Caldarium

The caldarium is the hot room of a Roman thermae, or public bath. It housed a hot bath and the greatest amount of heat in the bath complex, serving as the main site for sweating and cleansing during a typical bathing routine.

Heat was produced by a hypocaust system: a furnace or praefurnia heated air that circulated beneath a raised floor and through flues in the walls. The room was kept hot and humid, with heat supplied by flames and hot air directed into the space. Temperatures were high, typically around 40–50°C.

Architectural features included vaulted or domed ceilings, walls faced with brick, stucco, or marble, and a large hot bath basin. The caldarium might contain a raised rim around the pool for easier immersion, and seating or platforms for bathers to rest. Decorations and marble finishes reinforced its luxurious character.

In the standard sequence of thermae rooms, bathers moved from the apodyterium (undressing room) to the tepidarium (lukewarm), then to the caldarium (hot), and finally to the frigidarium (cold). The precise layout varied by site, with monumental baths like the Baths of Caracalla and the Baths of Diocletian offering extensive caldaria, while smaller establishments had more compact arrangements.

The term caldarium derives from Latin caldus, meaning hot. In modern spa contexts, caldarium is sometimes used to describe a hot, sauna-like space, preserving the Roman emphasis on high-temperature bathing and sweating.