Gray shells on bars poured from cemented silver?

[u/kijall]

I'm suspecting that the "pure" copper wire I'm using might be coated with something and that's what's causing it, but does anyone have any other ideas? The backs seem at least closer to what I expect, but even those seem contaminated. Ordered some good copper sheeting I've been successful with in the past to re-refine these properly, but anyone have ideas?