Chalk practice with skeletons

I used chalk and charcoal with these drawings, encouraged to use the technique of focusing and drawing around the negative space of the skeletons that were posed in front of me.
On the black paper I first drew the side of the ribs and spine of the skeleton with just chalk, the second time I drew the top of the spine and shoulders of the skeleton which didn't have the skull on top of it, this time using both calk and charcoal.
On the white paper the skeleton had some cloth draped over the top of it with an apron also tied around it, this time I only used charcoal.