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Introduction – How Frescoes Were Made

A fresco is a special type of mural painting where the artist paints on plaster (or intonaco) that is still fresh. The result is a painting in affresco or a fresco. Although a fresco is sometimes touched up when the plaster has hardened, painting on plaster that has already dried is not a fresco in the strictest sense.

The painter Cennino Cennini, who studied in the school of the Florentine master Agnolo Gaddi, explained in his treatise Il Libro dell’Arte in 1390 that in true fresco the paint must be applied to the damp or fresh (fresco) intonaco as soon as it is laid on the wall. The pigment then penetrates the wet plaster. As it dries and hardens, a chemical reaction takes between the plaster and the carbon dioxide in the air that holds the pigment in a hard crust of calcium carbonate – the principal component of marble, limestone and oyster shells.

When, an artist paints on dry (secco) plaster, a binding medium is needed to stick the paint to the wall. Egg white is used in tempera paintings; oil in oil paintings. Lime – calcium hydroxide, the reactive ingredient in plaster – is also used. In this case, the result resembles a painting executed afresco, as the same chemical reaction takes place between the lime and the air. This technique of painting is known as a secco, as it is performed on dry plaster.

Before the time of the great pioneer Cimabue, wall-paintings were usually painted a secco. But the great mural paintings of the golden age of the Italian Renaissance are true frescoes. Later the secco technique was used again. From the mid-sixteenth century a compromise called mezzo fresco, or half fresco, was developed. In mezzo fresco the painting is done on plaster that has partly dried. However, the colours penetrate less deeply than in a true fresco. But by the end of the seventeenth century the old techniques of true fresco painting were all but forgotten.

An advantage of painting a fresco is that the resulting surface is so smooth that rain, for example, will run off. For secco painting to adhere to the wall, the surface of pre-existing plaster has to be roughened, or new plaster has to be scored with a trowel or mixed with rough sand. This leaves it more vulnerable to the elements and later secco additions to Renaissance frescoes have gradually faded while the original fresco painting has remained.

However, the blues in skies and robes of true frescoes have often been lost, leaving the red preparation underneath. This is because the only types of blue known to early painters –costly lapis lazuli (ultramarine) and Alemagna blue (azurite) – were not completely soluble and so could not penetrate the wet plaster. However, the earth colours were soluble and dispersed easily. The blues had to be applied a secco, using glue rather than egg yolk which would have turned them green. Slight variations of tone occurred in the same colour applied on different days. These variations were remedied by later retouching in tempera. And colour refinements were made by retouching a secco, after the fresco was finished.

Almost all these additions have disappeared. Only in one case – some small figures of the Virtues around a window in the Cathedral of Pistoia by Alesso di Andrea – has this tempera retouching been preserved. This is because the window was walled up soon after the fresco was finished and has only been reopened recently.

So why did artists abandon fresco painting for techniques that were both less aesthetically less satisfying and less resistant to time and atmospheric conditions? One reason was simply that painting a fresco was much harder than painting a secco. Working on damp plaster, the artist could not see the exact tone of his colours. When the wall was wet, the colours did not look the same as they do when it was dry. As each brushstroke is immediately absorbed by the plaster, there is no room for error or changes. In secco painting mistakes can easily be remedied. But the main reason that fresco painting was abandoned was that painting a secco was much quicker. By the sixteenth century, the painter was no longer considered an artisan. They had a new prestige and had to work fast to handle a growing order book of the commissions. And pupils were not prepared to serve the long apprenticeship needed to become proficient in fresco painting when there was premium on speed.

However, Michelangelo swam against the tide. Though the altar wall of the Sistine Chapel had already been plastered so that his Last Judgement could be painted a secco in oil, Michelangelo insisted that the plaster be ripped out. He would paint a fresco instead.

“Oil painting is for women, and slow and slovenly people,” he said.

The fresco painters of the fourteenth and fifteenth centuries prepared every detail of their work. They personally spread the arriccio, the rough layer of coarse plaster which the upper layer of smooth plaster (or intonaco) stuck to. It was on this second layer that the painting was executed.

A grid was draw on the wall by pressing a fine cord soaked in red paint, fastened at each end and pulled taut, against the arriccio. The fresco was then sketched in charcoal. When the artist was satisfied, the outline was painted in a thin solution of ochre and the charcoal lines brushed out with a bunch of feathers. The faint ochre was then retraced with special red earth called sinopia. The resulting large mural drawings were called sinopie and have been rediscovered where frescoes have been removed for the purposes of conservation.

The sinopie were always executed by the artist himself, never a pupil, though pupils were often employed to paint the fresco itself. It was often used to give the patron who had commissioned the work some idea how the finished work might look. It is clear from exposed sinopie that patrons sometimes ordered changes.

The artist began his fresco by spreading the smooth intonaco on the arriccio. In true fresco, he applied only as much as he could paint in one day. The artist always began at the top and worked down. On the surface of frescoes, you can see the giornate – that is, the extent of each day’s work. The edges are marked by a slight bump where each new section slightly overlaps the previous day’s.

Although the sinopia disappeared under the new plaster, the lines were quickly retraced on the intonaco. Then the painting began. The size of the giornate varied with the complexity of the section and to the speed of the artist. In mezzo fresco, the giornate are much larger. In secco painting, they are not visible as all the intonaco was laid in one go and left to dry and harden.

When a large surface was be painting, the walls of a chapel, say, there are also a pontate divisions, which were determined by the level of the scaffolding. The intonaco was only laid as far down the wall as the artist could comfortably reach. When one section was completed, the scaffolding was lowered and new intonaco was laid below the section. This left horizontal joins, running parallel to each other at uniform intervals.

The use of sinopie declined after 1440, though it was taken up again at the end of the sixteenth century in Florence. It was replaced by the technique of spolvero (dusting). The fresco was sketched out on large sheets of paper. The outlines were pricked out with a needle. Then the drawings were cut into sections each corresponding to a day’s work. When the intonaco for the day had been laid, the section of the drawing was placed over it and dusted with fine charcoal powder. The dust passed through the perforations and stuck to the intonaco, making little black dots that are still visible today.

At the beginning of the 1500s, this technique was replaced by the cartoon. Here the section of the sketch was placed over the intonaco and traced with a sharp stylus, leaving the outline pressed in the fresh plaster.

While some fresco painters enjoyed the challenge of drawing their murals out full size, others divided the workspace up using a grid. This way they could scale up designs from smaller drawings. Leon Battista Alberti claims to have invented this method in his influential book On Painting of 1435. However, squares can be seen incised in the face of the Virgin in Masaccio’s Trinity, painted in Santa Maria Novella in Florence between 1425 and 1427. In the end, this technique took over completely, leaving art historians a valuable cache of preliminary drawings.

Even the paint is locked inside the plaster, frescoes do decay with age. Often inexpert restoration aggravates the condition it sought to rectify. Damp, seeping up from the ground or down from the roof, is the great enemy of mural paintings. Those at greatest risk are in the open air – on the façades of buildings, in cloisters, porticoes or tabernacles. It attacks the fabric of the intonaco and brings nitrates and other salts to the surface where they destroy the colour. The plaster crust holding the paint disintegrates and drops off in tiny flakes.

This happened on vast scale then Florence, the most heavily frescoed city in the world, was flooded in November 1966. Many of the frescoes had to be detached in sections from the walls where they had sat for centuries so they could be restored. Another great tragedy occurred in 1997, when a series of earthquakes hit Assisi, where frescoes began. They damaged priceless work that graced the walls of the Basilica of St Francis for over six hundred years. Painstakingly, they have now been pieced together again.