1911 Encyclopædia Britannica/Sudatorium

SUDATORIUM, the term in architecture for the vaulted sweating-room (sudor, sweat) of the Roman thermae, referred to in Vitruvius (v. 2), and there called the concamerata sudatio. In order to obtain the great heat required, the whole wall was lined with vertical terra-cotta flue pipes of rectangular section, placed side by side, through which the hot air and the smoke from the suspensura passed to an exit in the roof.