Buon Fresco

Buon Fresco Affresco, Italian for true fresco, is a fresco painting technique in which alkaline-resistant pigments, ground in water, are applied to wet plaster.

It is distinguished from the fresco-secco (or a secco) and finto fresco techniques, in which paints are applied to dried plaster.

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 is , intonaco. 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.

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.

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.

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.

...
Wikipedia