There is so much misinformation about casting on the internet, I will mention a few myths, as well as some safety items that must not be ignored.
Almost everyone you see on the internet uses a degasser and flux when casting aluminum, and they always skim the slag off the top of the aluminum before pouring.
If you use petrobond, there is virtually no gassing, and you don't need to add a degasser. If you use a high quality metal such as aluminum 356 available in the US from Budget Castings, then you don't need to add any flux, and I don't even skim the slag, since clean 356 does not have any appreciable slag.
Heat the metal as fast as possible, and do not stir it in any way, since stirring can introduce things that may cause gas (in the metal).
Use a pyrometer to get the exact pour temperature. I usually pour AL around 1,350 F.
If you pour at a temperature that is much less than 1,350, then the metal will solidify before if finishes filling the mold.
And of course, don't stand over the furnace and breath in AL fumes, they are not good for you.
Everyone on the internet says that steel crucibles add contaminates to the aluminum, but I see no evidence of that, and I have been getting perfect castings using a steel crucible, as do many others on the internet. You can really abuse a 1/4" thick steel crucible, and it just keeps on pouring. You can very easily damage a clay or silicon crucible just by handling it or putting metal in it.
Anything that contacts molten metal has to be preheated to remove all moisture, and you can expect an explosion if you mix molten metal with anything that has not be preheated to dry it completely, such as metal added to a hot crucible, metal poured into an ingot mold, and metal poured onto concrete.
Below is an ingot poured from AL356. You can see that the metal picks up the mill scale pattern from the ingot mold, and also perfectly copies the pattern left by the weld on the end of the ingot mold. This pour did not use flux, degasser, or slag skimming (none of my pours do).
The good part about clean AL356 used with petrobond is that it accurately copies the surface of any pattern. The bad part of clean AL356 is that it accurately copies the slightest of imperfections in the surface of patterns, but the solution is to just make smooth patterns.