Being a plumber I run across some lead in my business. The old cast iron pipe had joints that were caulked with lead to seal them, and lead pipe was often used for toilet flanges. I save it all and use the "thumbnail test" to determine it's suitability - if you can scratch it with your thumbnail (actually a deep scratch, not just a mark on the surface) it's close to pure. When you melt it for casting just heat it well beyond the melting point andany alloys an impurities will separate and float to the top. Skim them off, re-flux and cast.