When using wheel weights or linotype made from real type, it's fairly understandable that you will get crud floating to the top once fluxed. And it keeps coming up during a casting session. There isn't anything particularly "pure" about them.
I bought some 16-1 from a well known supplier, and you can get a certification of it if you pay for it. My question is, I still get the grey crud floating up from what I assumed was controlled components. And I started this in a new-to-casting pot which I scrubbed completely clean.
I also thought the grey stuff was antimony, but there isn't supposed to be any in there.