Andrew thanks so much! You must get back into casting. I forgot about the sheer fun of it (when it works!) -- breaking away some dirt to see what you've got -- and the playing with fire.... you know what I mean!
Pekka, thanks kindly! it's probably just being familiar with whatever materials you've got, through trial and error. Every sand is different, but I believe most are workable for aluminum at least. It's just doing it enough with your particular ingredients. The feel of the sand - tempering is very important, and the proportion of clay to sand also. But not necessrily the pedigree or cost.
This casting was made with 13 year old greensand made from fireclay and sand, not even Bentonite. The very same sand I cast my lathe, accessories, and everything else aluminum with.
I'm used to it, I know how it should feel, and how hard to ram it and how to vent it, etc. And I'm sure that would all be different if I changed to a different sand.
In fact casting was all brand new again the other summer when I did so with iron. Everything was different, and I had to re-learn it. Coming back to this stuff, wth aluminum in a foundry the size of my original, with my old sand, everything was familiar, and second nature.
Oz thanks! re. using a venting wire to poke holes. Everybody is different, but I would never vent with a hacksaw blade. I prefer just the opposite -- a fine straight wire, 1 mm (.040"), stainless steel spring wire.
I started out with coarser wire -- bare house electrical copper wire -- it was too thick, not straight enough, and dug out divots and tended to crumble the mold surface if poked through from the inside (I frequently do this -- not just venting from the outsode toward the pattern).
I later went to aircraft SS tie-wire -- still not stiff and straight enough, and too coarse, though better. Finally I found a a kitchen baking tool for poking pastry just perfect for the job. It had a coin sized flat plastic handle, 1 mm stainless spring wire, perfectly straight (so it didn't tend to disturb the sand when pulled out) and very thin. I think it cost $3. That's what I use. It really helps make a clean mold.
Seems a small point -- but it what works best for me.