Distinguish the various planes of a picture by circumscribing them each
in turn; class them in the order in which they present themselves to the
daylight; before beginning to paint, settle which have the same value.
Thus, for example, in a drawing on tinted paper make the parts that
glitter gleam out with your white, then the lights, rendered also with
white, but fainter; afterwards those of the half-tones that can be
managed by means of the paper, then a first half-tone with the chalk,
&c. When at the edge of a plane which you have accurately marked, you
have a little more light than at the centre of it, you give so much more
definition of its flatness or projection. This is the secret of
modelling. It will be of no use to add black; that will not give the
modelling. It follows that one can model with very slight materials.
_Delacroix._