Fresco Painting

While others went to the beautiful mountains or beach this past weekend, I returned to Florence to see the renowned Uffizi art museum and the Cathedral or Duomo. The museum was very impressive. I enjoyed the room for Botticelli the best. It was amazing seeing both “The Birth of Venus” and “Spring.” In comparison to other Duomo’s I have seen on this trip, the one in Florence is rather spare. It has the third longest knave in the world, but unlike others very little of the walls or ceiling is decorated. The exception is the expansive fresco on the underside of the huge dome. The fresco, designed by Giorgio Vasari and Federico Zucchari is called "The Last Judgment." It depicts the risen Christ dividing the people between heaven and hell. Despite the fact that this piece of art is over 400 years old, it is still vivid, and can be clearly seen from 90 meters bellow. We have talked some about fresco painting, and it is difficult to avoid here in Italy, but I do want to talk about the chemistry some, and how frescoes maintain their visual grandeur.

There are two types of frescoes. In buon fresco the pigment in applied directly to the wet plaster. As the plaster dries the pigment is incorporated into the wall making it both stable and sensitive to environmental conditions. In secco fresco, the pigment is applied to dry plaster so a binder such as egg white is needed. The plaster used is a combination of slaked lime and sand. Slaked lime is a paste or putty made of 1 part lime (CaO) to 2 or 3 parts water. It is a suspension of calcium hydroxide crystals (Ca(OH)2), and in order for it to be of the correct consistence it has to be produced over a period of at least 6 months if not longer. The first few layers of plaster are a 1 to 2 ratio of plaster to sand and are allowed to dry without pigment. A drawing is made on the last dry layer, and a paper guide is made from it. The most important layer of plaster has significantly less sand that the others, and is only applied in small areas at a time. For buon fresco, the pigment has to be applied within the first hour before the plaster dries. As the plaster dries the calcium hydroxide reactes with carbon dioxide to form calcium carbonate.
Ca(OH)2(s) + CO2(g)→ CaCO3(s) + H2O(l)
There were a limited number of pigments that maintain color in the basic conditions of the wet plaster, so certain colors, particularly blues, have to be added a secco.

http://www.mega.it/eng/egui/monu/buq.htm

http://www.wellesley.edu/Chemistry/Chem&Art/Topics/Pigments_Painting/Painting_pdf_material/fresco_lab02.pdf

http://en.wikipedia.org/wiki/Fresco