The structure found under the floor, near the tower's pillars, is the base of a muffle-kiln, the kiln used for the third firing of lustre ware.

Unlike the kiln of the first chamber (cut from the rock and used for first and second firings), this small kiln was built entirely of brick and tufa blocks without the use of mortar; the "a riverbero" or reverberatory kiln mentioned by Cipriano Piccolpasso in his work «I tre libri dell'Arte del Vasajo» (see figure), which was in circulation in Italy from the 15th to the 16th century.

The product of the third firing was lustre ware, the precious Renaissance ceramics, fragments of which can be seen in the nearby showcase, famous for its iridescent colours and sheen, comparable to gold and precious stones.

It's possible to see also a fragment of a plate made by the famous Mastro Giorgio da Gubbio.

The discovery of this muffle-kiln, in 1998, has added another extremely interesting chapter to the history of Orvieto ceramics, demonstrating that lustre ware was also produced in the city.