noun: (historical) A furnace or stove designed and used to maintain uniform heat, primarily used by alchemists.

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Giuliano Kremmerz
“The state of the magician begins with the first sublimation of the earthly being, that is, the acquisition of a state of sensitivity, understood not in a passive religious sense, but in an active and volitional sense, in which the purification of all other material actions of a lower level is possible, thanks to sublimation both through method and through consciousness.
The human body is an alchemical athanor for those who have become familiar with the occult truth, like a pot in which the earthly aspirations by which an ordinary mortal lives are digested and boiled. In the human body, which contains analogues of everything that is present in nature in all its fourfoldness, there is an instrument for the highest purification of the four elements themselves, so that you can gradually achieve a higher state, which the Jews called the state of Yod, since the mysterious Tetragrammaton is contained in the world and is reflected and comprehended in any of its particles and especially visibly in man, who is fourfold in all the infinity of his manifestations.” [Giuliano Kremmerz, "Corpus philosophorum totius magiae"]