When I teach drawing classes, I try and remember to bring along a can of reworkable spray fixitive. Once a student asked me why it's called "reworkable". I wasn't sure, but said that I thought you could "fix" your drawing, then add more marks over that. For the first time that is what I did with this drawing- when I'd almost had enough of it, I sprayed it and then added some additional layers of shadow. Charcoal is such an ephemeral (and sometimes maddening) medium that it was really cool to be able to make it stay in place and continue to work. I think I'll try this again.