What Is a Mosaic? Pictures Assembled From Thousands of Hard Pieces

The individual pieces are called tesserae, usually roughly cubic and typically between about five and fifteen millimetres across, with finer work using much smaller pieces to render faces and details. Stone supplies the natural palette, with marble, limestone and coloured hardstones cut into rods and then chopped into cubes, which gives durable and subtle colours limited to what geology provides. Glass extended the range enormously, since it can be made in any colour by adding metal oxides, and it also permitted gold tesserae, made by laying gold leaf on a glass backing and sealing it with a thin layer of clear glass, which is what produces the shimmering gold grounds of Byzantine work. Fired ceramic, shell and mother of pearl appear in various traditions. The pieces are set into a bed of lime mortar, and the work's survival depends far more on that bed and its foundation than on the tesserae themselves, since the stone and glass are effectively permanent while the mortar and the wall behind it are not.

Several features of the medium follow directly from working with discrete hard pieces:

• •Colour is mixed optically rather than physically, since adjacent tesserae of different colours blend in the viewer's eye at distance, which is the same effect painters rediscovered much later
• •Outlines are strong, because a hard edge is easier to achieve than a soft gradient when the smallest unit is a cube
• •Setting lines matter enormously, since the flow of the rows around a figure creates movement and separates it from the background, and mosaicists deliberately laid rows to follow contours
• •Wall mosaics were set at deliberate angles so that tesserae catch light unevenly and the surface glitters, which is invisible in photographs and fundamental to the effect in place
• •Floor mosaics use flatter setting and harder materials because they are walked on, which is why floor and wall traditions look different
• •Detail is limited by tessera size, so faces in fine work use much smaller pieces than drapery, and the transition is visible up close and invisible from the intended viewing distance

Pebble mosaics appear in Greece from the fourth century before the common era, using naturally rounded stones, and the shift to cut tesserae allowed far greater precision. Roman floor mosaic became a substantial industry with recognised regional workshops and pattern books, producing everything from geometric borders in ordinary houses to virtuoso figural panels, and the surviving corpus is enormous because floors survive when everything above them falls. The famous Alexander mosaic from Pompeii uses over a million tesserae and is generally understood as a copy of a lost Hellenistic painting, which makes mosaic one of the main routes by which knowledge of ancient painting survives at all. Byzantine practice moved the medium onto walls and vaults and exploited gold grounds and glass to create images that change with the light and are deliberately not naturalistic. Islamic traditions developed geometric tile mosaic to an extraordinary level of mathematical sophistication, and pre-Columbian Mesoamerican work used turquoise and shell on wood and bone in an entirely separate lineage.

Two methods dominate. In the direct method, tesserae are set face up into mortar in their final position, which gives the maker complete control of angle and surface irregularity and is how most ancient work was made. In the indirect method, pieces are glued face down onto paper or mesh in the workshop, transported, pressed into mortar on site and the backing removed, which produces a flat uniform surface and permits large-scale work made off site, and it became standard in the nineteenth century. Conservation is dominated by the problem of the bedding rather than the image. Water infiltration, salt crystallisation and structural movement destroy the mortar, causing detachment and loss, and the historic practice of lifting a mosaic entirely and remounting it on a new backing saved many works while destroying the original setting angles and archaeological context. Current practice strongly favours conserving in place where possible, stabilising the environment and consolidating the original bedding, which is slower and preserves more information.

Every colour must exist as a physical piece, so images are built from small hard cubes and colours blend in the eye rather than on a palette. Glass gave the medium a full palette and gold grounds made by sealing leaf between layers. Wall pieces were set at angles to catch light, which photographs cannot show. Floors survive when buildings fall, which is why Roman mosaic is our main window onto lost ancient painting.