The stick on WW's should be pure lead.

What you need to do is look at each WW. If you see a stamping with Zn on it it is a zinc one. Sort it out and never use it in your lead. There are also steel WW's that need to be sorted out. The steel ones should be marked Fe. One thing you need to do is make sure the temp never goes over 650*. Zn melts at around 780* so they should float on top like the clips. This is very important, Zn will ruin your lead.

One other way to check for the Zn is to use a pair of side cutters. The Zn will be very hard and should not crush or cut. Dropping them on a concrete floor is another way to check them. Regular WW's wil have a thud sound. Zn will ring.

Get a casting thermometer. Run your melt at 650* and everything will be OK.

Go over to the " Cast Boolits " forum and find out more. Everything you ever wanted to know about lead bullets over there.