Drawing from a skull cast.
This exercise was using subtraction. You start off with a mid tone using vine charcoal and then you erase or subtract the areas where it is lighter in value. It is a great exercise to let you see in 2 or 3 values.
One of the Bargue exercises.
I recently met and visited Tex Lecor at his Studio. He is one of the great artists from Quebec. Not only he paints, he is a famous folk singer in the 70's and had some TV shows too. A real renaissance gentleman.
Another subtraction exercise demo.