In alchemy, an athanor (Arabic: التنور, at-tannūr) is a furnace used to provide a uniform and constant heat for alchemical digestion. Etymologically, it descends from a number of Arabic texts of the period of the Califate which use the term "al-tannoor" in talismanic alchemy, meaning a bread-oven, from which the design portrayed evidently descends.

The athanor was also called Piger Henricus ("Slow Henry"), because it was chiefly used in slower operations, and because when once filled with coals, it keeps burning a long time. For this reason the Greeks referred to it as "giving no trouble", as it did not need to be continually attended. It was also called the Philosophical furnace, Furnace of Arcana, or popularly, the Tower furnace.