Research point Odilon Redon and tone

Was a French painter, printmaker, draughtsman and pastellist who lived between 1840 and 1916. He did a lot of work in charcoal from imagination creating some surreal rather Gothic figures many of which rely on creating dark pastel shadows to bring the light figures forward. I like his later pastel drawings which seem to me to be influenced by Gustav Klimt. I particularly like "Buddha" from 1906/7 and "Ophelia among the Flowers". Some of his oils look like they have been done in pastel as drawings rather than paintings.
He was born in Bordeaux, Aquitane Bertrand-Jean Redon, Odilon was a nickname from his mother Odile. He drew from when he was a child but started studying drawing when he was 15. His father insisted that he trained as an architect but he failed the entrance exams. He then studied sculpture and was taught etching and lithography by Rodolphe Bresdin. He served in the army in the Franco-Prussian war in 1870 then moved to Paris where he worked in charcoal and lithography using shades of black and published an album of lithographs in 1884. He was not widely recognised until he featured in a novel A Retours.
He became more interested in working in pastel and oil in the 1890's and stopped creating works in black (noirs) in 1900.
His work is influenced by his interest in Buddhism and representation of his internal thoughts and feelings. His noirs are very dark and seem to be the work of a troubled mind but his later work is lighter and more optimistic.
He describes his work as
"My drawings inspire and are not to be defined. They place us, as does music, in the ambiguous relm of the undetermined."
and "to place the visible at the service of the invisible"
Commentators have said that his noir drawings "defied classification, unheeding for the most part the limitations of painting"
He used charcoal that had been soaked in linseed oil, a medium that had gone out of fashion but which was good a sticking to paper. He also used black conte crayon and black pastel. He fixed his work intermittently during the drawing process and used fingers, fingernails, a sponge and pointed impliments to move the charcoal around before the fixative dried. Paper was usually coloured and could be treated with powdered charcoal before the drawing started. Redon was unusual in embracing the fact that paper changes colour over time and alters the hue of a drawing.
There was a very interesting paper  published in 2011 which documents the investigation into his techniques and has some good examples of his work http://cool.conservation-us.org/coolaic/sg/bpg/annual/v14/bp14-08.html Tadpole 1883 is lovely and very atmospheric

So what is the atmospheric potential of tone? A simple plain line drawing has very little atmosphere, more of a diagram. Even comics use tone to create a sense of atmosphere and give imformation without resorting to words. Lots of darker tones create a feeling of  forboding, mystery and depth. A few lighter tones can draw attention to a particular aspect of a drawing. The artist has the ability to control how a viewer reads a drawing by how they apply tone. Softly applied tone makes the picture look as though it has evolved from the paper rather than been drawn, maybe tone is where drawing turns into painting as in pastel paintings?