How they were painted:
Buon (true) fresco (fresh) This method was perfected in 16th century Italy but similar methods were used during the time of the Vatican Frescos (12th and 13th century). The method requires three layers of plaster and is done in daily sections. The section is fixed with wet plaster and paint is applied to wet plaster. A chemical reaction would occur between the drying plaster and pigment, binding the pigment to the wall. Because sections of the frescos could only be worked on while the paint was wet, the artist would only put plaster up for what he could accomplish in one day known as giornata, or day's work. A rough undercoat of plaster is first applied to the wall, and when dried, the artist'ws future fresco work is sketched onto the plaster using charcoal. These drawings are called "sinopia" and are covered with a very fine layer of plaster. Pigments mixed with water are applied to the nearly dry plaster. Artist worked from the top down, as to cover any dripping paint from above.
Some other facts about Frescos, taken from PBS.org(http://www.pbs.org/fresco/facts.html) fresco facts:
- If the plaster is too soft, the colors will be cloudy; if it is too dry, a crust will form and pigments will adhere poorly.
- In most painting techniques, the artist saturates pigments with oil to make them rich, dark, and glossy. In fresco, the artist mixes pigments with water and/or egg yolk, leaving them with much of their original mineral sparkle undimmed.
- Lime plaster can take a year to fully harden.
- When colors in a fresco dry, they are lighter than when they are applied and continue to soften over time.
- Particles of pigment then "carbonize" or "crystallize" as the plaster dries, producing the luminous matte finish that characterizes frescoes.