Fresco is the Italian word for fresh. In general, the term fresco is used to identify a method of mural painting in which water-based paint is applied to plaster. However, there are two methods of fresco painting. True fresco or fresco buono results from the application of paint to wet plaster so that the paint becomes part of the wall itself. Fresco secco, on the other hand, refers to the application of paint to dry plaster. For the true fresco method, a charcoal outline of the painting is traced onto a rough undercoat of plaster known as the arriciatto. Following, a fresh layer of plaster, the intonaco, is applied to an area suitible for one day's work. Finally, pigments are applied to the intonaco while the plaster is still damp. To complete a days work in the true fresco technique, a painter must work swiftly and decisively before the intonaco dries. Thus, the true fresco method lends to the painterly style we see in many fresco paintings. As the intonaco and paint dry, they fuse and form a durable matt surface. If kept dry, true fresco paintings may maintain their fresh appearance for incredibly long periods of time. Fresco secco, on the other hand, is obviously less durable as paint applied to dry plaster easily flakes or rubs off over time. For a description of the fresco technique by an ancient author, you may refer to Vitruvius' De Architectura (VII.3-7) of about 27 BC.