Mosaic is technically a more expressive medium than sgraffito. The number of colors used is not limited, nor is it any more difficult to produce a work in polychrome than in black and white. Only the preparation of the design and fullsize cartoon take longer with many colors than with few.
The strength of a mosaic is not affected by the number of colors, nor does it reduce the graphic effect. The network of divisions between the colored stones always looks like a drawn texture enclosing each small fleck of color. If this were not so, mosaic could become a painting with the softest transitions of color.
Classical mosaic is made from colored tesserae all of the same size, between 1/3 and 3/4 of an inch square. Only occasionally, for lines running to a point or very curved, are the small squares sloped off or split. As a rule, a variation in the width of the joins serves instead; it does not matter how the joins run.
The work begins by giving the wall a first coat of coarse plaster. This must be absolutely flat, but should not be too smooth, as it must give a good grip to the next coat. Into this upper layer of plaster, while soft, the tesserae are pressed. Unless it is quite flat the surface of the mosaic will be uneven. Its thickness depends on whether the tesserae are all of the same thickness or not. If they are uniform the bedding layer can be thinner than if it has to even out differences of thickness so that the mosaic surface remains flat.
The average thickness of mosaic, bedding layer and undercoat together, comes to about % of an inch. The plaster must be rather fat and very fine so that the tesserae can be pushed into it easily and lie as close as possible to each other. Only very fine plaster can press up into the narrow cracks and hold the tesserae fast. The cracks average about 1/16 of an inch if the edges of the mosaic stones are parallel. Very little plaster comes up into the cracks from underneath; after the mosaic surface is complete it is washed over with fine watery plaster. The attractive textured effect of the joins between the stones comes from the slight projection of each individual stone, and this is achieved by brushing away the plaster applied from above for about 2/3 inch down into the cracks, a process which cleans up the surface of the stones as well.
Mosaic is most attractive when each stone is fixed directly into the wall, showing the natural irregularities of work done by hand, as was done in the past, as long as 2,000 years ago. Recently
various methods have been evolved which enable larger sections to be composed in the workshop and then applied to the wall. These all produce a very smooth and close-packed surface which is technically perfect, but which loses much aesthetically. The result is even more negative when the artist sends his cartoon to be carried out entirely by artisans in the workshop which supplies the tesserae. However accomplished and tasteful these artisans may be, they cannot be expected to interpret the intentions of the artist exactly.
Nor can the artist be expected to exploit all the expressive qualities of the medium unless he has practical mastery of it, which means that he must carry out most of the execution himself. It is different if the design is purely decorative in intention and needs complete technical precision for its execution, for instance a tesselated pavement which has to be absolutely flat, or over-all ornamental decoration of walls and pillars. For these purposes the workshop can provide larger and closely shaped stones which can be fixed with scarcely. any space between them. This is very different from the Early Christian mosaics, whose expressive and tectonic texture is not conceived merely ornamentally. Their technical "imperfection" has much more personality than the smooth elegance of the precision work from a factory.
This "imperfection" is the same quality we required the artist to seek in sgraffito: work which bears the imprint of the artist's own hand, not the impersonal touch of the plasterer's. In mosaic it is also the constantly repeated building up of the design step by step to the final creation. No technique gives a clearer sense of finality than this composition of a picture stone by stone.
Before tackling our cartoon we must acquaint ourselves with the material. It is stimulating to play about with a collection of colored stones on a tray and arrange them in different ways and patterns. Mosaic tesserae may be artificially made from fired clay, either colored right through or glazed with color only on the top: or there are colored glass pastes which. are transparent and give a radiance to the work, and some with a layer of gold or silver leaf underglass. Many natural stones are used, cut to the right shape. Fragments of colored marble slabs and wall and flooring tiles of the right thickness are highly suitable. Even brightly colored pebbles from the seashore or the beds of streams can be used for a primitive, rather decorative style of mosaic.