Fresco and wall painting
"When the colours are carefully laid on the wet plaster, they do not on that account fade, but are eternally permanent."
"Therefore the plasterings which are properly done, neither become rough with age, nor, when washed, do their colours fade, unless laid on with little care, and when the plaster is dry".”
"When only one coat of sand mortar and one of fine marble have been spread on, this thin layer is easily cracked from want of strength, and from its lack of thickness it will not take on the brilliance, due to polishing, which ought to have [...] when, however, it has a solid foundation of sand mortar and of marble, thickly and compactly applied, it is not only brilliant after being subjected to repeated polishings, but also reflects from its surface a clear image of the beholder". (Vitruvius 7, 3,8-9).
Polishing by means of abrasives in order to achieve a mirror brightness indicates that Vitruvius is speaking of colored stucco, which effectively applies the colors on a fresh lime rendering. A similar polishing cannot be applied to a mural painting as it would eliminate many of the strokes. Modern texts on the execution of stucco describe a process very similar to that given by Vitruvius: "Apply the rendering coat to their lower surface; then lay on the sand mortar, and afterwards polish it off with crete or marble”(Vitruvius, 7, 3, 3). The application of pigments on the lime mortar rendering and their subsequent repeated polishing with crete or marble powder and polishing substances until a high degree of polishing is achieved, are traditional steps in the execution of coloured stuccoes that imitate polished marbles. Like Vitruvius and Pliny, stucco treatises recommend colouring them with pigments that are resistant to lime: “Preparation of colour tints is a particularly delicate matter, where only can be used pigments resistant to the action of light and lime". (Karl Lade and Adolf Winkler. Yesería y estuco. Barcelona,1960, 143). Treatises also recommend the final polishing with powdered minerals and polishes: "A very intense polishing can be obtained by rubbing the surface with talc applied on a wet cloth". (K. Lade, A. Winkler, 1960, 139). "When the stucco is well dry, one can proceed to polishing, rubbing it smoothly and ceaselessly; then a thin sand or silicon is laid, rubbing the surface with a piece of felt; afterwards the surface is washed with soapy water and finally it is rubbed with oil, producing an extremely shiny lustre” (M. Martínez Angel, Tecnología de los oficios de la construcción, Madrid 1930, 250-251).
It is important to bear in mind that coloured stucco technique cannot be used for the execution of artistic wall paintings, as it does not allow the painting of figures and detailed elements. In consequence, Roman wall paintings had to be done with a different technique, a painting technique applied on a dry endering that would allow themuse of pigments that cannot resist contact with lime.