My understanding is that the zinc is corrosive in some manner and is undesirable in the the alloy. I have always heard that it makes for ugly bullets due to not filling the mold properly. I don't really know because I don't allow it to melt in the mix. I get just enough heat to melt the lead and everything else floats. When I do large quantities I just scoop everything off that doesn't melt right away.
Alan
