The technique of fresco painting consists of painting colour pigments onto a layer of plaster that is still wet or "fresh" as the Italian term fresco suggests.
Walls to be frescoed were normally constructed from a single material, generally stone or brick to prevent damage to the frescoes from any movement in the wall due to settlement.
The artist would begin by applying a layer of plaster, made from a mixture of water, slaked lime, and large-grained river sand, to the wall.
This first layer, known as the arriccio, was applied to a thickness of one centimetre. The surface had to be very rough to allow the second layer of plaster to adhere to it easily.
On top of the arriccio, the outline of the fresco was drawn in charcoal, which was then erased using feathers after a second outline had been applied in ochre.
Over the ochre, another outline was added with a red pigment called sinopia, a term which came to be used to describe the preparatory drawings it was used for.
Finally, a fine layer of plaster called intonachino or velo was applied. This was transparent and much smoother than the arriccio and was usually made from one part slaked lime and two parts finely-ground sand. The surface had to be perfectly smooth surface and remain wet throughout the entire painting process and was therefore only applied to an area that the artist could complete painting in one day. For this reason, each one of these sections came to be known as a giornata, or 'day's work'.
The painter began by copying the sinopia outline, which showed through from the layer underneath, onto the wet plaster, and then started to paint using ground pigments mixed with water.
Faces were usually painted starting with light shades and progressing to darker ones. The opposite technique was used for clothing, in which dark shades followed lighter ones.
Not all pigments could be used. Indeed, when the plaster dries, the lime which it contained causes a chemical reaction called carbonation, which releases heat and can burn plant-based pigments. For this reason, some touching up was often required after the plaster had dried to perfect details. This technique is called a secco, or dry.
Blues, too, had to be painted a secco. This was true for both the extremely costly lapis lazuli or ultramarine, and azurite or "German blue".
By the mid 15th century, the sinopia technique began to be replaced with preparatory cardboard, or cartone. The drawings were done to a smaller scale on squared paper in the artist's workshop.
A larger-scale grid with the same number of squares was then drawn on a piece of paper the same size as the fresco. The smaller drawing was then copied onto the large one, enlarged square by square.
Holes were poked in the large drawing using a big needle and a loosely-woven sack of coal dust was passed over it.
This technique, known as spolvero or dusting, resulted in a continuous series of little dots which the artist would join together to create the drawing.
During the Renaissance a third method was also developed in which the drawing, enlarged using the grid method, was transferred onto thin paper. This was laid over the fresh plaster and a long nail was used to trace the lines and transfer the composition onto the mortar.