Among mural painting techniques, it is the most noted. It was widely practised from the Middle Ages to the end of the Renaissance and beyond, though the technique begins to take on various forms. It involves placing colours (for the most part mixed simply with water) onto an intonaco still damp and fresh or "fresco"; the slaked lime used in the intonaco, while drying, is combined with the carbon dioxide in the air, generating a film of calcium carbonate (carbonization). This causes the intonaco to become particularly compact and consistent; and the colour, incorporated in the crystalline structure of the calcium carbonate, becomes no longer soluble in water. Given the necessity to work in damp plaster, the fresco is laid in portions depending on the work that is foreseen to take place in a limited arch of time (giornata, v.; pontata, v.). In the execution, quickness and great certainty are necessary, however, the fresco may also include the application of final touches a secco (v.) or mezzosecco (v.). The execution technique and the material used obviously vary according to the period and the geographic area. Other than the cross-references already mentioned see: roughcast, arricciato, pouncing, cartoon, sinopia.