Albedo, the whitening, is the second of the colour-stages through which the alchemical Great Work (magnum opus) was said to pass. It names the washing and purifying of the blackened matter until it turns white.
The colour-stages of the Work
Symbolically Albedo marked the cleansing that follows dissolution, the first light after the darkness, and the recovery of a purified ground. The classic sequence, nigredo, albedo, citrinitas, rubedo, describes the transformation of base matter into the perfected end of the Work through a succession of colours seen in the vessel.
Many Latin-tradition sources place a further stage between the black and the white, the cauda pavonis or "peacock's tail," a brief passage through an iridescent display of many colours taken as a sign that the blackened matter is breaking apart and beginning to purify; not every text names it as a separate stage, and where it is omitted the shift from black straight to white is simply read as happening in a single movement. The stages carried at once a chemical and a spiritual meaning, and were read by the depth psychologist C. G. Jung, in his 1944 study Psychologie und Alchemie, as an allegory of inner transformation, treating Albedo as a stage of recovered clarity still short of full integration. The stage is described here within that documented tradition of European alchemy and its later interpretation.
Sources The Secrets of AlchemyLawrence M. Principe with Psychology and AlchemyC.G. Jung