Rub charcoal and parchment with a scent to get...information? Face of perp with more details based on quality of scent and degree of crime? Seems okay as an idea for a curio I guess.
Could be a good hide + coal sink (more use for good q. tar kilns). Would be fun if it would require you to use it against something, like you do with real charcoal rubbings (have to have something underneath). I wonder whether there could be a random object you use as an ingredient (not consumed during the craft) that would determine the output lp/hr/time/whatever.