Studies sketchbook: Light & shadow studies with sculptures✍?

Lately I've been a little bit obsessed with busts, it's something that happens from time to time. I think it is a magnific way of studying volume and shading. That's why I felt the need to draw some of the references I have saved these days. 

I also created a new Pinterest board with them, you can find it here! You will also find the reference pictures of these skethes attached at the bottom of this post, but if you like busts as much as I do I really recommend you to check the board?