FRESCO:
WALL PREPARATION
After clearing the wall to be used for the painting, any irregularities must be eliminated with a layer of smooth mortar called “rinzaffo”. At this point the “arriccio” is applied, with is coarse, rough layer of plaster composed of one parte slaked lime and two parts river sand, well washed, dried and sieved before use. Once the “arriccio” has dried, a much more refined and thinner layer of plaster, called “velo” is applied. On this layer, the artists drew the sinopia, a preparatory drawing wich served as an overview of the fresco and precisely defined the spaces to be painted. In the 15th century, they introduced the cartoon, namely a sheet transferred to the intonaco plaster either by pouncing or indirect incision. On a limited surface of the preparatory drawing, the artist spreads the “tonachino” so that it can be painted before it dries.

“GIORNATA” OR DAY’S WORK
The colour ha sto be laid on the still wet plaster to be part of the surface. The technique does not allow afterthoughts, because once the colour is applied, it is immediately absorbed. That is why the srtists identified the portions of the subject to be painted in one day: the so-called “giornata”, or day’s work. The “giornata” followed the course of the preparatory drawing and its surface could be of different sizes depending on the complexity of the image. It was possible to make corrections using the “ a secco” technique, for example with tempera or dry plaster, although they were more easily degradable.

PIERO’S “GIORNATE” OR DAY’S WORK AND THE PERSPECTIVE
The essential lines of the fresco of the Madonna del Parto are reproduced in this panel. They generally coincide with the preparatory drawing Piero della Francesca traced as a “guide” for the spreading of the colours. In some cases, in fact, the lines were traced directly onto the plaster, while the pouncing technique was used for the more complex parts repeated in different areas of the work (the curtain decoration for example). The broken line in the centre of the drawing represent the symmetry axis which guides the painting and exalts the central position of the Madonna. This line was used to “symmetrically” reverse the drawings, i.e. to maintain the equidistance from the central axis. The borders of the curtain decorated with pomegranates (magenta), its fur lining (beige) and the angels (gree-red) are placed symmetrically to the work’s central axis. The red lines trace the borders of the various “giornate”. In the work it is possible to see the border lines of the various “giornate” and understand their progression. They always proceeded from the top to the bottom (see numbering) in order to avoid spoiling the painting below.

MURAL PAINTING
We generally tend to define any type of mural painting a fresco, althoungh there are many differente techniques for painting on a wall. Prehistoric cave paintings were made “a secco”, drawing the images directly on the rock wall with coloured powders, derived from stones and coal, mixed with animal blood or fat. In the Roman era, encaustic painting was the most widespread; this technique was characterized by the use of colours with wax as a binder or the application of a thin layer of wax to protect and polish the painting.
Another technique widely used in ancient Rome, but in later periods as well, was “graffito”, which consisted in removing a layes of plaster lying on top of a previously coloured one. In the Middle Ages and the Renaissance, the most popular technique was instead fresco where pigments diluted in water are “encausto”, painted on fresh plaster, hence the name “fresco”. Very often frescoes were completed using the “a secco” technique or the “mezzo-fresco” painting on the nearly dry plaster.

FRESCO: PIGMENTS
The pigments used in fresco painting are all derived from minerals, which results in a limited colour range. The blues, for example, come from ground lapis lazul and from azurite. White is obtained from dried slaked lime. The backs come from stones such as graphite, or from charred plant twigs. In the artist’s workshop the preparation of the powders was entrusted to young students in the first year of their apprenticeship. When needed to be used, the pigments were dissolved in water and spread on the wet plaster. The wet plaster incorporates the colour particles through a chemical process which makes the painted film bright and durable.