Pure lead is damn hard to come by, I would mix it sparingly and save the rest for later. Fluxing in tin is easy but it needs to be done at the correct temperatures. Tin is expensive as you will find but if you go to scrap yards you will do well. Some tin can be bought and mixed from soldering wire but is expensive. When you get your sourse of tin I would mix in small batches until you get the hang of it.