Finish drawing detail

I would like to show a detail of the title image of this blog to give an idea of the texture in my work. This particular piece was a turning point in my artistic development. This Cinereous Vulture portrait (13.25"x11") was the first truly successful combination of charcoal and graphite I'd been working toward for several years. The birds beak is purely graphite which on gesso creates a mellow silvery tone. The area around the eyes is charcoal which offers a deep velvet black. Mid tones are careful combinations of the two depending on the effect I'm looking for. Carbon is reserved for the deepest black possible. I once heard of a theory of warm and cool value in B&W by using this combination that riveted me and changed the course of my work. I'll share what I understand of it as time allows.