Well, cast some boolits with a good alloy. The lead flows into the mold like water. Then cast some polluted with zinc, it's like trying to pour oatmeal into a mold!:barf:
The resulting boolits will have rounded corners, and look like a cheap job of zinc plating on Chinese steel. Also they will be very hard.
A little would be as small as 5% zinc will cause problems, or in other words, 5 zinc wheel weights melted with 100 lead ones. This is a guess on my part, I have never dealt with zinc contaminated lead. Just what I have heard from some others.