Jean,
Since you characterized your drawing as a quick charcoal study, which you attempted to heighten (in this case with white chalk), allowing the toned paper to serve as your middle value range, I would commend to your attention the many examples of this in Robert Beverly Hale's
Master Class in Figure Drawing. (I mention this reference in part because it's very accessible, both in terms of getting hold of a copy, and understanding his "lessons".)
Descriptive of the technique are pieces such as Durer's "Head of an Apostle in the Heller Altarpiece", a brush drawing with black and white ink. An image of that work (scanned from Hale's book -- this one's a bit dark, sorry, but I was trying to focus on the lights, anyway) is attached. The use of the technique is easy to spot here, in the white-ink lines in both parallel and crosshatched orientation (as well as, of course, the use of black lines to their own form-defining purposes). Incidentally, Hale is actually using the image in a section on drawing the nose, hence the structural lines around that feature.
There are, to be sure, other works in Hale's book depicting the rendering of planes and values to show form, but they are both more highly resolved than the "quick study" form you were working with and they employ techniques other than the very useful (but difficult to master) heightening or highlighting with white.
Cheers.
Durer's image:
Threaded Mode
