(Why) (is) ZINC "bad"?

[georgerkahn]

With apologies if this query has already been addressed elsewhere, I'm relatively renewing my interest/activities in the wondrous world of making cast boolits. In my earlier (young children, working) days wheel weights were pretty much all lead. Now retired, I recently learned that lead WW's have been verboten in New York State for a few years, and I need to sort through the buckets I just purchased ($0.25/pound) to get out the zinc and steel.

I've searched a bit, and have found many, many posts remarking that even a teeny amount of zinc in the pot is problematic; uses of sulfur to flux it out, and, possible muriatic acid to detect its presence. Further, I read one post which suggest zinc ruins moulds. (???)

Now (finally ;-) ) my question: Exactly WHAT does zinc in the alloy do, and what detriment is it really vis casting boolits?

Thanks -- geo (a 65 year old newbie, sort of...)

[clodhopper]

It can make your alloy "lumpy", difficult to pour boolits and boolits with very poor fillout.
Some casters use very small amounts of zinc to harden alloy, around or less than 1% zinc.

[btroj]

Small amounts aren't a problem but as zinc level goes up it tends to impede flow of molten lead giving poor fill out.
I wouldn't be paranoid about zinc but it is best to avoid adding it to the melt.