What is "laconicum"

n. A hot dry sweating room, next to the caldarium in a Roman baths

The laconicum (i.e. Spartan, sc. balneum, bath) was the dry sweating room of the Roman thermae, contiguous to the caldarium or hot room. The name was given to it as being the only form of warm bath that the Spartans admitted. The laconicum was usually a circular room with niches in the axes of the diagonals and was covered by a conical roof with a circular opening at the top, according to Vitruvius (v. 10), from which a brazen shield is suspended by chains, capable of being so lowered and raised as to regulate the temperature. The walls of the laconicum were plastered with marble stucco and painted blue with gold stars.

Sometimes, as in the old baths at Pompeii, the laconicum was provided in an apse at one end of the caldarium, but as a rule it was a separate room raised to a higher temperature and had no bath in it. In addition to the hypocaust under the floor, the wall was lined with ceramic flue pipes. The largest laconicum, about in diameter, was that built by Agrippa in his thermae on the south side of the Pantheon, and is referred to by Cassius (liii.23), who states that, in addition to other works, he constructed the hot bath chamber which he called the Laconicum Gymnasium. All traces of this building are lost; but in the additions made to the thermae of Agrippa by Septimius Severus another laconicum was built farther south, portions of which still exist in the so-called Arco di Ciambella.