Just looking through the back issues here. Surface area is mostly impurity (slag), and you remove the impurities before casting. Since pop cans are so thin, the are mostly impurity. It just makes a clump of crap in the bottom of the crucible! There is no such thing as pure aluminum, at least not offered to the public. Every aluminum that you see is alloyed. Some alloys have better charictaristics than others. 60 series is most commonly used for structural components, especially when the need to be machined, because it has good strength charictaristics and is just hard enough to machine easily. Check out a book on machining from your library, and read about the different metals and their machining charicteristics. Almost any part cast will have to be machined too.
Stop talking about it and do it already!!!!!
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)