The word foculus is a diminutive form of focus, which itself means "a hearth," "a fireplace," or a "coal pan." Foculus, a bronze or iron container, round or rectangular in shape, large or small in size, can be used for a brazier or a fire-pan. Filled with hot coals, it served as a portable chafing dish for food (see Plautus, Persa 1.3.24 and Captivi 4.2.67) or as a room heater in one of the hot rooms of the baths or in the home to ward off the cold in winter or, as here in this passage, to provide for a pre-dinner bath.
The model below is 7 feet long by 2 feet 6 inches wide. It was discovered in the tepidarium of the Old Baths at Pompeii, where bathers sat to perspire, be oiled and scraped clean with a strigil, toweled off, and annointed.