A caldarium is a very warm room for bathing, complete with a heated floor and hot plunge pool. In Roman bathing complexes, this was the hottest of a series of rooms customers would move through. When this word is used in a modern context, it usually refers to a spa room with a heated floor that keeps the environment warm and humid. In the caldarium, pores will open wide from the heat, and some bathers can become uncomfortable because of the extreme temperatures.
In the Roman era, bathing complexes were heated using a system known as a hypocaust, a series of furnaces below the floor to heat air and water. Plumbing delivered the water to plunge pools, while the air circulated under the floors to warm them. Some spas were attached to natural hot springs with geothermal energy for heating, in which case the plumbing might run through the floor to heat it with the spring water. The caldarium received the hottest water and air, warming it to just above regular human body temperature or even higher, depending on the spa.
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Bathers could dip into the plunge pool in the caldarium or take advantage of a dry heat area much like a sauna that was often included in the design. The high heat was supposed to facilitate sweating and promote skin health by clearing the pores. Bathers would rub scented oils and exfoliants on themselves, using a device known as a strigil to scrape away sweat, dirt, and bath oils. In some bathing facilities, the floor was so hot that bathers needed to wear sandals to protect their feet, and the air usually swirled with very hot steam.
Some modern spas may refer to a caldarium in the sense of a room with a heated floor and a high internal temperature. The room may also contain lots of warm steam to open the pores. A full Roman caldarium with heated floors and plunge pools is unusual for most spa designs. Spas also usually lack the traditional progression of rooms from frigidarium, the coldest room, through to the caldarium.
Bathers in very hot environments must be careful, because they are at risk of heat stroke. It is important to stay hydrated with fresh water, juice, or herbal teas and to pay attention to feelings like dizziness, extreme lethargy, or confusion. If a bather appears to be too hot, he should be taken out of the room, provided with cool fluids, and cooled in a plunge pool or cold shower.