One of the oldest art forms, the fresco, depends on a solid understanding of acid-base chemistry.
The first fresco-type murals date back 30,000 years and were originally done on the walls of limestone caves in France. Thousands of years later, about 1500 B.C., fresco artists adapted their techniques and began to apply plaster to adhere pigments to the walls of churches and public buildings, creating detailed and highly figurative murals that still survive today.
Curiously, what was true then remains true today: successful frescoes depend on a solid understanding of acid-base chemistry.
Fresco artists apply a layer of plaster to the wall, referred to as the intonaco, and then paint directly on the plaster while it is still wet. Only enough plaster is applied for each day’s work, and artists have only about eight or nine hours to work before the plaster dries. If the artist makes a mistake then the entire day’s work must be removed and redone.
The intonaco contains lime, or calcium carbonate, which means that painting a fresco innately involves acid-base chemistry. Acidic pigments frequently fail to retain their color when exposed to the basic intonaco; for example, the intense color of Prussian blue turns to yellow in a fresco. Modern pigment catalogs often contain a notation indicating which pigments are compatible with the fresco medium. Renaissance painters who prepared their own pigments understood these incompatibilities so they avoided base-induced color changes.
Buon fresco, the technique of painting onto the wet intonaco, offers the most permanence in protecting the image. Some artists turn to secco fresco, a technique using a dry intonaco. With this approach, artists wait until the plaster has dried, then apply a substance such as egg, and paint on top of that – this allows the artist to correct errors or use pigments that are otherwise incompatible with basic plaster. Unfortunately, this much weaker binder does not protect the paint, making the surface prone to flaking. Michaelangelo mastered the more challenging buon fresco style while painting the famous fresco ceiling in the Sistine Chapel.
A wide range of environmental conditions can cause serious damage to even the most brilliant frescos. In the lagoon conditions of Venice, water wicks up the walls carrying with it large amounts of salt. These salts crystallize on the surface of the frescos, causing the paint and the outermost layer of plaster to flake and fall off. Venetians have become quite adept at conserving frescoes using cotton gauze soaked in polyvinyl alcohol. Compresses of ammonium carbonate and barium hydroxide convert the problematic calcium sulfate salt into the less soluble barium sulfate, and a combination of lime putty, epoxy resin, and silica is used to repair cracks and flaking plaster.
For more information, see http://www.italianfrescoes.com/history.asp and http://www.italian-renaissance-art.com/Painting-Techniques.html