Like the Duomo Nuovo, the Baptistry of St John is another building with an unfinished facade, a seemingly recurring theme in Siena. The Baptistry was conceived as an extension to the Duomo but the challenges of the site meant it had to be designed as a separate building lying below the original cathedral. It sits next to the imposing stairs that climb up to the Piazza Jacopo della Quercia, home to the Duomo Nuovo.
The baptistry's design is credited to the master-builder Camaino di Crescentino and its construction started in the early decades of the 14th-century.
The Sienese-Gothic-style of the unfinished marble exterior, however, is believed to be the design of Domenico d'Agostino. Possibly, the decimation of the available workforce by the Black Death is what led to the exterior's unfinished state, but whatever the cause, the city and church authorities never resumed the project. The top part of the facade remains undecorated but what the Baptistry lacks in its exterior, the opulence of its interior certainly makes up for it.
With its font and frescoes, the Baptistry of St John is a fine expression of Sienese Renaissance art. Famous names from the period like Lorenzo di Pietro and Agostino di Marsiglio worked on the gospel-inspired scenes that cover the ceiling in a colorfully flamboyant style.
Later frescoes include paintings of Paul the Apostle and the Madonna along with the saints Joseph, Anne, and Elizabeth. These adorn the beautiful altar triptych. Dating from 1907 is a depiction of the Baptism of Jesus on the high altar, the work of Alessandro Franchi.
The highlight of the Baptistry's interior, however, is its font. This artistic masterpiece dates from the early Tuscan Renaissance and was designed by Jacopo della Quercia.
Carved from marble and featuring a hexagonal design, the font is finished with sculptures capturing key moments from the life of John the Baptist. Among these sculptures, critics generally agree that the emotional scenes from the Feast of Herod created by Donatello in 1427 are the most moving elements of the Baptistry.