Fragments of the slabs of the basin and enclosure of the ancient baptismal font of the Baptistery, made by Florentine artisans in the first quarter of the 13th century and then dismantled in 1577. Most of these were found in 1897-1907, reused as material in the roof of the Baptistery and the surrounding area.

The fragments are mostly rectangular panels, single or coupled, in white marble, carved and inlaid in green marble. Most are carved on only one side, with lacunar forms, including geometric frames and raised rosettes, adorned with geometric inlay motifs.

The shape of the ancient basin and its enclosure is uncertain. Most likely it resembled those still existing in Pisa and Pistoia. The masters who built it were the same ones who worked in the presbytery area and on the pulpit of the Basilica of San Miniato, which is decorated with almost identical slabs. Recently, seven marble slabs of the Baptistery were found in the church of San Francesco, in Sarteano (Siena).