Nigredo (Blackening)

Nigredo, or blackness, in alchemy means putrefaction or decomposition. The alchemists believed that as a first step in the pathway to the philosopher’s stone all alchemical ingredients had to be cleansed and cooked extensively to a uniform black matter. In analytical/Jungian psychology, the term became a metaphor "for the dark night of the soul, when an individual confronts the shadow within".