Begun in 1482, the chapel was ordered built by the Operaio del Duomo (head of the cathedral administration) Alberto Aringhieri to house the cathedral’s most illustrious relic, donated in 1464 by Pope Pius II, born Piccolomini, in a beautiful silver reliquary: the right arm of Saint John the Baptist, who baptized Jesus in the waters of the Jordan River. The furnishings and decoration of the chapel thus pivot around the figure of John, the last prophet and the forerunner of Christ, since he was the one sent by God to prepare the Hebrew people for the coming of the Messiah.
Erected on the left side of the church, the chapel has a round ground plan and is entered through a marble portal made by Marrina, with reliefs by Antonio Federighi and Giovanni di Stefano in the bases of the two side columns, adorned with motifs borrowed from antiquity.
The space is dominated by the large bronze statue of Saint John the Baptist, a masterpiece of Donatello’s late activity. He made it in Florence before 1457, the year when he brought the statue to Siena, where it was placed for years in the cathedral sacristy before being set in its current location. Intense and vibrant, the figure of Saint John, wearing a camel’s skin, expresses a beauty transfigured by asceticism and mortification of the flesh; the emaciated aspect, deep-sunk eyes, and nervous modeling of the tunic and hair give the image an expressivity that is powerful and dramatic at the same time. On the side walls are marble statues of Saint Ansanus, who first evangelized and baptized the Sienese, and Saint Catherine of Alexandria, carved respectively by Giovanni di Stefano and Neroccio di Bartolomeo in 1487.
At the center of the chapel is the marble font used to contain the water for baptisms blessed during the Easter vigil. Carved by the Sienese sculptor Antonio Federighi before the chapel was built (around 1460), like many baptismal fonts it is octagonal in shape, referring to the mystical significance of the number 8, the symbol of eternal life; the eighth day is the day of Christ’s Resurrection, the dies Domini, which comes after the Sabbath, the seventh day in the Jewish week according to the account in Genesis. Six of the eight reliefs around the sides of the font come right from Genesis: The Creation of Adam, The Creation of Eve, Eve Tempted by the Serpent, Eve Offering the Apple to Adam, God Rebuking Adam and Eve, and the Expulsion from Eden. The other two panels present Hercules Wrestling with the Lion and Cacus (or Hercules) Fighting with the Centaur, reflecting the taste for classical antiquity typical of Federighi and the Renaissance idea of uniting sacred subjects with others taken from mythology.
Along the walls unfolds the biography of Saint John the Baptist, painted by Pinturicchio between 1504 and 1506, before undertaking the Stories from the Life of Pius II in the Piccolomini Library. The six episodes illustrate: The Birth of the Baptist, Saint John the Baptist in the Desert, John the Baptist Preaching to the Crowd, The Baptism of Christ (repainted by
Rustichino in the early seventeenth century), John the Baptist in Prison Visited by the Disciples (redone completely in 1868 by Cesare Maccari), and The Beheading of John the Baptist (also repainted by
Rustichino). Together with these, two other pictures, also painted by Pinturicchio, portray: one, Alberto Aringhieri, who commissioned the frescoes, wearing the cloak of the Order of the Knights of Jerusalem, and the other a kneeling knight (probably a portrait of Aringhieri as a young man).