A sauna is room or house designed to induce sweating, for either hygienic or medical reasons. The sweat can be induced by wet heat (steam) or dry heat. In the Vinaya, the rules for monks and nuns, the sauna is referred to as a bathroom (jantaghara) or more specifically as a fire room (aggisala).
The Vinaya gives a detailed description of how these saunas were designed and used. They consisted of a room with tight-fitting doors, windows and ceiling to keep the heat in. Seats and benches were arranged around a fire and there were bowls or troughs of water for sprinkling on the body and on rocks heated by the fire. Clothes were hung on wall pegs, drains led excess water away and pipes let the steam out (Vin.II,120-1).
Monks were not allowed to be naked in the sauna. Nudity is not acceptable in Buddhism as monastic and lay people are expected to follow the normal conventions of society for clothing and appearance. See: Clothes & fashion and Buddhist fashion.
The normal procedure was to smear fine clay on the body, probably as an abrasive to remove old skin, and rub or massage the limbs until the clay was washed off by the sweat. There were four kinds of ‘sweating treatment’ (sambharaseda) – using steam made from water with certain herbs it, steam made from water with cannabis in it, ‘great sweating’ and udakakottaka, which may have meant soaking in a tub of very hot water (Vin.I,205).
Indian Buddhist monks introduced the sauna to China from where it spread to Korea and eventually to Japan. The Daito Seiiki Diary by Genjo Sanzo（602-664CE, mentions Chinese Buddhist temple with saunas that were open to the public. They also provided medicine and food for the benefit of the poor and the sick. From the introduction of Buddhism in Japan during the Nara Period, many of the larger temples had saunas from which the modern baths, the sento, evolved. In the beginning these baths were meant mainly for the monks but occasionally they were open to others. Records mention that the wife of the Emperor Shomu, Koumyou Kougou (701-760 CE) allowed the sick the opportunity to bathe six days every month and even personally washed them. From the Kamakura period (1185－1333), it was normal to make temple baths available for the sick. When public saunas and baths were established away from temples they continued to be built in the style of temples, a tradition that continues even today. Since the 1960s when Japanese houses were more commonly designed with bathrooms, public saunas and baths have declined in popularity.
- http://sdhammika.blogspot.com/ Feb. 23, 2010