Buon Fresco

Buon fresco (Italian for true fresco) is a fresco painting technique in which alkaline resistant pigments, ground in water, are applied to plaster when it is still wet, as opposed to fresco-secco (or a secco). The buon fresco technique consists of painting with pigment ground in water on a thin layer of wet, fresh, lime mortar or plaster, for which the Italian word for plaster, intonaco, is used. Because of the chemical makeup of the plaster, a binder is not required. After a number of hours the plaster reacts with the air in a process called carbonatation: this chemical reaction fixes the pigment particles at the plaster's surface in a protective crystalline mesh known as the lime crust. One of the first painters in the post-classical period to use this technique was the Isaac Master in the Upper Basilica of Saint Francis in Assisi.

Buon fresco contrasts with finto fresco, a technique in which paints are applied to dried plaster.

The advantage of Buon fresco is its durability. In fresco-secco, by contrast, the color does not become part of the wall and tends to flake off over time. The chief disadvantage of Buon fresco is that it must be done quickly without mistakes. The painter plasters and paints only as much as can be completed in a day, which explains the Italian term for each of these sections, giornata, or a day's work. The size of a giornata varies according to the complexity of the painting within it. A face, for instance, might take an entire day, whereas large areas of sky can be painted quite rapidly.

In medieval and Renaissance Italy, a wall to be frescoed was first prepared with a rough, thick undercoat of plaster known as the arriccio. When this was dry, assistants copied the master painter's composition onto it with reddish-brown pigment or charcoal. The artist made any necessary adjustments.