Gold turning greenish in pickle

[purplepepper8]

Hi,

I made a piece in 18KY gold and the piece has got a greenish colour to it. I think it is caused by leaving it in the pickle (I use a citric acid pickle). From my understanding, it is caused by the copper leaching out of the surface of the metal into the solution. If I polish the surface, the original, redder colour is restored to the surface. However, as you can see from the photos, the issue with this piece is it is that it has lots of grooves and small openings where it is hard to polish.

Does anyone know how I can deal with this issue and how I can avoid it in the future?

Thanks
Aman

 

[Chujybear]

Green gold is an alloy that is basically a silver rich alloy. So if you wanted, you could mix pure silver (and no copper) for the far end of a green gold. It could be that there is more silver than copper in your alloy.
the other thing that could be at play is so with if called depletion gilding (I think). As you note, the copper alloy is burnt out of the surface by the pickle. That may be what you are seeing.
Mathis is essentially erased by polishing. Use a brush with compound, or rubber wheel brushes to get down to the bottom of your crevices.
a berry fine wire brush will be the most effective tool to homaganize your colour…. But be aware it has a slight matting effect. Wire brush can work in conjunction with rubber brushes for a high polished effect on a detailed piece.

 

[rweigel]

Hi Aman,

this is a perfectly normal behavior for a Gold-Copper-Silver alloy. When soldering or annealing it, only the copper oxidises, your workpiece turns black. The citric acid pickle (as well as the sulfuric acid, sodium bisulfate or the sulfamidic acid pickle) dissolves the copper oxid but leaves the noble metals gold and silver untouched. Hence the top layer of your piece lacks Copper. It might become a pure Gold - Silver alloy, which looks pale yellow when fresh and greenish once the Silver part has captured sone Sulfur from the air. In german books, it is called the „Sud“, I‘ve got no clue about the corresponding english / american expression.

Three ways of dealing with it:

-remove this layer with abrasive tools, as mentioned in Chujybear‘s post

-after pickling in your usual pickle pot, wash the piece in water and dip it into warm diluted nitric acid. This will dissolve the silver in the top layer, leaving a rich gold color.
Provided you can get nitric acid. It’s banned in Germany now as it can be used the make nitrated organic compounds with explosive properties. Same‘s true for sulfuric acid „to protect the population from accidents“ Truth is, to nitrate organics, you‘ll need both acids, to make the process effctive. I wait for the day when propane and oxygen bottles will be banned.

-avoid oxidation: cover your piece with boric acid dissolved in ethanol or, as some sources prefer, in hot water. It will protect the surface during soldering or annealing. Boric acid is NOT a flux, it actually retards solder flow. You‘ll need very active flux in the places where you want solder to flow.
Other oxidation barriers are Pripps Flux (boric acid and trisodium phosphate dissolved in water, cheap but cumbersome to use) or Firescoff (VERY expensive)

I hope this helps

Ralf

PS. I once made three-color rings from different 18K gold alloys, they needed three days after polishing to develop the natural colors of the three alloys.

 

[purplepepper8]

Thanks Chujybear, I will try with the brushes

Green gold is an alloy that is basically a silver rich alloy. So if you wanted, you could mix pure silver (and no copper) for the far end of a green gold. It could be that there is more silver than copper in your alloy.
the other thing that could be at play is so with if called depletion gilding (I think). As you note, the copper alloy is burnt out of the surface by the pickle. That may be what you are seeing.
Mathis is essentially erased by polishing. Use a brush with compound, or rubber wheel brushes to get down to the bottom of your crevices.
a berry fine wire brush will be the most effective tool to homaganize your colour…. But be aware it has a slight matting effect. Wire brush can work in conjunction with rubber brushes for a high polished effect on a detailed piece.

 

[purplepepper8]

Hi Aman,

this is a perfectly normal behavior for a Gold-Copper-Silver alloy. When soldering or annealing it, only the copper oxidises, your workpiece turns black. The citric acid pickle (as well as the sulfuric acid, sodium bisulfate or the sulfamidic acid pickle) dissolves the copper oxid but leaves the noble metals gold and silver untouched. Hence the top layer of your piece lacks Copper. It might become a pure Gold - Silver alloy, which looks pale yellow when fresh and greenish once the Silver part has captured sone Sulfur from the air. In german books, it is called the „Sud“, I‘ve got no clue about the corresponding english / american expression.

Three ways of dealing with it:

-remove this layer with abrasive tools, as mentioned in Chujybear‘s post

-after pickling in your usual pickle pot, wash the piece in water and dip it into warm diluted nitric acid. This will dissolve the silver in the top layer, leaving a rich gold color.
Provided you can get nitric acid. It’s banned in Germany now as it can be used the make nitrated organic compounds with explosive properties. Same‘s true for sulfuric acid „to protect the population from accidents“ Truth is, to nitrate organics, you‘ll need both acids, to make the process effctive. I wait for the day when propane and oxygen bottles will be banned.

-avoid oxidation: cover your piece with boric acid dissolved in ethanol or, as some sources prefer, in hot water. It will protect the surface during soldering or annealing. Boric acid is NOT a flux, it actually retards solder flow. You‘ll need very active flux in the places where you want solder to flow.
Other oxidation barriers are Pripps Flux (boric acid and trisodium phosphate dissolved in water, cheap but cumbersome to use) or Firescoff (VERY expensive)

I hope this helps

Ralf

PS. I once made three-color rings from different 18K gold alloys, they needed three days after polishing to develop the natural colors of the three alloys.

Thanks for your post ralf.

I will try the abrasives first.

Nitric acid might be even harder to get in India if its not available in Germany so will have to check on that.

I do cover my piece with boric acid and alcohol but I think leaving it in the pickle results in cleaning that up plus pulling the surface copper out of the alloy. I’ll try reducing the time the piece spends in the pickle in the future

Thanks
Aman

 

[Chujybear]

Muriatic might do the trick of nitric... some interesting information ralf... the whys behind what works!

 

[rweigel]

Nitric acid attacks silver and copper, but leaves the gold alone. Hence the german name „Gelbbrenne“ (yellowing pickle).

Muriatic acid might actually work, as it attacks Silver, but is soon stopped by an impermeable layer of silver chloride. As there is very little Silver to dissolve, it might just do the job.

Cheers

Ralf