Pl.: testudines

A Latin term literally meaning a tortoise shell, it is also used as a technical term to denote the half-cylindrical metal container placed above a furnace, with its flat side facing down and in direct contact with the bottom of the alveus. The end towards the alveus was open: as the colder water in the pool sank towards the bottom, it was reheated inside the testudo and then rose again by convection. This cyclical process ensured that the water inside the alveus stayed warm.

An alveus in the Stabian Baths of Pompeii that was originally equipped with a testudo. The fact that the intrados of the arch of the furnace is at a higher level than the floor of the alveus, as well as the cutting in the floor of the alveus around the furnace, are clear indicators that this pool was equipped with an testudo that was later robbed (photo Konogan Beaufay).