The painting term fresco (plural, "frescoes," from the Italian "fresh") comes from the Italian phrase buon fresco, ("really fresh") a technical term in opposition to in secco ("on dry surface"). True, or buon fresco, technique consists of painting in pigment in a water medium on wet or fresh plaster. In secco painting is done on dry plaster and with the pigments in a binding medium, like egg. The difference between the two techniques is that the wet plaster as it dries absorbs the pigment and the painting becomes part of the wall surface rather than resting on top of it. This makes a very durable work of art; if the wall is destroyed the painting can often be reassembled because of the size of the plaster parts.
Because of the need to work on freshly laid plaster, careful study of the wall surface can reveal the area worked on in one day, commonly called in Italy in the Renaissance a giornata or a "daily amount." These divisions are perceptible with mild magnification (or even the naked eye if the plastering technique was not good).
Painters in fresco will often add details later in secco.
See also: Mural