Put the ingot in a HOT crucible, let it puddle and stir it. Dross and oxides will rise to the top. Skim quickly and pour. Dont dally, move with purpose.
Run your furnace rich, so it has a reducing flame that leaves less oxegen. Use a cover or lid to cover the vessel. Use talc to keep the lid from sticking. A bit of water to make a paste painted on the lid will do, it will flake off, its supposed to, in aiding the separation. Some guys wil turn a vessel upside down, on top of the bottom vessel, all full of scrap, it keeps you from adding a second backup charge, as it will all run down to the bottom, hopefully filling the lower primary vessel, and sealing off additional air.
I have never done so, but graphite will not absorb into aluminum or zinc, but will create a barrier for oxygen, as it will slowly burn away, creating a barrier on the molten metal.
I have not looked at the table for this compound , so I am shooting from the hip. Some zinc coumpounds do not do well with manual pouring, so beware that these metals can be severely limited in some applications, that are not injection molded.
I only use diecast, when there is a detail part that has little strength, and moderate detail, and then added to a basic aluminum stock of screen doors, (gummy) to condition the material more for my use. Any material, should be selected for its end use, machinability, strength, and finish. Not merely availability, unless your demand criteria are low.
Learn by doing, and strive to match your needs.
Be Alert! The world needs more Lerts.