In alchemical treatises, the formula L'Oeuvre au Noir … designates what is said to be the most difficult phase of the alchemist's process, the separation and dissolution of substance. It is still not clear whether the term applied to daring experiments on matter itself, or whether it was understood to symbolize trials of the mind in discarding all forms of routine and prejudice. Doubtless it signified one or the other meaning alternately, or perhaps both at the same time.

Author's note, p. 367 - The Abyss (1968)