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 . . .