My chair is solid! Not see through like Dales, so the negative shapes are only to do with tones. I did a sketch with Caran D'ache pencils, which looks like a patchwork & I will post it on my blog HERE
The sketch above was of the studs around one of the areas of the chair
Yesterday Ruth bought me a BLENDER pencil & I have used it here on the dark areas, it is a fabulous tool.
The shading in the centre part is a little bit lighter than around the edges, look how the blender grabs the pigment and accentuates the shading. I used just 4 pencils for this 2 browns, dark red & yellow & am pleased with a very hurried sketch - 5 minutes does not get any longer - phew!