II.

Fresco Techniques

In true fresco, pigment is applied to the top layer of several layers of plaster. To the penultimate layer of plaster, the painter usually applies a sketch, or cartoon, of the painting. The outlines of the various figures and forms of the cartoon are then reinforced with dark watercolour. Plaster is laid over the drawing in small sections, and colour is applied to the wet plaster, the painter often also working from a separate sketch of the colour scheme. As the plaster dries, the lime in the plaster reacts chemically with carbon dioxide in the air to form calcium carbonate; this compound forms a film over the colours, which binds them to the plaster, making them part of the wall's surface and giving the colours an unusual clarity. The colours of a fresco are usually thin, translucent, and light, often with a chalky look. In the Renaissance, methods were found to give the colours somewhat greater opacity.

In true fresco, the painting must be done quickly and confined to essentials. The artist must know precisely how much watercolour 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 colours are applied to this surface. The effect of fresco secco is inferior to true fresco; the colours are not as clear, and the painting is less durable.