Fresco (Italian: Affresco) - Type of mural painting on plaster used throughout the world since early times, but especially known from its high development in Italy during the Renaissance; hence the use of Italian terms to describe the technique. See also Buon fresco, Lime painting and Secco.
Buon fresco - True Fresco - Method of mural painting in which artist must start applying his colors on the wet intonaco (painting plaster layer) as soon as it has been prepared and laid on the wall/ceiling. The colors can thus be absorbed by the wet plaster. When it dries and hardens, the colors become one with plaster. Technically speaking the plaster does not "dry" but rather a chemical reaction occurs in which calcium carbonate is formed as a result of carbon dioxide from the air combining with the calcium hydrate in the wet plaster.
Secco Fresco - technique of applying colours which are not resistant to lime over an undercoat, putting the finishing touches on a fresco after it has hardened. Tempera is mixed with adhesive such as gum arabic, casein, animal skin glue or modern day acrylic mediums after dampening down the plaster (dumpening is needed for bringing the surface to a consistend tone - since painting is done in transparent layers and dry plaster would change color to a darker tone as soon as watery paint is applied making it difficult to judge the color tones) Results are not as durable as Buon Fresco and the painting will eventually flake off beyond repare - Last Supper of Lionardo is an unfortunate example of the limitations of the Secco technique.