Essential Elements

I found it hard to focus on tone rather than line and detail so it took me quite a few attempts to get any decent results. In the time suggested of 10 minutes per pose I found the best approach was to establish the basic shapes, then add some contour with a strong defined line, then shade.  I did some hatching but also, when using a charcoal pencil, found it helpful to just add more pressure to get darker tone.  Below are some of the sketches:
(below - light source from right)

      

    

 

 

Below I looked to use different media - conte, pen and ink, plus ink wash on the last one.