When someone mentions fresco painting, what always comes to my mind is Michelangelo painting in the Sistine Chapel on wet plaster. Of course, that’s not the only type of fresco painting, but it is arguably the purest and the most durable form.
Frescos painted in this manner two thousand years ago still retain their color. The photo below depicts a fresco from the ancient city of Pompeii, which was buried under volcanic ash in 79 AD during the eruption of Mount Vesuvius.
Painting on wet plaster is known as buon fresco. Buon means true or genuine and fresco means wet. In other words, buon fresco means true fresco. What makes this form of painting so durable is that the pigment is absorbed into the plaster. Another type of fresco painting is called fresco secco (dry), in which artist paints on dry plaster with pigment in lime water.
Fresco showing fruit bowl, jar of wine, jar of raisins, from the House of Julia Felix in Pompeii, Naples National Archaeological Museum.
What is so fascinating to me is the chemistry of buon fresco painting. In this unique process calcium carbonate (CaCO3) in the form of limestone is heated to 1652°F or 900°C. In this process, which is known as calcination, the calcium carbonate is converted to calcium oxide (CaO) called quicklime and carbon dioxide (CO2).
The term calcination comes from the Latin word “calcinare”, which means to burn lime. In spite of that meaning, the process of calcining is also used in the creation of pigments. For example, when raw umber is subjected to calcination, it becomes burnt umber.
CaCO3 + heat → CaO + CO2.
The calcium oxide is highly alkaline, which makes it very corrosive. It is so corrosive that it can actually cause a chemical burn on your skin. Next, water and calcium oxide are mixed to produce calcium hydroxide (Ca(OH)2), which is called slake lime. This chemical reaction releases a tremendous amount of heat.
CaO + H2O → Ca(OH)2 + heat.
The slake lime is mixed with sand to form the plaster. Instead of using sand, Michelangelo did things a little differently. He used volcanic ash, called “pozzolana”, in the mixture. The pozzolana reacts with the calcium hydroxide to form a harder plaster.
NOTE: What is also interesting is that the Romans also mixed volcanic ash with quicklime and water to create what we today call “super concrete”, which outlasts modern concrete. As the mixture cures, crystals form, filling any microscopic spaces and reinforcing its structure.
In fresco painting, the artist first applies a layer of plaster, which is allowed to dry for a couple of days. The underlying later of dry plaster is called the arriccio. Next, a layer of the wet plaster, called the “intonaco”, is applied to the wall or ceiling to be decorated.
The artist first pounces the pattern of the preparatory drawing, called the “cartoon”, onto the wet plaster, using the same technique that the old sign painters used. The artist then paints on the plaster using a simple mixture of water and ground pigment.
Michelangelo used a very limited palette of approximately seven colors. These were primarily earth colors, such as carbon black, ochres, sienna and terre verte. For blue, he used lapis lazuli. Because the plaster is highly alkaline, not all pigments are compatible with it and will fade in time.
The wet plaster dries in about six to eight hours, depending on the ambient temperature and humidity. That doesn’t give the artist much working time. For this reason, the artist only applies enough plaster that he is capable of painting in one day. The term that is used for this day’s work is “giornata”.
The wet plaster combines with the carbon dioxide in the air as it cools. As the plaster cures, the water evaporates. What remains is calcium carbonate, the same material that started this procedure. The chemical processes have come full circle.
Ca(OH)2 + CO2 → CaCO3 + H2O
The paintings of Michelangelo have endured for more than 500 years because the pigment is encased between the molecules of calcium carbonate of the plaster.