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.