Plaster is the final layer of limestone-based cement applied to create a permanent fresco. Different techniques are used, including adding eggs to pigment or applying color when the plaster is nearly dry. Lime’s alkalinity limits color options, and spolvero is used to transfer designs onto the wall.
Plaster refers to the final layer of plaster applied when creating a fresco. It consists of a smooth limestone-based cement that changes chemically 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 coats of plaster before the plaster finish. They created the plaster by mixing lime with sand and water. Lime is found in shells, marble, limestone and chalk. When lime is ground and heated, it becomes quicklime or hot lime made from calcium sulfate. Quicklime can be purchased at home improvement stores in powder form.
The initial layer of plaster used in making the frescoes is made up of coarse sand which creates a rough wall. This is followed by a second application, called a brown or arriccio layer, before the plaster continues. Each layer adheres to the previous application because the chemical composition of lime changes when exposed to air.
Frescoes must be painted over the plaster while it remains moist and before a lime crust forms. Artists typically applied the plaster layer in small sections to allow the color to soak into the wet plaster. Once it dries, it becomes permanent.
Another form of fresco painting is called dry fresco. In this method, the artist adds eggs to the colored pigment to make it adhere to the plaster. This artwork typically flakes off over time because it goes on after the plaster has dried. Mezzo fresco artists applied color when the plaster was nearly dry, so some of the color was still being absorbed. The color pigments came from natural minerals mixed with water.
Most frescoes using these application techniques consist of limited colors. Lime is highly alkaline and reacts with some minerals, making them unsuitable for use. Hues change and fade when exposed to the elements.
A typical method of transferring the desired design to the wall or ceiling involved a paper model of the scene. An artist punched small holes in the pattern and sprayed black powder through the openings to transfer the design onto the wall. This would leave a faint outline of the fresco on the plaster, a process called spolvero.
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