The later branch of esoteric neidan 'inner alchemy', which borrowed doctrines and vocabulary from exoteric waidan, is based on allegorically producing elixirs within the endocrine or hormonal system of the practitioner's body,[1] through Daoist meditation, diet, and physiological practices.

The Chinese compound wàidān combines the common word wài 外 'outside; exterior; external' with dān 丹 'cinnabar; vermillion; elixir; alchemy'.

Pregadio concludes that the semantic field of the word dan evolves from a root-meaning of "essence", and its connotations include "the reality, principle, or true nature of an entity or its essential part, and by extension, the cognate notions of oneness, authenticity, sincerity, lack of artifice, simplicity, and concentration.

It occurs in Du Guangting's 901 Daode zhenjing guangshengyi 道德真經廣聖義 (Explications Expanding on the Sages [Commentaries] on the Daodejing), which is quoted in the 978 Taiping guangji.

[3] Jindan zhi dao 金丹之道 (Way of the Golden Elixir) was a classical name for waidan alchemy, and wàidān shù 外丹術 (with 術 'art; skill; technique; method') is the Modern Standard Chinese term.

These texts show that while early waidan was mainly concerned with the performance of ceremonies and other ritual actions addressed to gods and demons, a shift occurred around the 6th or 7th century to the later tradition that used alchemical symbolism to represent the origins and functioning of the cosmos, which played a crucial role in the development of neidan.

[7] Despite a later tradition that Zou Yan (c. 305-240 BCE), founder of the School of Yin Yang, was an early alchemist, his biography does not mention alchemy, and no waidan text is attributed to him.

Emperor Jing of Han's 144 BCE anti-coining edict "established a statute (fixing) public execution for (privately) coining of cash or counterfeiting gold" that Dubs believes also made alchemy illegal.

[6] According to the c. 94 BCE Shiji, Li Shaojun told the Emperor: "By making offerings to the stove (zao), one can summon the supernatural beings (wu).

[12] Although Liu An's c. 120 BCE Huainanzi does not explicitly refer to alchemy, it does have a passage on the natural evolution of minerals and metals in the earth, which became a prominent idea in later waidan cosmological texts.

[17] The c. 60 BCE Yantie lun (Discourses on Salt and Iron) has the earliest reference to the ingestion of alchemical elixirs in a context criticizing Qin Shi Huang's patronage of anyone who claimed to know immortality techniques.

[18] "At that time, the masters (shi) of Yan and Qi set aside their hoes and digging sticks and competed to make themselves heard on the subject of immortals and magicians.

In 60 BCE, Liu Xiang presented the emperor with a rare alchemical book entitled Hongbao yuanbi shu 鴻寳苑祕術 (Arts from the Garden of Secrets of the Great Treasure)–which had supposedly belonged to the Huainanzi compiler Liu An–that described "divine immortals and the art of inducing spiritual beings to make gold" and Zou Yan's chongdao 重道 "recipe for prolonging life by a repeated method [of transmutation]".

Unlike the earlier Taiqing tradition, which focuses on ritual, the Cantong qi is based on correlative cosmology and uses philosophical, astronomical, and alchemical emblems to describe the relation of the Dao to the universe.

[34] Ge Hong emphasizes that waidan alchemy grants access to higher spiritual realms and is therefore superior to other practices such as healing, exorcism, and meditation.

Although the Shangqing texts used the waidan process mainly as a support for meditation and visualization practices, the language, techniques, and rituals in these works are mostly identical with those of the Taiqing corpus.

Advocates of the cinnabar-mercury waidan methods, such as the 8th-century alchemist Chen Shaowei 陳少微, described producing quicksilver in cosmological terms, without any reference to the Cantong qi system.

During the Tang dynasty, the lead-mercury tradition based on the Cantong qi acquired importance, and waidan alchemy was transformed from an instrument for communicating with supernatural beings to a support for intellectualizing philosophical principles.

[36] Several works related to the Cantong qi rejected cinnabar-mercury methods with the rationale that yang cinnabar and yin mercury alone cannot produce the true elixir.

[25] The Tang period is also known for intensified imperial patronage of waidan, even though elixir poisoning caused the death of Emperors Wuzong (r. 840-846), Xuanzong (r. 846-859), and likely also Xianzong (r. 805-820) and Muzong (r.

Early Taiqing tradition texts stress the performance of alchemical rites and ceremonies when compounding, and described elixirs as tools for either summoning benevolent gods or expelling malicious spirits.

Most post-Tang waidan texts related to Cantong qi alchemy stress the cosmological significance of elixir compounding and employ numerous abstract notions.

Emperor Zhenzong (r. 997-1022) established a laboratory in the Imperial Academy, where the Daoist alchemist Wang Jie 王捷 "produced and presented to the throne artificial gold and silver amounting to many tens of thousands (of cash), brilliant and glittering beyond all ordinary treasures".

[41] The alchemist's heating apparatus, interchangeably called lu 爐 'stove; furnace' or zao 竈 '(kitchen) stove', was placed on the highest tier of the tan platform.

[46] Ding 鼎 originally named a "tripod cauldron" Chinese ritual bronze, but alchemists used the term (and dingqi 鼎器) in reference to numerous metal or clay instruments with different shapes and functions.

[48] In addition to these basic tools, the alchemical apparatus also includes both common utensils (like mortars and pestles) and various specialized laboratory instruments for steaming, condensation, sublimation, distillation, and extraction.