s the scene:

I casted a ring out of 14k gold and had no issues with it. I fluxed
with borax during casting, and everything came out fine. No cupric
oxide, no cuprous oxide. I did the melt with a propane torch (no O2).

After finishing, when I soldered on the prongs, a 15 second process,
the ring went pink all over. Except for the prongs, which were 18k
gold.

This happened to me twice. Both times I was using Handy flux, and
used it liberally, and the ring was entirely covered.

I’ve had firescale before when I was making ingots (I don’t have all
the right equipment, so sometimes I do overheat). This I’ve removed
with a 50/50 nitric acid / water bath. 3 second dip followed by steel
wool removes it no problem.

So between the Sparex and the nitric, anything on the item that
wasn’t supposed to be there would be removed.

This pink just will not come off. The tone is almost like it’s been
copper plated with a very thin layer of copper. I’ve used clean
pickle, so it’s not a contamination-by-iron effect, and it was pink
almost immediately after I hit it with the torch for soldering.

The best I could do was polishing and buffing, which has removed
about 70% of this stuff. The ring still has a pink tint to it. In
yellow light, you can’t see it (you could before) but in daylight it’s
obviously pink.

In the archives, someone said to add peroxide to the pickle bath.
When I tried to buy the peroxide one supplier refused to sell because
I said I was going to add to the pickle (they said it wouldn’t do
anything).

They offered to bomb the ring for me as a favor, and told me to go
home and give it a finished polish, mount the stone, they would bomb
and that would be the last step. I did this, brought it back in, and
they then told me that after bombing I would have to polish and that
bombing only cleans the ring. I would have to polish again, at which
point the color would be uneven.

My understanding of bombing was that it removes everything that’s not
gold from the surface of the ring, leaving behind a 24k finish. I
said this to them, and they told me that I was wrong. I don’t know, am
I? I only read about bombing once a bit ago so I could be wrong.

Anyway, it bothered me that the story changed overnight, so here I am
with my finished, mounted polished pink ring, scratching my head.

I did go to another supplier and bought peroxide and added it to the
pickle. It did seem to help remove some, but without knowing what
kind of concentrations I should be looking at, or if it really
"officially" works, I can’t say for sure.

For the record, here is what I’ve tried:

(1) polishing (2) sparex (3) sparex with H2O2

result: between the three, removed about 70% of the tint

(4) nitric dip

result: nothing

What I can’t try because I don’t have the equipment:

(1) electroplating (2) electrostripping

Question: what the heck is this pink tint? It looks like copper, and
if it is, what mechanism is bringing it to the surface?

Anyone know a sure fire way of getting rid of it?

Thanks!

  • darcy

#2

Hi Darcy; Let me see if I can help you with this. Firstly, make sure
you are casting at the optimum temperature. For vacumn assist, I
take the flask out at 900 deg. F. (cooler for heavy articles), have
the metal already melted, and pour as soon as I reach maximum vacumn.
For centrifical casting, I’m taking the mold out at around 750 deg
F. (again, cooler for heavy stuff), the metal is already molten and
ready to release. This will minimize the effect of surface oxidation
on the casting. Pickle the casting well before you work on it. Some
people will electrostrip beforehand. Electrostripping would only
require you to have the fairly inexpensive electrostripping solution,
a stainless steel beaker, and a source of D.C. current. As some
have suggested, an auto battery charger, but you could look into a
door-bell transformer. It’s not perticularly dangerous, although the
solution is caustic. Bombing is an old trade trick, and I did plenty
of it in my time. It does strip everything but the 24K gold from the
surface of the article. It’s also a fairly dangerous operation,
involving potassium cyanide and industrial strength hydrogen peroxide
(17%). And it produces a large cloud of steam which carries a mist
of cyanide solution with it. As for soldering operations, with gold,
I coat the entire article with boric acid disolved in alcohol, having
made a saturate solution. Heat this coating until you get it to
glaze slightly. For flux, I prefer the self-pickling “batterns” flux
for gold work. Handy flux will almost guarantee you’ll have a lot of
oxides; although it works well at the solder joint, the piece will
need to stay hotter and for longer, and it certainly will not
substitute for boric acid/alcohol dipping. Have some ventilation
with the self pickling fluxes, the flourides in them are not good to
breath. As for the pink coating, it’s obviously a layer of copper
rich gold, the other alloys having been leached out in the soldering
process. If it were copper alone, the nitric would remove it easily.
You’l electrostripping. Seems to me, you’re not going deep enough
into the surface to remove it. As for the peroxide in the pickle,
this will improve the action of the pickle, and your supplier, like
the guys who offered to bomb, doesn’t know the facts. But the
peroxide pickle can only do so much. It will work for a superficial
copper coating, something heavier, it just can’t get deep enough.
Here’s a last resort. Assuming you haven’t set any stones in the
piece, you can heat it (without boric acid/alcohol) just enough that
when you quench it in the 50/50 acid-water solution, it will hiss
slightly. wear some safety glasses when you do this and it
wouldn’t hurt to have an apron on and some rubber gloves. If you
heat and quench in the the acid solution, after a half dozen times,
you’ll accomplish the same effect as bombing. It’s sometimes
referred to as “depletion gilding”. Good luck.

David L. Huffman