It looks like a natural patina caused by silver oxidation to me. Of course, all oxidation is a natural process so we can only speculate whether it was intentionally or unintentionally caused. The different colors are caused by the different thicknesses of the oxidation layer. The thinner layers have lighter colors, tending toward yellowish tints. As more oxidation occurs, the colors run the gamut and eventually get very dark, like the dark purple and black around the edges of your ingot. Your ingot is a little unusual in that it shows such a broad range of levels of oxidation. If I had to speculate, I'd guess that your ingot was in a fire and was stored in a way that allowed the highly corrosive (i.e., oxidizing) smoke to reach the edges much better than the center.
Posted 05/26/2014 07:00 am
