Zinc has a higher melting point (but not too much higher) so when you've got a good base of melted lead, anything that doesn't melt pretty quick, remove immediately. It tends to float as do the clips so it's not too difficult but if you let it sit, it will melt in your alloy.
http://castboolits.gunloads.com/showthread.php?t=66241
WA state outlawed WW lead, pretty bummed... they're all zinc now...l