FRESCO PAINTING. Painting in fresco is here defined as painting on wet lime plaster with pigments mixed with water or water and lime. As the water evaporates, the setting of the lime binds the pigment to the plaster, and the subsequent con version of the lime into carbonate of lime by the action of the carbonic acid gas in the air completes the binding of the pigment. This method of painting is very old—the wall paintings in the palace of Knossos are examples of genuine fresco painting.
The Egyptians, Greeks and Romans were familiar with the use of egg, gum arabic, size and wax as media for painting, and there has been a big controversy as to how the Pompeian frescoes were painted. Vitruvius gives a detailed account of the prepara tion of the plaster—a method that was obtained by the Romans from Greek practice. The Roman plaster was several inches thick, and beginning with sand, lime and broken brick, the final coats were made of lime and marble dust—the lime having been pre pared by prolonged treatment with water, pounding and mixing. The marble dust, lime mortar was beaten and trowelled until a highly polished surface was obtained. The evidence from Vitru vius' account is on the whole, in favour of the pigments mixed with water being laid on the wet plaster. It has, however, been pointed out by Prof. Berger that this description may apply only to the ground colouring and not to the final painting, which he believes was done with a wax emulsion. Walls were certainly treated with wax driven in by heat, in Roman times. Vitruvius describes the process, and there are other references in Pliny and elsewhere. The most recent conclusions are in favour of the process having been genuine fresco. The description of mediaeval fresco painting given by Theophilus and the Mount Athos manu script are of a method of painting by wetting the already dry plaster and painting with pigments mixed with water and a little lime.
In the 15th century, Italian painters distinguished between fresco secco—the pigments being mixed with egg as a medium, and buon fresco—the pigments being mixed with water and laid on the fresh wet plaster. The complete design for the fresco was drawn or pounced from a cartoon on the dry plaster. Over this was laid each day a thin coat of wet plaster of sufficient area for one day's work, and the pigments ground in water laid on this wet surface. At the end of the day, the plaster not painted upon was cut away, and the joins of the plaster can be seen on the walls to-day.
The proportions of lime to sand should be according to Cen nino Cennini (see trans. by C. J. Herringham, 1899), one of lime to two of sand. It is essential that the quick lime slaked to a lime putty should be given months or even years to mature. The pigments must be fast to lime and light. For white, either lime or Bianco Sangiovanni prepared by making slaked lime into little cakes and exposing them to the air for some months should be used. For the rest, the earth pigments—red and yellow ochres, terra verte, umber, raw and burnt sienna, black and the perma nent greens and blues, such as oxide of chromium, viridian, cobalt green, cobalt blue and cerulean blue should be used.
Italians before the invention of smalt were confined to azurite which is turned green by lime in time and the precious real ultramarine. They probably, therefore, laid on the blues mixed with egg.