Mosaic is the art of embedding small pieces of cut stone or pigmented glass in a plaster bed to serve as floor or wall decoration. Developed principally in ancient Greece, mosaic was a leading artistic medium in the Greco-Roman world and reached its greatest heights in Early Christian art and architecture and Byzantine art and architecture.

Solidity, resistance to moisture, durability, and color-fastness made mosaic a practical form of architectural decoration in the warm and humid areas of the Mediterranean basin. The process of constructing a mosaic begins with cubes of cut stone, pigmented glass, or gold or silver leaf sandwiched by glass. These cubes, known as tesserae (singular: tessera), average 1 to 2 sq cm (0.155 to 0.310 sq in) in size, although highly detailed and refined work can demand tesserae one-tenth as large. In preparing a ground for the tesserae, the mosaicist coats the surface of a wall or floor with three thin layers of plaster; on the second layer, the major lines of the design are sketched as a guide for embedding the tesserae in the damp plaster of the third layer. The size, shape, varieties, and color intensities of the tesserae, as well as their patterns of insertion, vary according to period, place, and monument.

Classical and Hellenistic Mosiacs.

The sophisticated mosaics of the classical and medieval worlds evolved from the practice, widespread in ancient Crete and Anatolia, of gathering pebbles from the beach and setting them in a cement bed to provide durable flooring in homes and temples. At first randomly scattered and set, the pebbles later were arranged in simple ornamental patterns. About 400 BC, pebble mosaics began to take pictorial form. In these early efforts, such as the Bellerophon mosaic (before 348 BC; Olynthos, Greece), white pebbles were set in a black-pebble background in order to depict mythological figures and scenes, and to provide ornamental border motifs. In the 3d century BC, cut stones were combined with pebbles for more detailed renderings and to widen the range of colors (see \TGreek art\t).

Although pebble mosaics continued to be used as simple and inexpensive floor covering, they were largely displaced in the Hellenistic era (323-31 BC) by tessellated mosaics of cut stone, colored glass paste, and occasionally of mother-of-pearl, shells, and terra-cotta. Once freed from dependency on the random shapes, sizes, and colors of beach pebbles, Hellenistic mosaicists executed works of great splendor, intricacy, and scale.

Important Hellenistic schools of mosaicists were located at Alexandria in Egypt and Pergamum in Anatolia. Prefabricated mosaic panels, called emblemata, were exported from these cities and were used as the centerpieces for mosaic pavements of lesser quality. These mosaic pavements became (3d century BC) the fashion in the homes and villas of the wealthy throughout the Mediterranean area and the Near East. Typical of the smooth, closely packed surface of Hellenistic mosaics is the Dionysus cycle (2d century BC) that decorates a villa on the island of Delos. Many Hellenistic mosaics, such as the Battle of Alexander from the House of the Faun in Pompeii (c.1st century BC; Museo Nazionale, Naples), present highly detailed scenes of violent action that were renderings of famous but long-lost Hellenistic paintings.

Roman Mosaics.

Hellenistic examples served as models for Roman mosaics until the 1st century AD, when changing aesthetic tastes and economic factors brought about the temporary displacement of polychrome pictorial mosaics by a black-and-white mosaic style (see \TRoman art and architecture\t). Beginning on a small scale in private homes, where black figures and decorative motifs were silhouetted against a field of white marble or limestone, this style soon carpeted the floors of public baths, marketplaces, and other areas of public assembly. Because it withstood the effects of humidity and moisture, and because the tesserae were color-fast, mosaic was often used to decorate garden walls, fountains, and baths in the ancient world. The pavement of the Baths of Neptune at Ostia (2d-3d century) is decorated with Neptune, the sea god, and his court of mythical creatures.

Roman settlers conveyed mosaic art to the farthest reaches of the Empire. Some of the finest Roman mosaics outside Italy have been found in North Africa and Syria. The central panel in a complex and elaborate pavement (AD c.4th century; Louvre, Paris) from the so-called Constantinian Villa in Antioch features hunting scenes set in trapezoidal sections that alternate with female personifications; the whole ensemble is bordered by illusionistic architectural moldings and a lush vine scroll. Athletic contests, the circus, seascapes, and scenes from mythology were also popular subjects in Late Roman floor mosaics. The pavements of the immense imperial villa in \TPiazza Armerina\t (AD 300), Sicily, display these and other themes in a style that presages the abstract character of medieval art in its patterned modeling of large figures distributed seemingly at random in crowded, spaceless fields.

Counterbalancing the extravagance of the Piazza Armerina villa are the geometric pavements that carpeted innumerable secular and religious buildings in the 4th and 5th centuries with colorful and intricate patterns, including the cosmographical and topographical designs favored in Greek and Near Eastern churches. A last, nostalgia-laden effort at emulating classical mosaics appears in the beautiful hunting vignettes bordered by a lush vine scroll that paved the huge court of the imperial palace in Constantinople (now Istanbul), a late-6th-century testimonial to the survival in Byzantine art of classical motifs and styles.

Early Christian and Byzantine Mosaics.

Mosaic as a form of wall decoration achieved its greatest expression in Early Christian and Byzantine art (see \TEarly Christian art and architecture\t and \TByzantine art and architecture\t). The earliest example of the extensive use of mosaic as wall covering is preserved in Rome's \TSanta Costanza\t (c.350), the domed, circular mausoleum of one of the daughters of Emperor Constantine I. Adorning the ambulatory vaults of the mausoleum are geometric and figurative patterns in mosaic, including a splendid version of the popular Hellenistic motif called the "unswept floor"--a scattering of branches, shells, pitchers, bowls, and birds that might have littered a floor after a banquet. The Old and New Testament scenes that once filled the dome have not survived.

Santa Pudenziana, a modest Roman parish church of the late 4th century, contains the oldest surviving apse mosaic. In this monumental but heavily restored composition, a golden-robed, regal Christ is enthroned in the heavenly celestial Jerusalem amid a court of apostles and personifications of the church. The background features a triumphal cross crowning Calvary and buildings that resemble Early Christian landmarks in Bethlehem and Jerusalem; symbols of the four evangelists appear in the cloud-streaked sky above.

In Santa Maria Maggiore, built in Rome by Pope Sixtus III (r. 432-40), mosaic decoration spread throughout the church. An Old Testament cycle rich in Christian prophecies and based in part on illuminated biblical manuscripts fills 27 panels (originally 42) on the nave walls of Santa Maria Maggiore.

Perhaps the richest Early Christian mosaics are those executed for the churches, baptisteries, and mausoleums of \TRavenna\t in the 5th and 6th centuries. In the Mausoleum of Galla Placidia (c.430; see \TGalla Placidia, Mausoleum of\t), patterns of rosettes, meanders, ribbons, and vine scrolls set against a brilliant blue ground cover the vaults, while figurative scenes fill flat areas of wall and a vision of the cross crowns the swirling, star-studded heaven of the dome. The play of light on the polished, tessellated, curving surfaces of the vaults in this and other buildings produces effects unmatched in \Tfresco painting\t. The shimmering surface, the impression of flickering images and of movement renders mosaic a lively and luxurious form of decoration. Because of this, mosaic was exploited as a mural medium even when its durability and moisture-resistance were not required.

In the octagonal Church of \TSan Vitale\t (526-47), Ravenna, the sanctuary is decorated with an elaborate mosaic program that ranges from purely decorative patterns to dogmatic imagery. In two large panels (c.547) the emperor Justinian and the empress Theodora are depicted offering liturgical vessels to the new church. In a naturalistic portrait, Justinian seems weary and troubled. In contrast, the bodies appear to be weightless and without volume, overlapping and impinging on one another like cutouts. This dematerialization of physical form, basic to Byzantine art, was used to great effect in mosaics, where beautifully costumed figures seem to hover weightlessly before a golden surface. This can be seen, for example, in the dome of the church of Saint George (5th century) in Salonika, where 16 saints with arms outspread in prayer (the orans gesture) stand before an exquisite architectural scaffolding that symbolizes the celestial church or heavenly Jerusalem against a golden background.

In Constantinople, the center of Byzantine civilization, relatively few schemes of mosaic decoration are preserved because of natural loss and the destruction wrought by iconoclasts, the Crusaders, and the Ottoman Turks. \THagia Sophia\t, the focus of Byzantine spiritual life, retains little of its original mosaic decoration, but what remains testifies to the unparalleled achievements of Byzantine mosaicists. High in the apse, above a band of windows, is a majestic image of the enthroned Virgin holding the Christ child on her lap and flanked by two archangels. The unevenness of the golden background documents the history of the apse mosaic. The original image (6th century?) was destroyed by the iconoclasts; only after iconoclasm was suppressed did Byzantine artists install (c.867) the present mosaic, in which carefully graded colors and meticulous setting both of stone and of glass-paste tesserae produced an image celebrated even in the 9th century as "lifelike imitation . . . with lips made flesh by the colors."

Also in Hagia Sophia, larger-than-life figures of the Virgin and Saint John flank the central figure of Christ in the Deesis, a mosaic probably executed in the 13th century. Small, finely graded tesserae are closely set in patterns and lines that suggest the sculptured planes of the faces and depict in detail the Virgin's locks of hair and the saint's beard. The mosaicists' precise placement of the tesserae adds to the quality and power of the design; by varying the cubes' angles of insertion into the plaster bed, the artists were able to differentiate, for example, between the golden halos and the golden ground.

In the 11th century, Byzantine artists perfected a decorative scheme in which the symbolic associations of different parts of a church are enhanced by the selection and hierarchy of figures and scenes. Epitomizing this canon of mosaic decoration is the interior of the small 11th-century monastery church at Daphni, near Athens. In the church's dome, the symbol of heaven, is a huge image of Christ Pantocrator, Lord of the Universe; circling the drum are 16 Old Testament prophets. The apse, the next-lofty surface, held an image of the Virgin. Prophets are situated in the drum of the dome. On the wall areas in and around the nave are scenes from Christ's earthly life, including, in the squinches beneath the dome, the Annunciation, the Nativity, the Baptism, and the Transfiguration. Lowest in the hierarchy and placed closest to the faithful are images of saints and martyrs. In addition to symbolizing the microcosm, this decorative scheme evokes the church calendar and local cults, for the scenes illustrating Christ's life parallel the major festivals of the Christian year, and the saints and martyrs represented include those both universally and locally revered.

The refinement of mosaic is evident in the small icons of miniature mosaic intended for private devotion. The tesserae, never larger than 1 mm (.039 in), are set in wax on a wooden plaque. The icons range in size from 5X10 cm (2X4 in) to 20X25 cm (8X10 in). One of the most exquisite examples is Saint John Chrysostom (12.7X17.8 cm/5X7 in; Dumbarton Oaks Collections, Washington, D.C.), executed in Constantinople in the first half of the 14th century.

Economic factors and a change in aesthetic standards contributed to the decline of mosaic after the 14th century. Because it emphasizes the surface it covers, mosaic and the new aesthetic stressing figural volume and spatial depth came into conflict. Fresco, more suited to this aesthetic and far less expensive, then became the leading form of mural decoration in Europe. Some late Byzantine mosaic schemes, such as those of the Church of the Holy Apostles (c.1315) in Salonika and of \TSaint Savior in the Chora\t (1303-20) in Constantinople, retained the brilliant color, graceful forms, and decorative richness that this technically demanding medium produces.

Other Medieval Mosaics.

The nature of its materials and production made mosaic a luxury art prized by Western emperors, princes of the church and state, and wealthy individuals. In medieval Rome the popes kept the art of mosaic alive by sponsoring mosaics in new churches and by restoring works in older churches. The attempt to revive the spirit of Early Christianity during the Carolingian period (see \TCarolingian art and architecture\t) resulted in the building and decoration of a number of churches following traditional architectural types and decorative schemes, as at Santa Maria in Dominica (early 9th century) in Rome. Charlemagne, who was crowned emperor in Rome, was so impressed by its art that he ordered mosaics installed in his octagonal palace chapel (792-805) at Aix-la-Chapelle (now Aachen) and elsewhere. Papal sponsorship again revived the use of mosaics in Rome during the 12th and 13th centuries, when themes such as the Coronation of the Virgin were combined with ancient and Early Christian motifs to produce rich and elegant images such as those adorning the apse (1295) of Santa Maria Maggiore in Rome.

Byzantine mosaicists were called to other cultures to beautify secular and religious monuments. Islamic caliphs, for example, employed Byzantine mosaicists in the 7th and 8th centuries to decorate the Dome of the Rock (c.700) in Jerusalem and the Great Mosque (c.715) in Damascus. During the 11th century, Constantinopolitan artists worked in new churches in Kiev, where, by adapting standardized decorative schemes and training local assistants, they helped implant Byzantine culture in Russia. The Norman rulers of Sicily also employed teams of Byzantine mosaicists to decorate their churches and chapels, as in Monreale Cathedral, whose walls and vaults are covered with traditional Byzantine figural and decorative mosaics (c.1180-90).

In northern Italy, Byzantine-inspired mosaics fill the churches of Venice and of the neighboring island of Torcello. The decorative program in \TSaint Mark's\t \Tchurch\t, executed in the 12th and 13th centuries by successive generations of local artists, has Byzantine antecedents, but it is largely a Venetian creation. Like the Islamic, Russian, and Norman rulers, the Venetians wished to emulate in Saint Mark's the richness and splendor of the the mosaic-sheathed spaces in Byzantine churches.

The Modern Era.

Since the 14th century the art of mosaic has been all but abandoned. Only with the \TGothic Revival\t, beginning in the 18th century, were mosaics used once again for architectural decoration, as in the Houses of Parliament (1840-65; see \TWestminster Palace\t) in London and in the basilica of \TSacre-Coeur\t (1875-1914) in Paris. In the 20th century, mosaics have been revived as appropriate decorations for harsh urban environments that demand durable surface adornment, such as the Mexico City subway system (opened 1968). SUZANNE \TSpain\t

Bibliography: Anthony, Edgar W., History of Mosaics (1935; repr. 1968); Demus, Otto, Byzantine Mosaic Decoration: Aspects of Monumental Art in Byzantium (1948; repr. 1976) and The Mosaics of Norman Sicily (1949); Fischer, Peter, Mosaic: History and Technique, rev. ed. (1971); Kitzinger, Ernst, The Mosaics of Monreale (1960); L'Orange, Hans P., and Nordhagen, P. J., Mosaics, trans. by Ann E. Keep (1966); Oakeshott, Walter F., The Mosaics of Rome: From the Third to the Fourteenth Centuries (1967); Rossi, Ferdinando, Mosaics: A Survey of Their History and Techniques (1970); Volbach, Wolfgang F., Early Christian Art, trans. by Christopher Ligota (1962).