
White charcoal has as much intensity of range as black charcoal, and it's important to never mix the two. It's a toned paper drawing for a reason, use the tone of the paper for the intermediate tone between the two contrasts.
In moving towards adding detail, such as the eye lids, its important to remember that the only reason for doing this is to make sure that the entire shadow shape around the eye is correct. This is the same for the tone on the side of the face running from the cheek bone to the chin.
This tone was added only to make sure the frontal plane of the face was the correct width.
At this point I'm going at the drawing with all the tones I can get out of my materials. Going for the darkest darks, and the lightest lights and at the same time constantly redrawing.
A note on technique:
It is of my opinion through my limited experience and my observations of my favorite artists, that charcoal strokes (and brush strokes) should follow the form. If the texture follows the form it is describing, it gives it a far more naturalistic look.
People who are obsessed with brush strokes for the sake of it have forgotten the purpose of them all together. Look closely at a Bouguereau, the strokes in his work resemble what they are actually trying to describe, than just outright brush strokes. The same is true of charcoal.