Starch paste

Starch paste is probably one of the oldest adhesives known to mankind. If you've ever cooked with cornstarch, flour, potatoes, etc. you know how sticky they can be. This stuff is very easily to make and completely non-toxic. If you wanted to gross people out, you could even eat this stuff.

Made without additives, this sort of paste is acid-free (archival), which is nice when you're working on a model that you want to keep around for a few years. While it's costuming work is limited (mostly to paper maché), it is inexpensive stuff to make. We keep an index card with a basic recipe around.

Note that if you live where it's very humid, your starch paste projects may grow mold. If this happens, you'll want to find wallpaper paste, which has special anti-fungal stuff in it.