Buon fresco pigment is mixed with room temperature water and is used on a thin layer of wet, fresh plaster, called the intonaco (after the Italian word for plaster). Because of the chemical makeup of the plaster, a binder is not required, as the pigment mixed solely with the water will sink into the intonaco, which itself becomes the medium holding the pigment. The pigment is absorbed by the wet plaster; after a number of hours, the plaster dries in reaction to air: it is this chemical reaction which fixes the pigment particles in the plaster.
The chemical processes are as follows:
In painting buon fresco, a rough underlayer called the arriccio is added to the whole area to be painted and allowed to dry for some days. Many artists sketched their compositions on this underlayer, which would never be seen, in a red pigment called sinopia, a name also used to refer to these under-paintings. Later, new techniques for transferring paper drawings to the wall were developed. The main lines of a drawing made on paper were pricked over with a point, the paper held against the wall, and a bag of soot (spolvero) banged on them on produce black dots along the lines. If the painting was to be done over an existing fresco, the surface would be roughened to provide better adhesion. On the day of painting, the intonaco, a thinner, smooth layer of fine plaster was added to the amount of wall that was expected to be completed that day, sometimes matching the contours of the figures or the landscape, but more often just starting from the top of the composition. This area is called the giornata ("day's work"), and the different day stages can usually be seen in a large fresco, by a sort of seam that separates one from the next.
Buon frescoes are difficult to create because of the deadline associated with the drying plaster. Generally, a layer of plaster will require ten to twelve hours to dry; ideally, an artist would begin to paint after one hour and continue until two hours before the drying time—giving seven to nine hours' working time. Once a giornata is dried, no more buon fresco can be done, and the unpainted intonaco must be removed with a tool before starting again the next day. If mistakes have been made, it may also be necessary to remove the whole intonaco for that area—or to change them later, a secco.
An indispensable component of this process is the carbonatation of the lime, which fixes the colour in the plaster ensuring durability of the fresco for future generations.
A technique used in the popular frescoes of Michelangelo and Raphael was to scrape indentations into certain areas of the plaster while still wet to increase the illusion of depth and to accent certain areas over others. The eyes of the people of the School of Athens are sunken-in using this technique which causes the eyes to seem deeper and more pensive. Michelangelo used this technique as part of his trademark 'outlining' of his central figures within his frescoes.
In a wall-sized fresco, there may be ten to twenty or even more giornate, or separate areas of plaster. After five centuries, the giornate, which were originally nearly invisible, have sometimes become visible, and in many large-scale frescoes, these divisions may be seen from the ground. Additionally, the border between giornate was often covered by an a secco painting, which has since fallen off.
One of the first painters in the post-classical period to use this technique was the Isaac Master (or Master of the Isaac fresco, and thus a name used to refer to the unknown master of a particular painting) in the Upper Basilica of Saint Francis in Assisi. A person who creates fresco is called a frescoist.