Its possible to cast zinc. If you mix zinc with lead, its completely uncastable. Had it happen once, think I had a zinc W/W get mixed in a pot full of lead W/Ws that I left unattended long enough for it to melt in. The results were 20 lbs of alloy that would only make badly wrinkled or partial bullets no matter what I did. The zinc ones melt at a higher temp and will float on top of molten lead, they are usually marked as 'ZN' or some other marking and usually come off BMWs or Japanese cars. If you can find a bucket full of semi-truck W/Ws, you've found gold- lots of good lead, very little dross/clips.