II.

Fresco Techniques

Pigment is applied to the top layer of several layers of plaster for a buon fresco. The painter usually applies to the next-to-last plaster surface a sketch, or cartoon, of the painting. The outlines of the various figures and forms of the cartoon are then reinforced with dark watercolor. Plaster is laid over the drawing in small sections, and color is applied to the wet plaster, often aided by another sketch of the color scheme. As the plaster dries, the lime in the plaster reacts chemically with the carbon dioxide in the air to form calcium carbonate; this compound forms a film over the colors, which binds them to the plaster. This makes them part of its actual surface and also gives the colors an unusual clarity. The colors of a fresco are usually thin, transparent, and light, often with a chalky look. In the Renaissance, methods were found to give the colors somewhat more opacity.

In buon fresco, the painting must be done quickly and confined to essentials. The artist must know precisely how much watercolor the plaster will absorb. Too much paint causes the surface to become “rotten.” Cutting away the defective portion, laying on fresh plaster, and repainting is then necessary.

In fresco secco, the dry plaster is rubbed with pumice stone to remove the crust, then washed with a thin mixture of water and lime. The colors are applied on this surface. The effect of fresco secco is inferior to true fresco; the colors are not as clear, and the painting is less durable.