Mosaics, works of art of surface decorations, composed of variously colored small pieces of glass, stone, ceramics, or other materials. Although mosaic decoration is most commonly found on floors and wall and ceiling surfaces, closely set colored components, or tesserae, may also be applied to sculptures, panels, and other objects, geometric patterns, and scenes from daily life and mythology.

In ancient times mosaics were a form of floor decoration made of small pebbles and later of cut or shaped pieces of marble, hard stone, glass, terra-cotta, mother-of-pearl, and enamels. The shaped pieces, in the form of small cubes, are called tesserae or tesselae. The tesserae are embedded in plaster, cement, or putty to hold them in place. Techniques for arranging the components in a design include opus tesselatum, simple geometric patterns; opus vermiculatum, small stones arranged in patterns of curved lines, including pictures of objects; opus musivum, mosaic decorations of walls; and opus sectile, a pattern composed of larger stones of varied shapes.

Knowledge of mosaic techniques is mostly derived from direct examination of specific examples; damaged mosaics in Istanbul provide explicit information. Walls to be covered with mosaics received a triple coat of plaster. The first layer, of lime, sand, and brick dust, was applied over the masonry to produce a smooth surface. The intermediate layer 1.25 to 5 cm (0.5 to 2 in) thick was made of lime, sand, and chopped straw. This surface was scratched or tooled to receive the third coat, called intonaco, of plaster of lime and marble dust, which was applied over a small area, as much as could be completed in one day. It was then painted in detail in true fresco and immediately set with colored cubes to match the painted surface.

To make tesserae, thin slabs of marble or of colored stone were cut into strips, which were then cut or broken into cubes. Molten glass was tinted in a wide range of colors with metal oxides and then poured on a flat surface such as a marble slab to form a disk of colored glass; this was scored with a sharp tool and broken into strips and cubes. Gold and silver cubes were produced by gilding glass slabs of pale shades with gold or silver leaf. The surface was then covered with a frit (thin layer of powdered glass) and reheated in a furnace to secure the gold or silver between the layers of glass; the slab was then scored and broken into small cubes. The cubes were set into the painted intonaco one at a time, with resulting deliberate irregularities of the surface. These variations in surface planes catch the light and impart vitality to the finished wall. In many backgrounds the cubes are angled downward in rows, with space between the rows; when viewed from a distance this gives the appearance of a solid background. The stone and glass tesserae in mosaics are relatively stable materials, so that many ancient mosaics have survived with the same brilliance that was part of their original conception.

Mosaics were made as either floor or wall coverings that could easily be washed and cleaned. Mosaics were assembled from thousands of tesserae (small pieces of marble) to create elaborate designs and pictures. Unlike fresco or wall painting, the mosaic art provided a medium of long-lasting beauty whose colors did not fade. The Romans practiced the art of mosaic as early as the 1st century BC. However, Byzantine artists used mosaic as a true painting medium with brilliant colors visible from great distances. Mosaics were adapted for walls using glass, ceramic, and gilded tesserae to stunning effect. The floors of fifth-century Early Byzantine churches in northern Syria often included scenes involving large numbers of birds and animals with geometric borders, and less frequently, human figures such as this example.