II.

Materials and Techniques

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 İstanbul 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.