A "charcoal sketch" refers to a drawing made using a black material called charcoal. It is usually a preliminary or unfinished drawing that captures the basic shapes and features of a subject in a quick and more casual style. Full definition
Starting with Foucault's Pendulum (1988), each of his subsequent novels has been concocted from a tattered shred of an interesting idea, a hasty charcoal sketch of a plot, and a warehouse full of dazzlingly obscure facts and boringly obvious intertextual allusions. (firstthings.com)
Raim ingeniously uses charcoal sketches that look like Harold's (but are actually drawn by Patrick Mate) to tie these pieces together. (leonardmaltin.com)
Along with the paintings are also preliminary charcoal sketches that reveal Garabedian's process behind these scenes. (bettycuninghamgallery.com)