Ya can`t do it ,or should I say the lead won`t allow it !!
The lead as soon as the impurity is coated with carbon from what ever source will push paticles out !!!
Wether up sideways or down it`ll push it out , true the majarity will "float" to the top with a bit of help , but as we stir the lead is pushing out impuritys on the path of least resistance .
Ever notice the stuff is very fine dust like , that`s because the bigger stuff floats .
I can`t say enuff about fluxing & stiring it while in the smelting pot !!
I first skim the big stuff & keep the melt at a lower temp (in case I missed a zinc weight or someting I don`t want melted into my alloy) flux a couple of times then turn up the heat & flux some more , how many times ??? I let the melt whisper to me
I used to clean my bottom pours every 5 pot fulls (boiling water in em cleans em like ya would`nt believe , until ya try it)I have extended casting furnace service to every 8 pots .