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Depletion Gilding, and Tarnish Prevention in low karat metals
Current question:May 23, 2021
Q. Hello everyone!
I have a question about gold alloys and I need some help. I have a gold ring that I cast, it's 75% gold, 15% silver and 10% copper. I'm experimenting with a process called depletion gilding. This is a process where you heat the metal in a kiln, let the copper and silver oxidize, then bathe the metal in acid to remove the oxides, leaving behind a pure gold layer. You can repeat this process several times and you will eventually have a pure gold surface layer. I understand the pure gold layer will be extremely thin.
My process is as follows:
1: Heat in kiln to 1100 °F for 20 minutes.
2: Remove from kiln and allow the metal to cool.
3: Submerge in a warm bath of 3 parts water 1 part 70% nitric acid for 20 minutes.
4: Neutralize in a bath of sodium bicarbonate.
5: Repeat 6 or 7 times.
After the 3rd or 4th time I have a very beautiful yellow, pure gold surface that looks incredible. Here is my issue. No matter how many times I do this the fine gold, beautiful yellow layer disappears in a kiln soak. It reverts back to a green tint. I have a suspicion that I'm successfully removing the copper oxide but I'm leaving behind the silver. Im trying to figure out a way to create a pure gold layer on my ring that will not 'disappear' in the kiln. I understand that if I can successfully achieve this the fine gold layer won't be permanent, one swipe of sandpaper will remove it.
If your wondering why I'm going to this trouble it's because I'm applying glass enamel to this metal and I want the glass to fuse to pure gold and not a green tinted metal.
Any insight will be really helpful.
- Baltimore Maryland
^ Reply to Ryan_Plummer 's posting ^
A. Hi Ryan. I had heard of depletion gilding but didn't actually know the mechanics of how it is done. Thanks for the education and the report that it actually works :-) However, I suspect that the problem doesn't lie quite where you think, but in the fact that gold, copper, and silver all very readily inter-diffuse. At room temperature it's probably a very slow process, but at the kiln temperature for your glass enamel coating, and considering the extreme thinness of the pure gold surface layer, it probably happens fast. I wonder if here is another way of applying the glass enamel or something similar which does not involve kilning the ring?
Luck & Regards,
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey
Closely related Q&A's, oldest first:July 23, 2008
Q. We are a jewelry manufacturer who has developed a yellow low-karat metal that has significant amounts of copper and silver as well as other precious metals. We would like to significantly improve the tarnish-resistance, but are prohibited from using any lacquers or plating - and we have to maintain the yellow color of the metal.
Can you suggest any other treatments or additives that will hold up to wear, yet provide improved tarnish resistance?Jonathan Seidel
Product Engineer - New York, New York
July 24, 2008
A. Hi, Jonathan. I know exactly what you need, I just don't know the details of exactly how it's done: "surface enrichment", sometimes called "depletion gilding". This is a process whereby you dissolve away the copper and silver on the surface, leaving a surface condition of much higher purity gold. You can probably start at Wikipedia.
I once read a claim that no one could properly account for where most of the Aztec gold went after the Spanish conquest, and the author's explanation was that the majority of it was very low carat gold that had been surface enriched / depletion gilded to look like it was very pure gold; so when it was melted down there was far less gold than expected :-)
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 15, 2008
A. Ted's advice is very good -- you must have a solution that can dissolve copper and silver (or any other metal less noble than gold). You can use even Oxalic Acid [affil. link to info/product at Rockler] rich plant extracts (like Aztecs and Incas). According to one old German book you can use diluted nitric acid(1 part acid/1 part distilled water). Hope it helps and good luck!Goran Budija
- Zagreb, Croatia