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Ancient Bronze Tarnishing/Oxidizing

Hello
I have ancient bronze casting grain from Rio Grande. 706-051
I can leave the grains sitting out and they never change colour, staying the bright bronze colour.
When I use the grains as elements on my sterling silver, I solder them on and then pickle. Of course the bronze develops a copper film that I then remove by sanding and polishing.
As this finished jewellery is exposed to air or placed in hot water even, it oxidizes and turns a deep copper colour that can be removed with a polishing cloth.
I am wondering why this happens when it doesn’t happen to the casting grain?
And any suggestions for prevention?
I am not interested in using a wax or lacquer coating.
Thanks in advance!
Melody Armstrong

Hi Melody

I have ancient bronze casting grain from Rio Grande. 706-051
I can leave the grains sitting out and they never change colour, staying the bright bronze colour.
When I use the grains as elements on my sterling silver,

The grains change color because the bronze alloy is less noble than the sterling silver it is connected to. A galvanic cell is set up where the bronze is more likely to corrode/oxidize than the silver. I don’t know how to prevent this other than coating.

Karen

3 Likes

Thanks Karen!
That is very interesting, I have not heard of this Galvanic Corrosion before.

It is actually a big problem for electricians when they find copper and aluminum wiring mixed together in humid environments. Over time, the galvanic corrosion will cause problems with the connections.

It just dawned on me that the strange annoying occurrence when a tiny piece of aluminum foil or even metalized chocolate wrapping hits my metal fillings I’m experiencing a galvanic reaction! Not fun