Fresco painting is the most famous of all mural techniques. People who should know better misuse the term "fresco," using it to describe every type of wall painting.
However, it means only a picture painted on a freshly plastered wall while still quite wet. It is more correct to speak of "fresco buono."
The Italian word "buono" (good) is used to distinguish authentic fresco technique on fresh plaster from "fresco secco," in which the plaster, though fresh, is quite dry.
The latter process is actually the more ancient. It was known to the Egyptians, Greeks, and Romans; and its technical peculiarity is the admixture of a binder, in this case, size.
Byzantine and Late Roman paintings in fresco secco are celebrated for their use of egg yolk; their work was often given a protective coat of wax varnish, but this was possibly not until a later date.
There are many fresco secco techniques, but all require wall plaster specially prepared for the purpose which led to the discovery of the fresco buono process.
Lime was known as a thin liquid binding agent at a very early time. After a number of intervening developments, the first attempts were made, in Italy about 1300 A.D., to paint "al fresco" on fresh plaster with unbound watercolor. The first to appreciate this seemingly modern technique and to make brilliant use of it was Giotto.
The technical preparations center around the production of suitable plaster, capable of taking up the colors from underneath. The reason for this is that the colors are mixed with water alone, or with lime water, which binds them better.
For the mortar, only pure lime mortar will do. Cement or diluted cement mortar is quite unsuitable. Fresco mortar is always made out of burnt lime slaked in the quarry. It must stand in the quarry in at least four inches of water for a minimum two months, and much longer if possible. The mortar is composed of this quarry or bog lime, mixed together with sand and water. Spiky river-grit and sand are suitable, but for the best fresco mortar, marble grit or marble sand is used, as they produce the whitest and hardest plaster. You may like to think of lime plaster as a primer on a brick wall.
This primer holds the colored pigment, but it will do so only as long as it is quite fresh, which, in practice, means soft and wet. Thus, one of the chief points is that the brick wall should be thoroughly wetted before the plaster is laid on it; if possible this should be done some days previously. To give a better hold, the joins between the bricks may be scraped out about half an inch.
The first layer of plaster consists of about four to five parts grit and coarse sand and one part lime. It should set very hard indeed and if need be can be made thicker still by beating it with a stick. It should be about one inch thick. If beating makes the surface too smooth, it should be roughened to provide a good hold for the next layer; this consists of about three parts finer sand and one part lime. Lastly, a layer known as the fine plaster, consisting of two parts fine and finest sand and one part lime, is applied. Rub this surface down with an emery board to make it really smooth.
This kind of plaster will be dry on top in at least three days. When the colored pigments have been applied with water, a thin transparent film of crystalline carbonate of lime is formed. The pigments become more or less petrified together with the plaster. At feast this is the aim, but it succeeds only if the carbonic acid in the atmosphere penetrates the entire layer of plaster by about one and one-half to two inches. The carbonic acid in the atmosphere can reach the lowest layer only if the plaster is sufficiently porous. The consistency to aim for is one which appears to contain too much water. Some of this water will be used up in crystallization and the rest will evaporate, thus inducing a certain porousness.
The success of these hardening and evaporation processes depends on factors which can be neither predicted nor regulated with accuracy. What tips the scales is the composition of the limestone and the atmosphere after the application of the plaster. Heat causes the water to evaporate too fast, so that it is no longer available for crystal formation. The most favorable conditions are an average temperature of about 60 degrees F., as this enables the evaporation and the absorption of acids from the atmosphere to proceed together. It is worthwhile providing artificial means of supplying
these acids, as the air contains only a small percentage.