What the Heck is this??

[km101]

As I was heating up the casting pot today I added some old "lead" bullets and minie balls that someone gave me. As the pot was heating up I noticed something that looked like cheese curds, or really lumpy oatmeal on top of the melt. It floated on top of the lead and would not mix in. I thought it might be zinc, but it melted at about 530 degrees like the lead, so I dont think its zinc. But I dont know what it is. Could it be something contained in the bullets and minie balls? Maybe bismuth?
This is the first time I have seen this, so I cleaned it off and put it with the dross. Oh yeah, it is a dull silver, not shiny like lead.

Any ideas?

Thanks!