Understanding lead alloys is a subject of much debate on bullet casting forums for many years. There are some things you just can't get away with concerning some alloys. Such as putting too much antimony in a lead alloy WITHOUT the correct percentage of tin. Tin is required, in the correct percentage, to keep antimony in solution with the lead.
That last sentence is the key to understanding lead alloys. It's a solution more than anything else. Look at it like a water/salt solution. Once you put salt in water, you have a solution. Overly simplified, because you CAN separate the salt from the water by by distillation. You can only get just so much salt to dissolve in water, it becomes a saturated solution.
Same with lead and antimony. Not enough tin, the antimony shows up as separate crystals surrounded by pure lead. The bullet feels hard, but will lead horribly. Linotype is a eutectic alloy. It melts all at once at a very specific temp. Some solders, like 37/63 lead tin are also eutectic.
Now that you're thoroughly confused, want to continue? Then this is good reading over a lasc;
http://www.lasc.us/