...under development...
One can cast concrete against just about any surface. Assuming one wishes to detach the concrete at some point, selecting a reliable release agent becomes an important consideration. The materials listed here are cheap and easy to come by. One can, if one wishes, purchase professionally made release agents. It's hard to imagine they don't confer a few advantages for some situations. But I wouldn't know. I've never used anything but stuff from the local big box stores.
This is a quick run down on items I've used over the years (more discussion & pics to come....Jan '17, BH)
- petroleum jelly
- vegetable oil
- plastic (technically, this is more "barrier" than "release agent", but sometimes it's useful)
- liquid soap
- bar soap (this has to be soaked in water in order to get a loose, slimy goo that can be used to coat surfaces)