I've been thinking about the process for making molds and I found this helpful tutorial at Dan Perez Studios. There are three pieces to this model for Slimer from Ghostbusters. The artist is using Klean Klay to support the object to be molded. The clay is probably soft and easy to work with so that you can get a smooth and even seal around the prototype object. There are instructions here on how to create key holes, air vents, and pour spouts. The process here looks much more complicated than the method we learned in class, but it's a good visual aid for me to see how to make a complicated object with casting. Here are a few of the cool pics in the tutorial:

Slimer's body supported by globs of Klean Klay

Slimer supported by a bed of clay

Detail of how the artist forms an airtight seal around the prototype

Air hole and pour spout

Finished 1/2 of the cast with key holes and walls

I'm still kind of confused about how some molds are made though. I have a stone gargoyle sculpture that came from The Gargoyle Statuary and I know there were plenty of duplicates around. The object has a lot of "overhanging" features though, and I suspect there must have been several pieces to the mold as far as I can figure, the gargoyle below probably has 4 or 5 pieces for it's cast. I might try to make a cast out of this guy just because I think it would be a cool learning experience. If it worked I'd be able to reproduce this guy in jello. Well, jello is one possibility.