Frescos

Fresco (plural frescos or frescoes) is a technique of mural painting executed upon freshly-laid, or wet lime plaster. Water is used as the vehicle for the pigment to merge with the plaster, and with the setting of the plaster, the painting becomes an integral part of the wall. The word fresco (Italian: affresco) is derived from the Italian adjective fresco meaning "fresh", and may thus be contrasted with fresco-secco or secco mural painting techniques, which are applied to dried plaster, to supplement painting in fresco. The fresco technique has been employed since antiquity and is closely associated with Italian Renaissance painting.

Buon fresco pigment mixed with water of room temperature on a thin layer of wet, fresh plaster, for which the Italian word for plaster, intonaco, is used. Because of the chemical makeup of the plaster, a binder is not required, as the pigment mixed solely with the water will sink into the intonaco, which itself becomes the medium holding the pigment. The pigment is absorbed by the wet plaster; after a number of hours, the plaster dries in reaction to air: it is this chemical reaction which fixes the pigment particles in the plaster. The chemical processes are as follows:

In painting buon fresco, a rough underlayer called the arriccio is added to the whole area to be painted and allowed to dry for some days. Many artists sketched their compositions on this underlayer, which would never be seen, in a red pigment called sinopia, a name also used to refer to these under-paintings. Later,new techniques for transferring paper drawings to the wall were developed. The main lines of a drawing made on paper were pricked over with a point, the paper held against the wall, and a bag of soot (spolvero) banged on them on produce black dots along the lines. If the painting was to be done over an existing fresco, the surface would be roughened to provide better adhesion. On the day of painting, the intonaco, a thinner, smooth layer of fine plaster was added to the amount of wall that was expected to be completed that day, sometimes matching the contours of the figures or the landscape, but more often just starting from the top of the composition. This area is called the giornata ("day's work"), and the different day stages can usually be seen in a large fresco, by a sort of seam that separates one from the next.

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Wikipedia