I buy my lead from roto metals, I buy both pure lead and 5% antimony and there is a big reason why I no longer use wheel weight lead for antimony. The shops I use to get weights from don't separate new ones from old weights, and the new ones have a high zinc content. If you want to ruin 10 pounds of lead, add 30 percent wheel weight lead that had a high zinc contenet and watch as it gets thick and clogs up your pot. It is so bad that new LEE furnaces now come with a warning against using it. That said, I run a 70/30 soft to hard ratio for spinnerbaits and a 60/40 for jigs, what happens is the lead will sometimes, not always freeze faster, I'm not 100% sure but I have a feeling humidity places a roll as I have very few bad pours during the winter than in summer. In order to combat that problem, make sure your mold is hot, and try to keep hooks in a place that they are at least room temp, if they are cold it can cause problems. But there isn't too many problems when making leads hard, and if I do have a little problem I'll bump up the temp of my pot and it usually works.