Mosaics are perhaps the most representative art form of Byzantine art, and were the most elaborate and expensive form of mural decoration employed in Byzantium. Unlike the fresco technique, which involves painting on wet plaster, mosaic is an additive medium that relies on the placement of small pieces of material, the tesserae, to create an image. This results in a strong emphasis on line and contour, which can be challenging to overcome.

However, the use of microscopic cubes, as seen in miniature mosaic icons from the 11th century onwards, can help to soften the overall effect. Despite these limitations, mosaic is capable of great subtlety and complexity, with artists using hundreds of shades of color to create intricate and lifelike images.

From the 4th century onwards, the tolerance of Christianity allowed for the construction of churches and the expansion of mosaic art beyond floor mosaics. It was deemed more appropriate to depict sacred figures and biblical scenes on walls and ceilings rather than on the ground, where they might be walked upon. In late antiquity, wall mosaics were less extensive than floor mosaics and were typically limited to domes and apses. However, during the reign of Justinian I, a new model was established at Hagia Sophia in Constantinople, which featured marble floors and mosaic-covered upper surfaces. This period marked the peak of mosaic usage, with some of the finest examples surviving at the monastery of St. Catherine at Sinai, Porec, and Ravenna, as well as at other sites in the Byzantine Empire.

Mosaic art soon became an important Byzantine export, with the Arabs importing mosaic cubes and workers for the decoration of the Umayyad Mosque in Damascus in the early 8th century. Pope John VII also employed Byzantine mosaicists for his oratory in St. Peter’s in Rome. By the late 8th century, mosaics of holy figures were a common feature of sacred decoration, and the economic revival of the 9th and 10th centuries saw the frequent use of mosaics in churches and private chapels in Constantinople.

Mosaics were also a popular choice for palace decoration, as evidenced by the Kainourgion at the Great Palace built by Basil I and the epic of Digenes Akritas. Itwas the preferred technique for imperial portraits in Hagia Sophia from the 9th to the 11th century and was also favored by Emperor Manuel I for historical scenes in the 12th century. However, none of the secular mural mosaics have survived, even though the Ruggero room in the Norman Palace in Palermo may provide an idea of the lavish decorations of the Byzantine palaces.

In emulation of Empress Helena, Manuel I may have sent mosaic cubes and craftsmen, such as Ephraim, to Bethlehem for the Church of the Nativity. Clavijo describes mosaics in the church and cloister of the Peribleptos monastery in Constantinople, as well as in St. George of Mangana, which date to the 12th or 13th century. Large areas of the Holy Apostles in Constantinople were decorated with mosaics by Eulalios in the 12th century. The 11th and 12th centuries were a high point for mosaic art, with artists and materials being transported over great distances for projects such as Hosios Loukas, the Nea Mone on Chios, and Daphni. In the early 11th century, mosaicists were sent to Kiev to embellish St. Sophia and teach local workmen the craft. A similar importation likely occurred during the decoration of San Marco in Venice, and mosaicists were among the craftsmen brought from Constantinople in the 11th century by Desiderius of Montecassino.

From the 13th century onward, mosaic was used only in the most lavish projects in Constantinople and, exceptionally, in Arta. The mosaic of the Deesis in Hagia Sophia, dating to the late 13th century, may have been an imperial commission, while later programs, such as those at the Chora and Pammakaristos in Constantinople and the Holy Apostles in Thessalonike, were generally sponsored by the bureaucratic or ecclesiastical elite, often in conjunction with fresco decoration. The last major mosaic project in the capital was at Hagia Sophia following the partial collapse of the dome in 1346. Shortly after 1355, the Pantokrator in the dome was restored, and images of John V Palaiologos, John the Baptist, and the Virgin were installed on the great eastern arch. The mosaics on the eastern arch, which had been covered by plaster for centuries, were rediscovered in 1989.

The increasing use of mosaic for wall decoration allowed for the incorporation of a wider range of fragile materials in the tesserae. In addition to the multicolored stone and marble commonly used in floor mosaics, artists also used brick or terra cotta, semiprecious gems, and opaque colored glass. Gold and silver tesserae were created by layering foil between translucent glass. The size of the tesserae varied greatly, with the smallest being used for modeling faces and other important details. Mosaicists typically followed preliminary painted guidelines and impressed the tesserae into a setting bed laid over previous plaster strata.

While tesserae could be produced in small local workshops, such as at Masada in the early 5th century, large-scale mosaic decoration required significant financial investment and industrial organization. The mosaic in the apse of Hagia Sophia in Constantinople, for example, required almost 2.5 million tesserae “smeared,” as Photios described it, “with gold.” The amount of space a mosaicist could cover in a day depended on the size of the tesserae used, but one mosaicist could cover up to four square meters per day.