This is an octagonal building, part of the Florence Cathedral complex, and is dedicated to St. John.
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The outside of the Baptistery has the same polychrome marble decorated as the rest of Florence Cathedral. The first floor level has 3 arches, each containing windows, and is the same across all 8 sides. Similarly, on the second level, every side has a small window within a 3-part panel.
The Baptistery is famous for it's 3 doors - on the North, South and East side:
The North Doors
The North Doors were completed in 1422, after 21 years of work by Lorenzo Ghiberti.
There are 28 square panels on the doors. The top 20 of which depict scenes from the life of Christ. The bottom 8 are of the 4 prophets (or Evangelists) and the Church Fathers - St. Ambrose, St. Augustine, St. Jerome and St. Gregory.
Surrounding these panels are busts prophets and sibyls. Additionally, surrounding the actual doors is a framework of foliage.
Above these doors, is a bronze statue by Francesco Rustini depicting St. John preaching to the Pharisee and Sadducee.
The South Doors
The South doors were designed by Andrea Pisano. They were completed in 1336, after it took 6 years for the bronze-casting and gilding to be finished by Leonardo d'Avanzano.
These doors consist of 28 square panels. The top 20 of which depict scenes from the life of St. John the Baptist - to whom the Baptistery is depicted. The bottom 8 are the Virtues - Charity, Faith, Fortitude, Hope, Humility, Justice, Prudence, Temperance.
In 1571, Vicenzo Danti designed a bronze statue which sits above the doors and depicts the beheading of St. John.
The East Doors, 'Gates of Paradise'
The East doors were named by Michelangelo 'The Gates of Paradise'. They were designed by Lorenzo Ghiberti, who completed them in 1452 after 27 years work (his workshop included Michelozzo and Benozzo Gozzoli).
The panels on these doors are slightly different to the others. Here there are 10 rectangular gilded panels. Furthermore, each panel depicts stories from St. Joseph's life but they all show more than one episode in the panel.
These doors were sculpted after the other two and is the most celebrated. Ghiberti here uses many new sculptural techniques where the individuals in the panels are practically free-standing - an extraordinary feat.
Similar to the other two, the doors are surrounded by a foliage framework, and the panels are surrounded by busts of prophets.
The interior is divided into two levels. The ground level has columns supporting the first floor where you can walk through. The inside is also decorated similarly but not exactly like the polychrome marble of the exterior - here it is there is more gilding than white but a similar dark green is used.
On the floor there is a very complex marble mosaic pattern of shapes - particularly octagonal shapes echoing the design of the building. Additionally there is an octagonal font.
The Baptistery has the most incredible mosaic ceiling. The earliest mosaic pieces date back to the 13th Century.
This mosaic cycle starting from the inside out the ceiling depicts Choirs of Angels, Genesis stories, Stories of St. Joseph, Stories of Mary and Christ, and lastly stories of St. John the Baptist.
In the two sections either side of the large depiction of Christ are scenes from the Last Judgement. On Christ's left are the damned, and on his right are the saved.