In high school metal shop class we used old Briggs and Stratton crankcases. We made wood patterns allowing for the aluminum shrinkage, and we tamped damp "sand" around the pattern that was set inside the steel alignment frames. We removed "sand" by hand to let the aluminum in, and to get vents out. On on long or large patterns you would have to add more vents depending on intricacy. I forgot what chemical we added to clean the molten metal before the pour, but it worked well and the contaminents were easly skimmed off. We would pour the moulds full, as fast as we could safely pour, so the aluminum would not try to stop flowing and make bad casts. Good part of getting a bad cast, was that it just wasted your time. The bad cast was returned to the crucible, and the pattern re-done, and re-poured.
The most common mistake I saw happen was too small of sprue openings, that did not let the metal enter fast enough, and did not have enough head pressure to make the metal flow out on thinner areas. It is a bit of an art, some kids just never got it right, even with extensive coaching. Intricate items take a lot of practice.
I had a lot of fun with it.
Good luck!
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