Skulls on toned paper

Today in drawing class we worked on one of my favourite techniques for creating representational form and dimensionality using tonal values.  The assignment was given after a demo on how to squint and see the darks apply them, then erase out the lights and to go back building up the forms with adding or taking away material but not relying on line to draw what they see. In no particular order are some of the images I snapped during class and to share with anyone interested or who missed class, you know who you are, hopefully this is helpful ;P