On the decline of the Western Empire, this, like the other arts, was practised principally in Constantinople, where a school of workers in mosaic was formed, which for many centuries furnished artists for Italy, and through Italy for the rest of Europe : and, until the revival of art, mosaic was the usual method adopted for mural decoration in the churches of Italy. The Byzantine type of Christian mosaic dates from the 4th century, and continues down to the 12th, when it began to be superseded by a native Italian style. The walls and vaulted ceilings as well as the pavements of several of the basilicas in Rome and its vicinity are decorated with Byzantine mosaics of the 4th and rth centuries. One of the best known of these is that of Santa Maria Maggiore, hut the mosaics have been much repaired. An older mosaic is the representation of Cluist and two Apostles, in the church of Sta. Constantia, erected by the Emperor Constantine. Some remains' of the Roman school, however, seem to have lingered on, and the Christ in the church of SS. COSMO and Damiano, of the 6th century, is believed to be quite free from Byzan tine influence. Later, however, this influence was paramount, and extended even to England. Churches now mostly had in their apses a huge mosaic of Christ, his hand resting on a book, either above or surrounded by his apostles, technically called a Majesty. Some of the best examples of these are in the churches of Ravenna ; but the same uncouth, expressionlees, conventional type is apparent in all. By the 8th century, Byzantine mosaic had degenerated into mere rigid manu facture, carried on by fixed monastic rules, and it did not revive till the time of the Comneni in the 12th century, and then soon to relapse into its previous state. _ Some of the Byzantine mosaics, however, of this date have a certain splendour of effect from being of large size, and executed in vivid colours on a gold ground. The tesserre of which the Byzantine mosaics are formed, are very irregular in size, and the work manship is often extremely coarse. Many of these Byzantine mosaics are remarkable for their large amount of Orientalism of character. Among the rich examples in the cathedral of Monreale, near Palermo, in the church of San Miniaro of Florence, and elsewhere, aro patterns almost exactly like those found in Saracenic tracery.
One of the most remarkable mosaic works in Italy consists of the decorations, external as well as internal, of the church of St. Mark at Venice, which extend in all over some thousands of square feet of surface. For this great undertaking a very large number of Byzantine workmen were in the 11th century brought from Constantinople; and even some of the mosaics were, it is said, imported also. But native art was beginning at this time to revive in Italy, and this and other great undertakings no doubt acted as a strong stimulus to native genius. In the 13th century, Andrea Tafi, Gaddo Gaddi, and even Cimabue and Giotto, designed mosaics, and in a newer and nobler style than the people had been accustomed to see. Italian musaicisti began to he employed upon the works of St. Mark, which were, with all the wealth of Venice, prolonged over several centuries ; and eventually a native school was formed, which carried the art to its utmost perfection, so that, as Vasari observed, "it would be impossible to accomplish greater things with colours." Indeed, as Lanzi remarks, " The church of St. Mark remains an invaluable museum, where, commencing with the 11th century, we may trace the gradual progress of design belonging to each age up to the present, as exhibited in many works in mosaic, beginning from the Greeks and continued by the Italians."
Mosaic was in its most flourishing condition in the 16th century, when the Byzantine had come to be throughout Italy, with the exception perhaps of Naples, superseded by a native school. At this time the most distinguished painters—as Titian and Veronese in Venice, and Raffaelle in Rome—did not disdain to -make designs for execution in mosaic. But it eoon after declined from its pre-eminence. Before fresco painting, in the hands of the great masters who practised it, mosaic was compelled to give way ; and though it found ample patro nage from the popes for the decoration of St. Peter's—the most mar vellous work executed in mosaic subsequent to St. Marles,—and has continued to he prosecuted with continued success down to the present time, it has long been employed mainly in producing copies of pictures, and not in embodying original design.
Mosaics of this kind came into vogue in the 17th century, when, in consequence of some of the frescoes of Raffaelle, Da Vinci, and other great painters, having already decayed from being painted on damp walls, it was desired to have comparatively imperishable copies of them ; and this kind of work is that now almost exclusively practised. In the Vatican there is a workshop devoted exclusively to preparing the mosaics of St. Peter's, and a similar establishment is maintained in connection with the cathedral of Monreale in Sicily. Mosaics of this kind are facsimiles of the original pictures, and have merely the effect of paintings produced in the usual way, although beyond all com parison more laborious and tedious in their process, and necessarily, from the nature of the process, lacking their freedom and play of hand. As each separate piece of glass is of the same colour-through out, the graduation of tints, the melting off of any one coldur from its highest light to its darkest shadow, can be obtained only by an immense number of small pieces, of which those contiguous to each other exhibit scarcely any perceptible difference to the eye. It has been said that no fewer than ten thousand different tints, all of which must be kept methodically sorted and arranged, are requisite for this kind of mosaic-work ; the preparation of such a palette there fore, for anything upon an extensive scale, must of itself be a task of great labour and time, as well as expense ; besides which the execution is so entirely mechanical, that it is fit only for copyists. Some of the larger and more elaborate works aro said to take fifteen or twenty years In copying. The sole advantage in any degree proportionate to the cost attending it, is the extreme durability of the work when once! accomplished, as its colours can hardly be changed by any length of time ; nor is it liable to decay or injury, except what may happen to the structure In which it is fixed. The mosaics in St. Peter's, which are masterpieces of their kind, may be expected to last as long as the building itself stands. When Sir Christopher Wren designed St. Paul's it was his earnest wish to have the cupola decorated with mosaics, like the cupola of St. Peter's, and he had actually entered into negotiations with some Italian inusaicisti for their execution, but the authorities would not give their eanction. Quite recently proposals have been lamed by the present dean and chapter for carrying out Wren's intentions, according to a plan drawn up by Mr. Penrose, the present architect to the cathedral Many tesselated pavements of good design and excellent workmanship have been laid down in this country within the last few years.