People draw with charcoal sticks, right? Charcoal sticks are expensive. Why can't I pick up some charcoal from a fire and draw with it?
Well, I can. And it works.... okay. The problem is it's hard to get a really deep black. Straight charcoal doesn't dig in to the paper, it sits on top of the paper. If I draw over a line I've already made, I'm pushing the other charcoal out of the way. There gets to be a bunch of it on the surface of the paper. And, of course, it's really easy to smear.
I guess the next step is to crush it and mix it with some kind of binder. If I use wax I'll get crayons, if I use clay I'll get conte crayons (broadly speaking).
At any rate, the kids had fun, and clean up was super easy because the dust didn't stick to anything and we tried to keep them from grinding it into the carpet.