Waidan, "external alchemy," is the older, laboratory branch of Daoist alchemy, which sought physical immortality through the compounding and ingestion of elixirs prepared from minerals, metals and herbs. Where the later "internal alchemy," neidan, turned the alchemical work inward to the refining of the body's own energies, the external alchemists worked with actual furnaces and crucibles, heating and transforming substances, cinnabar, mercury, lead, gold and many others, according to elaborate procedures recorded in a large technical literature, in the belief that an elixir could be produced whose consumption would confer longevity, immortality, or ascent to the ranks of the transcendent beings. The transformation of cinnabar, which on heating yields mercury and can be recombined, held a special fascination as a visible image of death and renewal.
Waidan flourished for centuries and enjoyed imperial patronage, but it carried a lethal flaw: many of its prized elixirs were compounded from mercury, lead and arsenic, and their ingestion poisoned and killed a number of practitioners, including, it is recorded, several Chinese emperors who sought immortality and found death. This bitter record contributed to the gradual eclipse of external alchemy from around the Tang dynasty and to the rise of the internal alchemy that reinterpreted the whole enterprise as a spiritual and meditative discipline, treating the crucible and elixir as metaphors for processes within the body. Waidan nonetheless left a lasting legacy in Chinese pharmacology and proto-chemistry, and its detailed experimentation with substances forms an important chapter in the early history of the chemical arts.