A Romanesque octagonal structure, built at the end of the 11th century over the ancient ruins of a fifth-century Roman temple dedicated to Mars. Clad in the exterior with two-toned marble and dedicated to St. John the Baptist, patron saint of the city of Florence, the structure served as the place where all Florentine citizens were baptized. In the 13th century, a major decorative campaign to embellish the building's interior was launched. The octagonal vault was covered with mosaics that depict scenes from the Creation, the Last Judgment, angels, apostles, and the lives of Patriarch Joseph, St. John the Baptist, and Christ, all arranged in concentric bands. Above the altar is a large figure of Christ as judge, sitting on a rainbow and enclosed in a heavenly circle. These mosaics have been attributed to Coppo di Marcovaldo, though some believe them to have been executed by masters from Venice, where the medium was more commonly used. In 1330-1334, the sculptor Andrea Pisano was commissioned to create a set of gilded bronze doors for the Baptistery, composed of 14 rectangular reliefs. The upper 10 panels represent scenes from the life of the Baptist, from the announcement of his birth made to his father Zacharias, to his decollation and burial. The remaining lower panels contain the Virtues.In 1401, a major competition was launched for the execution of the Baptistery's east doors. It was won by Lorenzo Ghiberti who submitted a Sacrifice of Isaac panel that conformed to the new classicism expected in art. In 1425-1452, Ghiberti also executed the final set of doors that led from the Baptistery to the Cathedral of Florence, called the Gates of Paradise.
Historical dictionary of Renaissance art. Lilian H. Zirpolo. 2008.