II.

Materials and Techniques

Mosaics in their classic form were made from small pebbles and later of cube-like pieces of marble, hard stone, glass, or terracotta, known as tesserae (Latin, dice).

To make tesserae, thin slabs of marble or of coloured stone were cut into strips, which were then cut or broken into cubes. Molten glass, which could be tinted in a range of colours by the addition of metal oxides, was poured out on to a flat surface to cool and harden. The resulting slab of coloured glass was scored with a sharp tool and broken into strips and cubes. Gold and silver cubes were made by applying gold or silver leaf to slabs of glass in pale shades. The surface was then covered with a frit (a thin layer of powdered glass) and heated in a furnace to seal the gold or silver between the layers of glass; the slab was then scored and broken into cubes.

Different styles of mosaic include opus tesselatum, simple geometric patterns; opus vermiculatum, small stones arranged in patterns of curved lines, or depicting people, animals, plants, or objects; opus musivum, mosaic on the wall in a house; and opus sectile, a pattern composed of larger stones of varied shapes.

Knowledge of ancient mosaic techniques is derived mostly 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 (a mixture of lime, sand, and brick dust) was applied over the masonry to give a smooth surface. The intermediate layer (of lime, sand, and chopped straw), 1.25 to 5 cm (0.5 to 2 in) thick, was then applied, and was scratched or tooled to receive the third coat, called intonaco, of plaster of lime and marble dust, which was spread over a small area at a time; as much as could be completed in one day, for instance. It was then painted in detail in true fresco and immediately set with coloured tesserae to match the painted surface.

The tesserae 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 instances the tesserae are angled downwards in rows, with space between the rows; when viewed from a distance this gives the appearance of a continuous background.