The Treasure Room, so called because it houses the most precious items in the Cathedral’s Treasures, is the first room in the Diocesan Museum. It houses the beautiful De Firmonibus Missal, illuminated in 1436 by Giovanni di Ugolino of Milan, commissioned by Giovanni De Firmonibus. It is open on the representation of the Cavalcade of the Assumption, an ancient city procession celebrating the Assumption of Mary into heaven, a tradition that still takes place every 15 August in the form of a historical re-enactment. This codex, an extraordinary example of Gothic art, contains thirty-one illuminated, decorated or historiated initials, two pages with large friezes and marginal miniatures and two full-page illustrations.
Later, visitors can admire the staurotheke, a reliquary cross donated in 1503 by Francesco Todeschini Piccolomini, the future Pius III. Its construction, with a filigree decoration of clear Venetian manufacture, is however much older. Miniatures are inserted in the tips of the arms. Theprotagonists of the Crucifixion are depicted on the front, with the symbols of the four Evangelists on the rear.
Another precious object is the crosier that belonged to Sixtus V, later donated to the diocese. It is a precious tortoiseshell staff inlaid with mother-of-pearl and with rock crystal in the knot.
Originally located above an altar, the 16th-century bronze tabernacle was made for the Cathedral by the Lombardi-Solari workshop. Its octagonal band depicts scenes from the life of Jesus, while the central drum is decorated with leaves and grape shoots. At the end of the ciborium are statues of the Twelve Apostles and Christ the Redeemer.