Here are a few progression shots of the drawing, Callisto. I use a 0.5mm mechanical pencil with 2B lead and a kneaded eraser on Mylar (polyester drafting film). I prefer to use line only to create the gradations relying on changes in pencil pressure rather than a blending stump.
Finished drawing of CALLISTO (Jan 2019). Graphite on Mylar.
Turning form.
Contour drawing.
CLOSE-UP in process.