noun: (historical) A furnace or stove designed and used to maintain uniform heat, primarily used by alchemists.
Oswald Wirth
"Man is the self-feeding furnace, the athanor in which the philosopher’s pure gold is ripened. Bring about in yourself the ideal of creation to make your Microcosm conform to the harmony of the Macrocosm, for such is the final objective of the wise man." [Tarot Of The Magicians by Oswald Wirth]
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"]
Raymond Lulle
"We will now speak of our Furnace, but it will be very unfortunate for us to report here the secret of our Furnace, which the ancient Philosophers hid so much; for we have depicted in our Books various Furnaces: nevertheless I sincerely declare to you that we only use one Furnace, which is called Athanor, the meaning of which is to be an immortal fire, because it always gives the fire equally and continual in the same degree, vivifying and nourishing our compound from the beginning to the end of our Stone.
O children of doctrine, listen to our words, and hear; our Furnace is composed of two parts, they must be well sealed in all the joints of its enclosure; such is the nature of this Furnace; whether the furnace is made large or small, according to the quantity of matter requires a large Furnace, the small one small; it must be made in the manner of a distilling furnace with its lid, that it be well closed and closed; so when the Furnace has been composed with its lid, make sure that there is a ventilator at the bottom, so that the heat of the lighted fire can breathe there; for Furnace this nature of fire requires and demands this only Furnace, and not another; and the closure of the joints of our Furnace is called the Seal of Hermes, as it was known only to the Sages, and is in no place expressed by any of the Philosophers; for it is reserved in Wisdom, especially as it guards it by a common power." [Elucidation or Clarification of the Will of Raymond Lulle]