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The cameraThe hypocaust, or "fire underneath", was below ground and stoked by slaves. It consisted of a heated tank of water, which was transported by pipe to the appropriate pool. Specifically, the heat was provided through the stone channels of the aqueduct. Hence, the hypocaust can be seen as an early form of the furnace.
Its origins lie in a story that is greatly disputed amongst historians. It is believed that a man named Sergius Orata, who wanted to sell oysters and figured he could do so only if they were sold in warm water, came up with this system. So, he designed a system in which he put the oysters on pillars and built a fire underneath them. Then, he made it possible to adjust the size of the fire to the preferred heat for the oysters.
The hypocaust's heating system was used mainly for the Caldarium. Generally, three or four walls of the Caldarium and one wall of the Tepidarium were heated by this system. In particularly large baths, there was a dry-heat sweating room in addition to the Caldarium called the Sudatorium. In these two hot rooms, the hot water was supplied by the tanks integrated with this heating system.
Fig. 1 Hypocaust
Photo Courtesy of: http://www.vroma.org
The system itself consists of a few levels, as seen above. The first is a series of pillae that raised the floor about 3 feet above the foundation. Slaves needed to be able to fit between the pillae for cleaning and repair so they were spaced accordingly. Next was a layer of tile called the bipedalis. The Romans typically left these tiles to dry for months or even years on end. Then, there was a layer of concrete 4-5 inches thick. Finally, there was a layer of marble on top. Builders made sure to have four chimneys that helped promote the circulation of heat and gases as well as to let out the smoke. Until the 1st Century A.D., the heating came solely from under the floor. After that, beginning in Turkey, there was an additional system that consisted of lining the walls in "tubuli" that circulated heat vertically and horizontally throughout the walls.
Fig. 2 Pillae
Photo courtesy of: http://www.dirtydexterdesigns.com/bath.html
*For much more information on the Hypocaust and its usage today, please visit my page on the Caldarium.
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