Fresco technique

The term "fresco" is commonly used in a broad sense, to indicate a mural whatever the technique used.
The etymology of the word fresco helps us to identify its meaning: it means a painting technique performed on a surface of pluster  still wet, through the application of colors predominantly inorganic simply crushed and diluted in water without any glue.
The plaster is made of seasoned lime, and sand.  The "nature" of the binder is the main indicator of a painting technique.
The binder of fresco painting is the lime contained in the plaster or rather its transformation process during drying (carbonation). The pigment finds the binder in the pluster. This process is called  " lime carbonation".
The hydrated lime plaster, in contact with the air dries slowly and hardens the sand and incorporating the pigments in a hard microcrystalline film.
The pigments spread out on the plaster are physically incorporated in the  crystallization surface film of the lime contained in the plaster, thus remaining fixed to plaster, in a process similar to the formation of natural rocks. Fresco tecnique was practiced since prehistoric times,  but often it was not distinct from the technique of tempera. In the other painting techniques the binder is mixed with the color.
Often the two techniques were adoperate together, merging and complementing each other, in many cases, in fact the fresco paintings were completed by tempera retouches. The use of both techniques allowed the extension of the color palette allowing the application of dry colors alterable by lime.
The ideal pigments, in fine powder, are the natural lands of the group of the oxides or hydroxides of transition metals such as iron (Fe), aluminum (Al), manganese (Mn), or hydrous silicates. The palette of the fresco thus has a limited range of colors than other types of painting.
The qualities that must have colors for the fresco are: resistance to lime, preservation of the quality, stability over time, growth of tone - purchase in brilliance. The colors tend to lighten wiping.
The pigments for the fresco should be chemically stable, free of acidic components, organic or inorganic which would tend to react with lime decomposing or altering the tint, and of molecules sensitive to water, air and light.
The air, pollutants and moisture can change the properties and chemical composition of a pigment through chemical reactions of oxidation etc. that transform the compounds in others, with different composition and coloring. In addition, the chemical character of a particular pigment may preclude its use joint to another with which would be incompatible and therefore unstable. The beauty of the fresco is due precisely to this process of maturation of the painting called "carbonation".
The final outcome of the work is unpredictable, even by the most experienced artist.
The carbonation time is about 50 or 60 years, but much of its success can be checked within a year, six months for taking the plaster. Plaster colors have hue, saturation and brightness  very different from the one that will have when after a long time the process of carbonation is completed . Time reveals the excellent work or condemns it.
Features:
-simplicity of materials used,
-difficulty of predicting the outcome of the colors,
-resistance in time,
-difficulty of execution: precision and speed of gesture, no second thoughts.