Blocking out A4 page with charcoal was followed by erasing surface with putty rubber and drawing over this with willow charcoal. These steps were repeated. Here is an intermediate stage where I have swiped lightly with the charcoal and combined this with some very thick defined areas.
I then continued the process. My final picture (below; I kind of arbitrarily stopped when it began to look messy) did not look at striking in terms of objects disappearing into the page as the previous stage, although it is more interesting. I was able to layer up the erased lines, such that one appeared to lie on top of the other. It began to get more difficult to make a definite line as the paper became more impregnated with charcoal.
The nex drawing required 6B pencil with putty rubber. I found I had to sharpen a lot of the painted surface of the pencil away with a knife in order to shade uniformly and quickly. I am getting an idea that the subtraction can be applied to bring objects into light, to signify depth, light and shadow, and textured surfaces.