The only time you should have lead oxidizing and forming a crust / skim is if you are not smelting at the correct temperature. Use a thermometer.
Good clean pure soft lead at correct temp, should have very little skimming.
Of couse there will always be a very fine discolored skim. That is usually just the lead it self that has cooled on the surface of the pot.
I'm not sure this is so.
Lead oxidizes just sitting solid on your desk, and quickly at that. It's why your bullets turn from silver to gray.
Heat accelerates oxidation.
I usually cast at around 750F. But as soon as my lead melts and you skim the dross, the lead on the surface begins to oxidize. It starts off with discoloration, but if you skim this you will get a dusty dross residue.
I see this even with 99.9% pure lead ingots.