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Intonaco refers to the final layer of plaster applied when creating a fresco painting. It consists of smooth, limestone-based cement that chemically changes as it dries, making the fresco a permanent part of a wall or ceiling. Buon fresco, a popular method of decorating walls during the Renaissance Era, incorporated art into the final layer of plaster.
Artists typically applied several layers of plaster before the intonaco finish. They created the plaster by mixing lime with sand and water. Lime is found in seashells, marble, limestone, and chalk. When the lime is ground and heated, it becomes quick lime or hot lime with a calcium sulfate base. Quick lime can be purchased at home improvement stores in powder form.
The initial layer of plaster used in creating frescos consists of coarse sand that creates a rough coat on a wall. A second application, called a brown layer or arriccio, follows before the intonaco goes on. Each layer adheres to the previous application because the chemical composition of lime changes when exposed to air.
Frescos must be painted onto the plaster while it remains wet and before a lime crust forms. Artists typically applied the intonaco layer in small sections to permit color to penetrate into the moist plaster. Once it dries, it becomes permanent.
Another form of fresco painting is called secco fresco. In this method, the artist adds eggs to the color pigment to make it stick to the intonaco. This artwork typically flakes off over time because it goes on after the plaster dries. Mezzo fresco artists applied the color when the intonaco was just about dry so some of the color was still absorbed. Color pigments came from natural minerals mixed with water.
Most frescos using these application techniques consist of limited colors. Lime is highly alkaline and reacts with some minerals, making them unsuitable for use. The hues change and fade when exposed to the elements.
A typical method of transferring the desired design onto the wall or ceiling involved a paper pattern of the scene. An artist would poke tiny holes in the pattern and sprinkle black dust through the openings to transfer the drawing onto the wall. This would leave a faint outline of the fresco on the intonaco, a process called pouncing.