Art historians usually say that, in Italy, Byzantine mosaics were done after Emperor Justinian. They tried to reunite the Byzantine empire’s two halves during his reign from 527-565. Before that, mosaics still were in the ancient Roman, classical style. So it wasn’t until the 6th century that the Byzantine style really was codified. It was characterized by a severe, ethereal style that was much different from the naturalism of ancient Roman art.
