If you heat the scrap lead too fast, you can have some zinc trapped at the bottom where it melts before the lead on top melts. Don't ask me how I know this.
On an unrelated topic,
I had about 7 pounds of zinc-contaminated lead on my bench, mocking me. I probably should have thrown it out, but I can't throw anything out (you'd think I was raised during The Depression.) So I cast a bunch of silver dollar sized ingots, and I can put one or two of those in a 20# batch of lead without hurting anything. Three would be pushing my luck. I've used up a couple of pounds of the stuff that way.