The gold mosaics that cover a big part of the cupolas, walls and floors were started in 1063. These early mosaics were made by mosaic workers from the East. The mosaic decoration of the twelfth and thirteenth centuries altered profoundly the original Byzantine iconographical orientation because it provided for a Western-style narrative rather than a theological synthesis in the Byzantine manner. The divinity, for example, is represented in three domes (Ascension, and Pentecost) and in the apse (The Pantocrator), thus investing the basilica lengthwise with themes that mature Byzantine tradition reserved for holy days (Ascension, and Pentecost), and certainly would not have placed on domes (Web Gallery of Art mosaics).
In the vestibule or narthex are on entry (west side) and on the left mosaics visible from the 12th and 13th century. These mosaics depict several events from the Old Testament, such as Creation and the lives of Abraham and Joseph.
“The mosaics present scenes from the life of Saint John the Baptist on the walls and, in the ante-baptistery, the infancy of Christ. Directly above the bronze font, designed by Sansovino, the dome contains the dispersion of the Apostles, each shown in the act of baptizing a different nationality in reference to Christ’s command to preach the Gospel to all people. The second dome, above the altar, presents Christ in glory surrounded by the nine angelic choirs. The altar is a large granite rock, which according to tradition was brought to Venice from Tyre following the Venetian conquest. It is said to be the rock upon which Christ stood to preach to the people of Tyre.” Cited from Wikipedia
The end of the mosaic art was heralded when a lot of well-known Venetian painters (Titian, Tinoretto, Salviati) started to make designs for mosaics and a part of the original mosaics was replaced. The artists forgot that a good mosaic does not have the same requirements as a good painting. Mosaics became paintings effected in mosaic. Despite the many additions and destructions, the original medieval scheme has largely been preserved.
The mosaics were cleaned and restored to their original state in the 1970s. Furthermore, art historian Otto Demus ‘The mosaics of San Marco in Venice‘ conducted a large study. In the cathedral, we will not only pay attention to the mosaics above us, but also to the 12th century floor, comprised of a colourful geometric mosaic of antique marble, purple stone and glass with pictures of animals and birds. Click here at Web Gallery of Art for more images of the mosaics (11th-13th centuries).
“The picture shows the mosaic on the right side of the barrel vault of the Mascoli Chapel in St Mark’s. It represents the Visitation and the Dormition of the Virgin. Although Jacopo Bellini and Michele Giambono also worked on the cartoon of the mosaic, the composition of the mosaic is by Andrea, who probably produced the cartoon for the mosaic before leaving Venice in 1442. The project was left to languish until the late 1440s, when Venice was more receptive to Florentine style. Then Bellini and Giambono added their own figures, suggesting either that Andrea left the work unfinished or that it was damaged in the intervening years.” Cited from: Web Gallery of Art