Piranesi’s technical rendering of the hypocaust, or central heating system, for the Baths of Caracalla exhibits his understanding of the structure and function of Roman engineering. It is also part of his fierce commitment to the superiority of Roman engineering and aesthetics. Furnaces on the perimeter of the chamber burn wood or coal, filling a chamber with hot air that rises to heat the room above it. In thermal structures, a hypocaust is used to heat reservoirs of water used for the hot bath, or caldarium. A series of piers fill the chamber and direct hot air flow. The floor and water above are heated as the heat distributes across the chamber and rises. Chimneys are built into the walls surrounding the hypocaust, releasing smoke from the chamber.  

As Piranesi’s explanation moves from the top to the bottom of the page—beginning with the aqueduct above ground and descending to the hypocaust below—so too does the sequence of his alphabetic annotations, first addressing the cross-sections above and then the detailed perspective of the hypocaust below. Beginning in the top-left corner, the aqueduct is shown in cross-section, a technical convention of the architect, and labeled as such in the caption as “spaccato.” The aqueduct (A) delivers water atop its arcuated system into a “bottino” (reservoir) (B). The water then flows down into the chambers below through a hole in the vault (C). In this chamber, the vaulting is not visible, and the ceiling looks flat. Underground is the network of furnaces (F), which heat the water. The space is composed of rectangular piers, visible in the section and in the magnified perspective view at the print’s center.  

At the top righthand corner, another cross-section of the chambers is shown in line with the first, but rotated at ninety degrees, revealing the vaulting of the chambers. This section also reveals a curious feature of the hypocaust at ground level: the floor is on a slight incline. In the book of De Architectura dedicated to public buildings, Vitruvius refers to hypocausts as “suspended floors,” necessary to build a caldarium. He provides the following explanation for the sloping floors: “First, the floor is laid with one and one-half foot tiles that incline toward the furnace so that if a ball is thrown in it cannot stay in place but returns to the furnace of its own accord. In this way, flame will circulate more easily under the suspended floor” (72). Piranesi’s print faithfully reproduces this distinctive feature, which is visible in section.   

A detailed, zoomed-in perspective of the hypocaust furnace occupies the center of the image, beneath the two sections. The floor level of brick (N) covers a level of tiles (M). A series of masonry blocks support the floor above it and distribute the flow of heat. The walls of the reservoir, shown both at a distance (I) and closeup (P), are built in the opus testaceum manner, which was common for monumental structures such as baths. Triangular cut bricks are placed into a concrete center. The “tubi” (pipes) (K) function as chimneys in the reservoir walls, releasing smoke from the hypocaust.  

Piranesi’s choice to focus heavily on waterworks in Le Antichità Romane carries particular significance in his aesthetic theories. Aqueducts and baths were distinctly Roman, promulgating Roman superiority in the Greco-Roman debates (Kantor-Kazovsky 117). Romans had aqueducts and the advanced engineering that accompanied them—in this case, the ability to heat water for baths. For Piranesi, hypocausts are a feat of Roman engineering, and worthy of a detailed, technical study. As opposed to Piranesi’s more Romantic views of ruins across Le Antichità, this image provides both a reconstruction and explanation of how this heating system worked at the Baths. While the majesty of bath architecture is viewed above ground, it is the complex, subterranean world that made bathing possible. Later in the volume, Piranesi includes a reconstructed plan of the sprawling Baths of Caracalla complex. But before the footprint of the bath’s architecture can be imagined and reconstructed, Piranesi demonstrates the ingenuity involved in the practical consideration of heating the bath’s water. (SAH)