Well, going by your description, it could be anything lead based. If you get pure lead really hot, it will do exactly as you describe. And, a lead/tin alloy has a lower melting point (so I hear), so it will stay liquid longer, only at a lower temp.
To ascertain whether or not it's pure lead, here's a couple of things to look for. If the balls start getting a dull grey look, like the next day after pouring, they have a high lead content. If the balls stay shiney indefinately, they have a harder alloy mixed in. Also, if the balls are coming out of the mold with a frosty look, they are not very pure lead, and you are running the metal too hot. Pure lead can be poured red hot, and the balls will not get frosted.
Hope this helps. Bill