This was the hottest room in a Roman bath. At the Baths of Caracalla, the room was 115 feet wide and crowned with a concrete dome.
The hot water and steamy air were designed to open your pores, and water and air temperatures may have risen well above 100 degrees Fahrenheit, with a sticky 100 percent humidity to exaggerate the effect. At the Baths of Caracalla, the caldarium consisted of a large hall that contained a large pool a little over three feet deep. If you had slaves attending you, they might use a pouring dish called a patara to refresh you with cool water.
This room and its waters, like the tepidarium, were heated by the hypocaust, the system's furnace. The hypocaust, below ground and stoked by slaves, heated a tank of water transported by pipe to the appropriate pool.
The furnace heated the air drawn underneath the floor of the caldarium to heat its tiles. You would have probably worn sandals or wooden clogs so as not to scorch your feet. Hot air then rose up through hollowed-out bricks that lined the walls before exiting through chimneys.