Fresco is a technique of mural painting executed upon freshly-laid, or wet lime plaster. Using water as the medium the paint is absorbed into the plaster as it dries and the pigment is bound into the structure of the plaster. Wall painting was very popular in the renaissance era and fresco was the standard method of decoration. The word fresco (Italian: affresco) is derived from the Italian adjective fresco meaning "fresh or wet". Frescos are mostly permanent because of their chemical composition: The active ingredient in fresco is Lime Paste, which is produced by heating Calcium  Carbonate with limestone.  Lime Paste + Air  changes back into calcium carbonate, hard crust (carbonatation). If pigment is applied to this when wet, it becomes trapped into the wall an it is very chemically stable.