Ash is the incorruptible substance left in the alembic after the matter of the Stone has been subjected to the purgatorial fire. The ash can no longer be set on fire, and is, psychologically speaking, free from the turmoil of the passions.
Abraham defines ash as the definitive alchemical-psychological symbol of purification achieved through calcination: the incombustible, passionless residue synonymous with the albedo.
, A Dictionary of Alchemical Imagery, 1998thesis