A tessera (plural: tesserae, diminutive tessella) is an individual tile in a mosaic, usually formed in the shape of a cube. Also known as an abaciscus, abaculus.
In antiquity, mosaics were formed from naturally colored pebbles, but by 200 BCE purpose-made tesserae were being used. Marble or limestone was cut into small cubes that were arranged into the design. Later, tesserae were made from colored glass, or clear glass backed with metal foils. The Byzantines used tesserae with gold leaf, in which case the glass pieces were flatter, with two glass pieces sandwiching the gold. This produced a golden reflection emanating from in between the tesserae, causing a richer and more luminous effect.
Another kind of tessera was the ancient Roman equivalent of a theater ticket. Stamped into a clay shard was an entrance aisle and row number for spectators attending an event at an amphitheater or arena. Above the doors of the Colosseum in Rome are numbers corresponding to those stamped into a spectator's tessera.
Contemporary types of Tessera
Vitreous Glass: These are manufactured glass tiles made to a uniform shape and size. They are made by molten glass being poured into trays and fired. An imprint of grooves is made on their underside for help with adhesion to cement when fixing.
Ceramic Tesserae: These are the cheapest range of bought materials and can be glazed or unglazed. The glazed ceramic tiles have the colour painted onto the top of the clay and then fired to a high temperature in a kiln. The unglazed or body glazed version has the coloured mixed into the wet clay so the colour runs them. They vary in size.
Smalti: This is the classic mosaic material. It is opaque glass fired in large slabs in a kiln and then hand cut with a hammer and hardy(chisel) into small cubes. Their irregular finish makes them a wonderful reflector of light and this material is best used working straight into cement. It is produced in Venice and sold by colour and weight.
Gold Smalti: This tile is made with real gold and silver leaf sandwiched between two layers of glass and fired twice in the kiln to embed in the metal.
Mirror: Mirror adds great depth and sparkle to a mosaic. It is cheap as offcuts from a glass cutting shop are often free. Use mirror glue as this protects the silver on the back of the mirror.
Stained Glass: Known for its translucent qualities stained glass is also available in opaque form. It comes as large sheets that can be cut into smaller sections with a glasscutter. It can provide areas of larger tesserae pieces for variety and contrast.
Household Ceramic Tiles & China: Colours and surfaces are limitless and can add wonderful texture and contrast to mosaic work.