A fresco is a method of wall painting in which powdered pigments mixed only in water are applied to wet plaster freshly laid on the wall. The word ‘fresco’ is Italian for ‘fresh.’ As the wall dries an irreversible chemical reaction occurs that binds the pigment with the plaster, making the picture an integral part of the wall (the lime -calcium hydroxide-of the plaster combines with carbon dioxide in the atmosphere to form a crust of calcium carbonate). This technique is also called buon fresco or fresco buono (true fresco) to distinguish it from painting on dry plaster, which is called fresco secco or simply secco.