Greek mosaic floors: 4th century BC

In 348 BC Philip of Macedon attacks and destroys Olynthus, whose inhabitants have been unwise enough to rebel against him. Any survivors abandon the town, which is forgotten until it is excavated in the 1920s.

The excavation reveals a fact previously unknown. Well-to-do Greeks of the 4th century have their floors covered in elaborate mosaics, consisting of pictorial scenes set within a succession of borders, much like the design of a carpet. Many of the houses in Olynthus have floors of this kind. Simpler versions of patterned floors are known from several hundred years earlier in Crete and in mainland Greece, but the Olynthus designs are much more advanced - constituting virtually a new art form.

The mosaicists of Olynthus use natural pebbles, limited mainly to black for the background and white for the figures. A few stones of different colours are included to improve the effect, but they are used complete - showing a rounded surface.

During the next century Greek mosaicists become more ambitious. They use small cubes cut from stone, to give a greater range of colour, and sometimes they add fragments of coloured glass. These are the two varieties of small cube, known as tesserae, which become the basic ingredients of all subsequent mosaic. As the tesserae become brighter and smaller, there is little limit to the pictorial effects which can be achieved.

Mosaic in the Roman empire: 1st c. BC - 3rd c. AD

Mosaic spreads through the Hellenistic world, and is brought by Greek craftsmen to Italy - as revealed in the amazing examples from Pompeii (for example, the dramatic image of Alexander and darius in battle).

The Romans carry the art further afield. Soon, throughout the empire, rich villas have impressive mosaic floors. They are often laid by local craftsmen (invariably the tesserae are from materials of the surrounding district). Many of the views are charming scenes of life in and around a villa. The images are copied from existing patterns rather than being original works of art, but the results are often impressive - particularly in several north African villas, and in one spectacular example in Sicily.

The great Roman villa near Piazza Armerina in Sicily, built in about AD 300, has mosaic floors which were probably laid by craftsmen from north Africa. Originally they covered some 4200 square yards. Of their many lively scenes none has given more delight than the group of bikini-clad maidens playing a musical game with a ball.

The mosaics of Piazza Armerina are of the early 4th century. By that time the bishop of Aquileia in northern Italy is adapting this Roman art form to his own polemical purposes. He commissions for the floor of his church a splendid mosaic depicting all the scenes in the dramatic story of Jonah and the whale. It begins the great tradition of Christian mosaic.

The Christian tradition: from the 4th century AD

The turning point for mosaic, as an art form, is the use of it by Christians to decorate the walls of churches rather than the floor. Two of the earliest examples are in Rome. Santa Costanza, built in about AD 350 as the tomb for a daughter of Constantine, has lively mosaics on pagan themes decorating its vault. More significant, as a foretaste of things to come, is the mosaic in the apse of Santa Pudenziana.

Dating from about 390 (though much restored), it shows Jesus on a throne. His apostles support him on either side. The regal nature of the image, very different from the good shepherd of the early Christians, prefigures the Christ in Majesty depicted so forcefully in later Byzantine tradition.

Even more significant are the mosaics in a Roman church of the following century, Santa Maria Maggiore. It is built in about 435 by pope Sixtus III, who commissions mosaics to decorate spaces on its walls. These spaces are small and far from the ground (for this is essentally a Roman basilica, with two great rows of columns providing the main feature), but the content and treatment of the mosaics prefigures much in later Christian art.

Rectangles above the columns depict scenes from the Old Testament. Such narrative panels will produce rich glories in late medieval frescoes (as for example at Padua). But the triumphal arch over the altar of Santa Maria Maggiore offers an even more inspiring example for the future.

The dominant figure in the scenes on the arch is the Virgin Mary. In the top-left corner (the beginning, when reading the sequence as a narrative) she sits enthroned for the Annunciation, bejewelled and in a golden robe. Her pride of place and her regal appearance reflect the fact that the church is dedicated to her. But this is also a political gesture by the pope. The most controversial issue in church politics of this period is the nature of the Virgin Mary.

Is she theotokos (the 'bearer of God')? Nestorius, who denies that she is, has been condemned as a heretic only a few years previously at the council of Ephesus.

In commissioning Santa Maria Maggiore and its mosaics the pope makes plain where he stands. A regal Mary in the first scene, surrounded by angels, receives the news that she will give birth to Christ; and the following scenes concentrate on her son's childhood.

The sequence introduces one of the most productive themes of Christian art - Mary and the infant Christ, whose scenes form only a small part of the Gospels but who will inspire countless painters and sculptors. Here they are formal figures in the golden convention of mosaic. It remains for the painters of later centuries to develop the emotional side of this most human of Christian themes.

Ravenna city of mosaic: 5th - 6th century AD

The town of Ravenna becomes a place of importance early in the 5th century when the western emperor, Honorius, moves his capital there from Rome to escape the advancing Huns. Well fortified and with a safe harbour, it remains until 751 the place from which Byzantines and barbarians in turn administer Italy.

The Byzantine rulers and the greatest of the barbarians, Theodoric, decorate the holy buildings of Ravenna in glittering mosaic, the medium which by now almost symbolizes the might of Christian rule within the Roman empire.

The earliest of the surviving mosaics in Ravenna are in the simple brick building, in the shape of a cross with an interior dome, which Honorius creates as a mausoleum for his sister Galla Placidia (she dies in 450). The images here are more natural and informal than in the later art of Ravenna. A friendly stag drinks at the fountain of life; a young-looking Jesus, the Good Shepherd, almost reclines in a wooded landscape while he strokes an adoring sheep.

From the same period is the baptistery (variously known as 'orthodox' or 'Neon') with its superb dome mosaics of the twelve apostles, in relatively informal poses, around a circular scene of the baptism of Jesus.

The best-known building in Ravenna associated with Theodoric, the Ostrogothic king, is his own mausoleum - with its dome carved from a single block of stone. But the Arian cathedral built for him in the early 6th century also survives (under its new name as the Roman Catholic church of Sant'Apollinare Nuovo), and some of its mosaics are from his period.

They include scenes from the life of Christ together with views of Ravenna and Theodoric's palace. Originally there were also portraits of Theodoric, his family and officials, until these were replaced by later Byzantine rulers - objecting to the personnel rather than the principle, for Ravenna's most famous Byzantine mosaics are of an emperor, his empress and their retinues.

On one wall of the choir of San Vitale in Ravenna, built for Justinian and consecrated in AD 547, the emperor stands with crown and a golden halo. On his left are his bishop, Maximian, and two priests holding a bejewelled book and a censer. On his right stand other priests and soldiers. One holds a great shield decorated with the Chi-Rho, the symbol of Christian armies ever since the victory of Constantine in 312.

On the facing wall is a more unusual group. Justinian's wife, the empress Theodora, stands in equal dignity, with magnificent crown and jewels, accompanied by her own priests and superbly robed women.

In the half-dome of the apse, above and beyond the imperial retinues, Christ sits on the globe of the world with saints beside him. He is still the youthful, unbearded Christ of early Christian art. But he is also now Christ in Majesty - the role in which, in mosaic, he will dominate the interior of so many Byzantine churches.

The imperial Christian hierarchy of Byzantium is never again so tellingly depicted as in San Vitale in Ravenna. But the mosaic medium in which it is expressed will have many other such peaks of creative splendour - in particular in parts of western Europe influenced by Byzantine culture, such as Sicily and Venice.

Capella Palatina in Palermo: 1132-1189

The small palace chapel in Palermo, with its walls covered in bright pictorial mosaic, is one of the most exquisite buildings of the Middle Ages. Known as the Capella Palatina (Latin for 'palace chapel'), it is begun in 1132 and completed in about 1189.

The mosaics are in the Greek tradition, created by craftsmen from Constantinople. Christ pantocrator is in the apse and cupola, in traditional Byzantine style. Round the walls are sequences of scenes from the Old Testament, and from the lives of St Peter and St Paul. This is a narrative convention which will later be much used in Italian frescoes.

The roof of the Capella Palatina, by contrast, is unlike anything in a Byzantine church. In vaulted wood, carved and painted in intricate patterns, it would seem at home in a pavilion of a Muslim palace or in a covered section of a mosque. The sturdy round arches supporting the walls are from yet another tradition - that of European Romanesque. Classical pillars, inherited from an earlier period of Sicily's rich history, complete the influences seen in this eclectic building.

It perfectly encapsulates the merits of Norman Sicily.

Monreale: 1172-1189

The cathedral of Monreale, close to Palermo, has mosaics on a much larger scale than those in the Capella Palatina. They date from the same period (though now considerably restored), but they are less delicate than the scenes in the royal chapel. They are probably the work of Sicilian craftsmen trained in the Byzantine tradition. Nevertheless the interior of Monreale is an impressive sight. The figure of Christ Pantocrator dominates from his position in the apse above the altar. The church glows like a golden treasure trove because of the background colour of the mosaics.

This sense of golden profusion can be felt even more powerfully in the last great church interior of the Byzantine tradition - St Mark's in Venice.

Venetian mosaics: 12th - 13th century AD

Venice's long link with Constantinople is evident in the mosaics, in the Byzantine style, for which the islands of the lagoon are famous. The earliest are on Torcello, the first centre of the Venetian state, where the cathedral apse contains a superb 13th-century image of the Virgin and Child.

The entire west wall of the cathedral is occupied by a vast mosaic of the same period depicting the Last Judgement. The subject is more characteristic of the western church than of Byzantium, as is the somewhat radical manner in which figures of authority are prominently displayed among the damned - including the Byzantine emperor, the Venetian doge and the German emperor.

When the Torcello mosaics are being installed, this cathedral is no longer the most important one in the Venetian lagoon. That honour has passed to St Mark's, where craftsmen in mosaic are busy at the same period. Their labours produce probably the most sumptuous church interior in the world, with every corner a sombre glittering gold. It has been calculated that the mosaics of St Mark's cover an area of about an acre.

Dating mainly from the 12th and 13th centuries, these Italian mosaics represent the culmination of a great Byzantine tradition. But at the end of the 13th century Italy also provides a new beginning in an equally great theme in the history of art - that of the European Fresco.