In many illustrated alchemical manuscripts one may find images of furnaces. Starting from the sixteenth century they are often depicted allegorically as well as vials, alembics, and other laboratory utensils. Furnaces were sometimes symbolized by characters connected with fire: hell mouth, demon, or dragon. In other cases, alchemical ovens were depicted as buildings due to their similar form. A tree was also a symbol of an alchemical oven. These iconographical types connected alchemy with something significant outside of alchemy: be it technology, religion, or mythology. This paper argues that the sources for such allegorical imagery could lie in manuals on military technology as Bellifortis or De re militari.