The art of mosaic consists in an artistic composition obtained by the use of tesserae, (small pieces of glass, stone or ceramic), of different sizes, different types and colours as stones (including the precious ones), glass, gold, ceramic, shells. Widely spread during the Roman times, the mosaic technique reaches new shine with Byzantine art: just think of the marvellous monuments of Ravenna, or the golden cycles of the San Marco’s Basilica in Venice and Santa Sophia in Constantinople. Depending on the epoch and nature of the tesserae, also the techniques of preparation and installation are different: for example, the Roman floors were made of stone tesserae into a bed of three layers, constituted of specific screeds. The golden tesserae instead had a vitreous support, sometimes red coloured, over which was applied a gold leaf, with a glass protective superficial folder.
Depending on the state of conservation, it may decide to conduct the operations without to remove the wall or the floor, or removing tesserae after a careful survey, catalogue and temporary shelving. In the first case, the surface is cleaned to remove any over putted layers (concretions, dirt, vegetal organisms), the parts in the detachment are fixed by injections and the gaps filled by different techniques as appropriate, with a reconstruction of the tesserae (as much as possible similar to those old, but distinguishable at a close examination) or fill-neutral; the joints are plastered. In the second case, once removed the tesserae, removed the old bedding layer, cleaned the substrate and spread a new layer on which the tesserae are placed, cleaned, with any re-integrations in a new tessera or a gap.