text: Via the process known as spontaneous resolution, 5-10% of all racemates crystallize as mixtures of enantiopure crystals.  This phenomenon allowed Louis Pasteur to separate left-handed and right-handed sodium ammonium tartrate crystals. These experiments underpinned his discovery of optical activity.  In 1882 he went on to demonstrate that by seeding a supersaturated solution of sodium ammonium tartrate with a d-crystal on one side of the reactor and a l-crystal on the opposite side, crystals of opposite handedness will form on the opposite sides of the reactor.
  Spontaneous resolution has also been demonstrated with racemic methadone. In a typical setup 50 grams dl-methadone is dissolved in petroleum ether and concentrated. Two millimeter-sized d- and l-crystals are added and after stirring for 125 hours at 40 °C two large d- and l-crystals are recovered in 50% yield.
  Another form of direct crystallization is preferential crystallization also called resolution by entrainment of one of the enantiomers. For example, seed crystals of (−)- induce crystallization of this enantiomer from an ethanol solution of (±)-.