Granulation puzzle

Apologies if this issue has been aired previously.
I melted 2 washers diameter 3/4" of new .3mm sterling sheet for granulation . One was a knobbly copper coloured oval with a slight patch of silver the other was a normal silver coloured ball. They stayed the same even after a time in the new/clean silver safety pickle pot.
Was this as someone suggested the result of heating in oxodising conditions? This does not seem chemically correct. I do melt and granulate fairly often and have never experienced this copper surface before. I use both charcoal and magnesite??? blocks.
I repeated the experiment with other off cuts from the same sheet and had the same mixed reults.Even filing the surface did not reveal shiny silver underneath.
Please can anyone tell me what is going on here.
Thank you

If silver turns copper coloured in pickle solution, it is usually as a result of contamination with iron in the pickle, or using steel tweezers, or wire.
That said, true granulation works better with a thin coating of copper on the balls, and can be polished back later. Dennis.

The copper was there immediately the melted silver cooled on the block without anything touching it. The surface showed only speckles of silver. I was using the fresh side of a charcoal block the first time this happened. Subsequent experiments were with my normal magnesia block.The lump looked like a thick copper sandwich with the thin silver top and bottom.It was not spherical as I should have expected.
I shall try to workout how to attach a picture which shows the problem.

Have you had previous success with that particular sheet, it is not unheard of to have dodgy Silver especially from places such as Ebay.

The results were mixed and the problems came from one end. The 925 came from a reputable UK source which we all know. Their technical team suggested the oxidising theory.
Oxides should have been black not copper. My guess is a bad ingot which when rolled out had patches of copper rich metal.
I only initially granulated 2 misshaped washers which I had cut out. The subsequent granulation experiment was from carefully numbered pieces from the edge of the sheet. I wonderd if there was contamination on my disc cutter, however that seems unlikely to have caused the large ammount of copper appearing on what had been the second washer to be cut out. It was not a huge piece of sheet and I have 2 usable discs from it so all is not lost.I am just puzzled.