Of all of the painting techniques available, the technique of fresco painting involves the greatest number of chemical reactions. The study of the science behind frescos gives us the opportunity to discuss these reactions and chemical equations.
The technique of fresco is one of the oldest and most durable forms of painting. Frescos have been created for thousands of years, and thanks to their robust nature many have survived the millennia. Not only have frescoes been painted for thousands of years, they are also geographically widespread. This is due to the fact that the materials used to make a fresco (lime, sand and colored clays) are widely available and easy to process.
The term fresco means “fresh” in Italian. The essential feature of a fresco is that pigment is applied to a wet (fresh) surface of lime plaster. Some of the earliest frescoes found in Greece date back to 200 B.C.. During the Italian Renaissance of the 15th and 16th centuries, the fresco painting technique was perfected. One of the best known examples of a fresco from this era is Michelangelo’s painting of the Sistine Chapel ceiling.
Fresco Anatomy: To understand the chemistry of a fresco, we first must understand what a fresco is. Frescos are most often painted on walls and ceilings. Typically a layer of plaster is applied directly to the wall to smooth the surface of the wall. This first layer is called the arricio layer. Once this layer has dried and hardened, a second layer of plaster is applied called the intonaco. It is in this layer that the pigments are applied. However, the pigments must be added before the intonacco layer dries. For this reason, the artist must work fast and without attempting to do too much. If this layer dries before the pigment is added, the artist is out of luck.
Fresco Chemistry: The chemistry of a fresco starts with the reaction used to make the materials needed for the fresco. The basic raw material for a fresco is limestone. Limestone is calcium carbonate (CaCO3) and is an abundant mineral found throughout the world. The first step is to convert the limestone into lime (or quicklime). Lime is calcium oxide (CaO) and is formed by heating calcium carbonate to over 800° C. Carbon dioxide is a by-product of the reaction. The chemical equation describing this reaction is shown in reaction 1.
The lime is a white solid that has a very high melting point. Long before the advent of electricity and on demand lighting, huge chunks of lime were heated until they glowed white. This light was used to illuminate theater stages, thus the origin of the term “limelight.” The carbon dioxide by-product is a gas and simple diffuses away.
The next step involves a process known as “slaking.” Water is added to the calcium oxide in order to “slake” it. The product of this reaction is calcium hydroxide (Ca(OH)2) also known a “slaked lime.” See reaction #2 above
Lime is allowed to slake for years. This “aging” process allows as much of the calcium oxide to react. Calcium oxide is not very soluble in water. As a result, it takes a long time for the water to interact fully with the calcium oxide. Traditionally, the lime was slaked in large pits and is often referred to as “pit lime.” The slaking process produces heat as a by-product. A reaction that produces heat is called an “exothermic.”
The calcium hydroxide that forms is only slightly soluble in water. This results in a white, milky suspension with water. This suspension is a very caustic substance as calcium hydroxide is a strong base. Care must be taken when dealing with this substance since prolonged exposure can result in serious damage to the skin.
It is at this point that the artist uses the material. Slaked lime (calcium hydroxide) is mixed with sand to form the actual plaster that is applied to the wall. Typically, course sand is used for the arricio layer of plaster and fine sand is used for the intonaco layer of sand.
Once the intonaco plaster has been applied the pigments are added. The pigments are usually a suspension in water. What that means is that the pigments do not dissolve in water. When the pigments are applied to the plaster, they get absorbed into the plaster and eventually become part of the intonaco rather than a coating on the surface. After the intonaco plaster has been applied, the artist must finish the painting before it dries. Usually an artist will only attempt to paint a section 3-5 square meters in area. In addition, since the calcium hydroxide is very basic (alkaline), the pigments must be stable in an alkaline environment.
The drying of the plaster is a physical change. In this process, the slaked lime does not change chemically. The water simply evaporates leaving the dry slaked lime on the wall. What makes a fresco so robust is the chemical reaction that occurs slowly after the water is gone. Carbon dioxide from the atmosphere reacts with the calcium hydroxide to form calcium carbonate as shown in reaction 3 shown above.
The formation of the calcium carbonate (identical in chemical composition to the original limestone) results in a very hard and stable surface that traps the pigments. This reaction is very slow and occurs over the course of decades and even centuries.
