Fresco is a painting technique wherein the paint is applied to a plaster wall, or the “ intonaco,” that is still wet, so the wall absorbs the color of the paint while drying. This method makes the painting permanently set on the wall, evidenced by a matte, less shiny finish, as opposed to applying paint to an already-dried wall.
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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).
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.
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.
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.
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 paintings of Michelangelo have endured for more than 500 years because the pigment is encased between the molecules of calcium carbonate of the plaster.
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.