Depletion gilding question

[ErichCDesigns1]

Greetings all,

Years ago I was given this formula for depletion gilding 18K gold:

2 parts potassium nitrate
1 part alum
1 part table salt
10 parts water

Heat until steaming (not boiling) and use gold wire to suspend piece until desired effect is reached.

I've experimented with this once or twice to no definitive effect and after searching through the forums, it would seem that this mix would just perhaps remove copper oxides from the piece, but still leave the silver behind creating a greener look rather than richer yellow. So far, I haven't been able to find any concoction for depletion gilding gold that doesn't involve nitric acid. So my question is this:

1. Does anyone have any experience with this recipe (or something similar) that can perhaps speak to its effectiveness or if I have perhaps misunderstood it in some fashion?

and

1. Does anyone otherwise have a recipe for not just removing copper oxides (which any pickle will do anyway), but also silver to raise the surface karat without using nitric acid? (Franky, I'm a bit nervous about messing around with that in my shop)

Thanks for any insight/help you can offer!

Erich

[WADEDESIGNS1]

I think it is a case of Gold plating the color you are looking for is easier and safer than depleting the surface to get the effect. So the process has gone out of use and out of mind. Sort of like cyanide bombing.

[ErichCDesigns1]

That's what I keep reading as well, but I thought I'd at least check it out since, with the exception of nitric, I've got all the materials and tools to depletion gild. Whereas with plating, I've got nothing (not the space for the equipment). Nonetheless, thanks a bunch for the reply!

[ErichCDesigns1]

So I just found this in archives, but I'm no chemist. Can anyone speak to the validity of this assertion?

"The potassium nitrate supplies nitrate ions while the
potassium aluminium sulphate (alum) supplies hydrogen ions, all in
aqueous solution. This is equivalent to having dilute nitric acid"

Thanks!

[janetb]

Here is a very old post of mine on this subject with one reply:

Has anyone done pickle-depletion on standard 9K gold? I was wondering if it would come out yellow---have a feeling it would have too much white metal in the alloy and would come out pale. The pre-Colombian depletion gilding of Central and South America got very yellow surfaces from very low carat gold, but they mostly used tombaga which was gold/copper with only trace elements of silver. But there were also greenish-white silver/gold/copper alloys with a high silver content which were much more common (especially in Peru) which were (properly) depletion gilded. I thought heating and pickling 9K many times might give a low cost gold look with minimal effort. Also, are there a lot of different 9K gold alloys on the market? It would be nice to be able to get a mostly-copper one for depletion-pickling.
Janet in Jerusalem

The only reply:

Christine_PymanJul '04
Hi janet, I have done pickle-depletion on standard 9K gold, not
intentionally - I was doing many soldering operations on the one
piece- and it came out very yellow. Looked good. I don't know what
the mix of other metals was, it was just what I bought from the
dealer, cheers, Christine

So 'Orchid experience' seems to show that you can get even 9K up to "very yellow".

Standard jewelers' sulfuric acid removes silver oxides as well as copper. The process of removing the base metals from non-pure silver by heating and pickling even has its own name: blanching ("whitening"). I often work on 14K and 18K pieces which undergo very many solderings (pickling formerly in sulfuric acid and presently in Sparex), and they always come out very yellow. So oxidizing the piece and pickling a few times should give you your desired results quickly, easily, and without requiring any special equipment....:-)....

[ErichCDesigns1]

Thanks Tom! It was my understanding that Sparex would only remove copper oxides. Are you saying that it also removes silver as well?

Thanks!
Erich

[TomArnold]

Hi Erich, I don't know much about the science of this. It is just a method passed on to me as a way of accomplishing the desired finish..Sorry I don't know the why of it. There are a lot of things like that. tom

[ErichCDesigns1]

No worries. Thank you nonetheless!

[TomArnold]

I forgot to mention that mercury was used for a long time in depletion gilding. When applied to the surface, it created an amalgam. This could be heated to remove other metals including silver; leaving only fine gold which then was burnished. This is how the work was down when I was a kid. Don't do it this way. It kills jewelers and we need them. tom

[ErichCDesigns1]

Ah yes, butter of gold. Given the fact that nitric acid makes me a bit nervous, mercury is completely out as an option. . . though I actually have a small jar of mercury that I've been tempted to experiment with. Still, I think I forego any potential options that could turn me into the next Mad Hatter.

Thanks again though!

[TomArnold]

If you have the patience, get the piece red hot and throw it in pickle (sparex), rinse and repeat this process about 10 times, hence the patience. It will have a great color. The pickle has to be fresh and very strong. I have done this many times. A cross come to mind that is worn daily and 10 years later it looks new. The surface has a satin look. Rub it up with baking soda to highlight it. Have fun. tom

[ErichCDesigns1]

Thanks for the replies guys. I appreciate it, but I think some further clarification is needed on my part. I understand depletion gilding in so far as the heating of the piece, consequent oxidizing of the copper, and then removal of oxides via pickle, followed by burnishing. The problem that I have is that I used an 18K green gold alloy (high silver content). Depleting the copper from the surface only brings out more green in the piece which means I need to find something that removes the silver as well which I'm attempting to do without having to use nitric acid.

I think that the techniques described above would work very well for about 95% of depletion gilding applications due to the higher copper content of standard alloys. Unfortunately though, I'm the other 5% that still has to deal with a significant amount of silver. I'm going to experiment some more today with the recipe I mentioned and see if I can't get further with it. I'll let you know what I discover.

Thanks again all!

[ErichCDesigns1]

I think this is probably the case for standard alloys as what is being removed is almost entirely copper oxides. I need to find something that will remove the silver as well since I'm working with a green gold alloy so the more I use this technique, the greener my pieces tend to get.

[janetb]

As I wrote above, sulfuric acid (also Sparex) does remove silver oxide. If it didn't, one would not be able to blanche sterling. So long as the silver in your green gold is oxidizing, you can deplete it (i.e., by removing the silver in the form of removing silver oxides). Make sure you are heating in an oxidizing atmosphere/flame. If you are using, say, the reducing part of a torch flame, the silver will not oxidize and consequently cannot be depleted.

[ErichCDesigns1]

Hi Janet,

Thanks for the reply. I think I may be misunderstanding something then which perhaps you could clear me up on. My understanding of "blanching sterling" is that you are whitening it by removing the copper that it was alloyed with to create the sterling. . .creating a layer of fine silver on the surface (also the idea behind reticulation). If Sparex removed the silver as well, it wouldn't blanche it, but just essentially remove the entire surface layer instead keeping the color the same. . .?

[ErichCDesigns1]

Okay, quick follow up as I had to pull out Brepohl's "Theory and Practice of Goldsmithing"

Regarding silver and oxidation:

"Silver reacts in the solid state with oxygen, forming a coat of silver oxide on the surface. Because the coating is only one molecule thick and therefore of no practical importance, we generally say that pure silver is resistant to oxygen in the air. On the other hand, when there is hydrogen sulfide in the air, silver develops a dark coating of silver sulfide."

So it would seem that the vast majority (if not the entirety) of the oxides being created during heating are copper oxides. . .which is why sterling can be whitened, silver can be reticulated, and why standard gold alloys can be gilded easier as they have a much higher percentage of copper in them.

[janetb]

Acid/Sparex only removes silver oxide--not silver. In this sense it would remove some of the silver on the surface, i.e.,the part that oxidized. If it didn't, silver oxides formed during your repeated oxidizing copper-removal-process would leave the piece gray/black (with silver oxides). So yes, blanching removes mostly copper, but it does also remove any silver which has oxidized, which is good vis-a-vis your gold depletion. SIlver doesn't oxidize any where near as easily/fast as copper, so you would have to 'work harder' if silver depletion is what you are after in an item with a lot of copper.

[ErichCDesigns1]

I understand what you are saying, but I guess I'm questioning whether or not what you are describing as silver oxide isn't just copper oxide based upon the aforementioned quote from Brepohl. I'm no chemist, but from what I've read so far, I can't find any information that confirms that silver will oxidize if simply exposed to heated oxygen in the form of a torch. We all know our pieces turn black when we heat them whether gold, silver, etc, but it's my understanding that nearly all (with the exception of perhaps a few trace amounts) of that is copper oxide. . . which is why our pickles turn green over time, they'll plate with copper once iron is introduced, etc. It would seem so far that all roads have led to it just being copper oxide unless you introduce chlorine or sulfur (with the exception of the one molecule thick layer which is too insignificant to account for anything).

Incidentally, because context can be lost in type, I'm NOT saying you're wrong. I'm saying, I don't understand and the above is why.

And as much as I hate to use Wikipedia as a source, there's this little bit in the Chemistry section:
"Silver does not react with air, even at red heat, and thus was considered by alchemists as a noble metal along with gold. Its reactivity is intermediate between that of copper (which forms copper(I) oxide when heated in air to red heat) and gold. Like copper, silver reacts with sulfur and its compounds; in their presence, silver tarnishes in air to form the black silver sulfide"

[janetb]

"Silver reacts in the solid state with oxygen, forming a coat of silver oxide on the surface"

I agree 100% with the Brepohl quote....:-).... Not sure where the problem is.

Doesn't it follow from the above quote that "silver will oxidize if simply exposed to heated oxygen in the form of a torch"? It in fact says that silver reacts with oxygen even without heat!

When we heat sterling silver or karat golds and they turn black, the vast majority of the black are copper oxides, then (much less) silver sulfides, then (even less) silver oxides.

The most significant point, however, may be that in actual fact I always find that repeated soldering and pickling on a karat gold makes the piece yellow, and if I want to get the original color of the alloy back, I have to polish it, i.e., remove the outer layer of pure-ish gold. And even with white golds, a lot of soldering and pickling make them yellower!

Mind you, green gold is a pretty unusual alloy, so it would help to know what its composition was. Green golds normally have a very high percentage of silver (15%-46% in my own personal alloys list--most alloys with over 25%), and thus it stands to reason that it would be difficult to remove all/most of it. Since the other ingredient often found in green golds is cadmium (4%-12.5%), you might want to look into what would remove that.

[ErichCDesigns1]

Well, the important part is the very next sentence: "Because the coating is only one molecule thick and therefore of no practical importance we generally say that pure silver is resistant to oxygen in the air."

It certainly makes sense that you would experience a yellowing of most karat gold since most of them have a much higher percentage of copper. With white golds there's also a higher amount of base metal (usually nickel) responsible for the color that can oxidize leaving the yellow mix behind. On the flipside, the alloy I use is 75% gold, 19% silver, 6% copper and the more I heat and pickle it, the greener it gets. Hmm. . . . I'm starting to think that I'm just going to have to bite the bullet and experiment with some nitric acid as even if there is some silver oxide forming that is being removed, it must be a significantly disproportionate ratio to the copper oxide. It never ceases to amaze me how often I pick the one odd thing to play with that is almost always the "exception to the rule"