A lot of the "crud" forming on top of the lead isn't impurities. It's the good stuff ie the alloying metals. When the lead alloy is heated, the tin, antimony etc come out of solution and form oxides on the surface. If you scrape it off and discard it then you are lowering the alloy content. The role of the wax, or other agent (I use sawdust), is to setup a redox reaction whereby the metal oxides are reduced back into solution and the carbon in the wax etc is oxidized into CO and CO2 (Note: hence the need for good ventilation).