I've got some 30 lb lead bricks, too. I was told they were salvaged from a nuclear lab that was decommissioned (no, they don't glow in the dark). If that's what yours are, too, they are pure lead and unless you want to cast ball for cap and ball or musket, you'll want to add tin and antimony to bring the hardness up a bit.
A common strategy is to add Linotype metal to pure lead, but how much depends how hard you want it to be. 2:1 is a good general purpose mix. Modern lead-free plumbing solder is usually 95% tin any more, and will serve as a close-enough your tin source. Elmer Keith used 16:1 lead:tin casting the bullets he developed the .44 magnum with, so you can actually use tin alone. It just costs more.
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Last edited by Unclenick; October 8, 2011 at 08:23 AM.