Frescos refers to any of several related painting types,done on plaster on walls or ceilings. It comes from the Italian word 'affresco' which derives from the adjective "Fresh". The Buon fresco technique consists of painting in pigment mixed with water on a thin layer of wet,fresh,lime mortar or plaster. Because of the chemical makeup of the plaster,a binder is not requires. The pigment mixed solely with the water will sink into the plaster,which itself becomes the medium holding the pigment.The pigment is absorbed by the wet plaster;after a number of hours,the plaster driees and reacts with the air;it is this chemical reaction which fixes the pigment particles in the plaster.