Recently, I drew a monochrome study, as part of the course. The aim was to get a feel for the different values between white and black.
Bromeliad - click to enlarge
It was recommended charcoal and gesso (thick white paint) be used together. Although I found it really difficult to work with them. The gesso went over the charcoal, no problem, but the charcoal ate into the gesso - leaving indent marks, instead of black.
I switched to an oil pastel however and it worked much better on the gesso. If I were to do this again, I would just use my white and black charcoal blocks. It was a fun exercise nonetheless.