Nero pencils contain a colour strip which is made from a blend of soot, clay and binding agents, plus oil. The addition of makes their stroke extremely soft and smooth and provides a very rich coverage. It is not soluble although partial wiping is possible in the softer grades. The soft grades leave a strong dense mark so that great depth can be achieved. Coarse strokes can be achieved by using a rougher grainy paper and the harder grades allow for a sensitive rendering of fine details.