If it is cooling with a blue tint it would indicate that it is lead a pure as we can get it. If it hardens with a frosty appearance then I would suspect there is a small amount of tin in there.

You do not need to flux pure lead. That is only done when using a tin/alloy mix and the tin separates out.
Anything that floats to the top of the melt with unalloyed lead is dross and needs to be skimmed off.