Fresco and wall painting
Fresco technique consists of the brush application of pigments mixed with water onto a final coat (intonaco) of fresh lime mortar and finely powdered aggregate, generally marble powder. Once the mortar has set, which takes about 8 hours, colour can no longer to be fixed by the intonaco, and the painter must consider his work finished for that day. The following day leftover fragments of the unpainted intonaco are removed by cutting an edge around the finished figures and backgrounds; then fresh mortar is applied, taking care not to cover the already painted surface. In consequence, fresco wall painting is painted on giornate, or surfaces of mortar that the artist is able to paint in one day of work. The surface area of the giornate varies greatly depending on the complexity of the painting, and can be small, covering the size of a single head, or very large depicting a simple landscape or sky. Joints between giornate are easily visible as mortar contraction widens the contact joints with the previous work. The presence of giornate is a characteristic feature of wall paintings executed with fresco, thus allowing distinction from paintings executed "a secco" –with an organic binding medium on dry mortar– where the mural support is not divided into giornate.