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Ancient Bronze Tarnishing/Oxidizing


#1

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


#2

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

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


#4

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.


#5

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