Preheat your molds, make sure you use some flux in the lead. It will help get out the impurities. If flux isn't available, I have used beeswax as a substitute.
Personally, I add a little tin to make the lead flow better. I add some babbit when making hard cast hunting bullets and I water quench them. It's a very fine line between hard and too hard. Too hard and the bullets will crack and fracture.
A good thermometer will be one of your best and relatively cheap investments.
Good Luck!
