Thanks Neubert.
If too soft, it probably means you're using extrusions or other unknown scrap. Try using only aluminum castings for scrap. They have a high silicon content.
Porosity means different things to different people. Occasional very tiny bubble dots (needing magnification to see), are usually the result of dissolved gas -- often ascribed to melting in steel or iron crucibles. But many scrap castings have steel in them already (pistons for instance have internal steel reinforcing clips) or rust washed over them from outside junkyard storage, used motor oil sludge, varnish, piston rings, etc. so it's pretty much impossible to avoid ALL ferrous contamination unless you're using virgin ingot material and a non ferrous crucible. Or you can add a clay and sand liner to a steel or iron crucible. I don't usually bother doing that for my own aluminum castings, and nothing I've built has fallen apart yet. But to each his own in that department -- many internet casting discussions go back and forth repeating the same technicalities, discuss de-gassing, etc. Not many using involved methods actually build much of interest to me or detectable superior quality from what I've seen. But again, the game is the important part, not necessarily the result.
If by porosity you mean bigger bubbles and hollows, the cause is often too moist greensand, or failure to skim the melt, or some other sand contamination. Pouring too hot, is also a cause. Unless you're pouring super thin castings, or lost foam, aluminum should just be fluid, in my opinion, and not super-heated. Venting with a wire can help fill difficult cavities, though I often forget to, and usually things work out.
