All through the Middles Ages artists painted their pictures directly on the walls of churches. Their “colors” were ground inorganic matter, such as the semi-precious stone lapis lazuli for blue, or malachite, a copper ore, for green, or earth scooped right out of the ground (sienna, umber, terra verte, Naples yellow). White was lime and black was charcoal.
To make their colors stick to the wall, they mixed them with eggs—with egg-white, egg-yoke, or both.
An older way was fresco. They spread a layer of sand mixed with lime over the wall and painted their picture while it was still wet or fresh—fresco in Italian. With fresco they didn’t need to mix their colors with eggs: plain water was good enough. It was like water painting: the wall absorbed the paints like modern water-color paper does. And when it was dry, the colors were somehow (a chemical change) fused with the lime and couldn’t even be rubbed off.
An illustration from from Techniques of the World’s Great Painters, edited by Waldemar Januszczak, QED Publishing, Ltd., 1980
How to paint in fresco
Preparing the brick or stone wall for the fresco had to be done just right. The painter applied several layers of the lime-and-sand plaster: the first he mixed with rough sand and maybe ground brick; the second with sifted sand; and the third, the layer that had to be smoothest—the one he would paint on—he mixed with his finest sand.
Cracks would appear if the lime and sand mixture was too thick; a mold would form on the surface of the picture if the wall stayed damp or if there were certain chemicals in the water. Condivi tells how Michelangelo fought with these problems and was so discouraged that he wanted to give up painting the Sistine Chapel:
“…in the midst of all this, he was not without anxieties because, when he had begun the work and completed the picture of The Flood, it began to mildew so that the figures could barely be distinguished. Therefore Michelangelo, reckoning that this must be a sufficient excuse for him to escape such a burden, went to the pope and said to him, “Indeed, I told Your Holiness that this is not my art; what I have done is spoiled. And if you do not believe it, send someone to see.” The pope sent San Gallo, who, when he saw it, realized that Michelangelo had applied the plaster too wet, and consequently the dampness coming through produced that effect; and, when Michelangelo had been advised of this, he was forced to continue, and no excuse served.” (Condivi, Life of Michelangelo)
You have to work fast when painting fresco—that is one of its peculiarities. You must finish your picture before the lime sets up, which will happen in only a few hours. That means you have to plan your work carefully: divide the wall up into sections only as large as you can finish in a day.
And you must know exactly what you will paint: there is no time to consider changes as you work. This is what the cartoons are: your full-size versions on cloth or paper of the wall-paintings. You draw these at home or in your workshop. Then you carry them to the church or wherever you are working, and haul them up the scaffold. As soon as you have applied the last layer of lime on the wall, you spread the cartoon over it, tack it to the plaster, and trace its outlines—scratch them right into the surface of your fresco with a stylus.
Here in this close-up of Michelangelo’s famous Adam the stylus grooves are clearly visible in the eye-brows, eyes, nostrils, hair-line, and collarbone.
(See the comments to this post, especially those by an expert fresco painter.)
The painter carefully trims the edges of his plaster each day; but as there is no way of blending the new day’s lime with the old, a seam between them is always visible; and so, by these seams, you know how many days it took him to paint the fresco, and just what he painted on what day.
Michelangelo’s Delphic Sybil was painted in six days, according to a study of the divisions shown in this diagram, published in La obra píctorica completa de Miguel Angel in the Clásicos del arte series, Noguer-Rizzoli editores.
The black lines show the limits of each day’s plaster (each day’s work). The dashed lines were traced into the wet plaster as general guidelines. The crosses show where Michelangelo hammered in nails to hold his cartoon while he traced it on the plaster.
He painted the Sybil’s head and some of the background on one day; her arm on the next, and so on. Did he spend a whole day on that single little foot? Maybe he was dissatisfied with the first one he or a helper had painted, and did it over.
After a fresco is finished it can be touched up with the other method of painting: mixing your colors with eggs (called tempera). Michelangelo did this here and there, and meant to do more; but Pope Julius was too impatient to wait for him to finish the ceiling as he wanted and ordered him to take the scaffolding down and open the chapel to the public.
“What was lacking,” says Condivi, “was the retouching of the work a secco (Italian for “dry”, the opposite of fresco. It is the painting you do after the plaster has dried) with ultramarine and in a few places with gold, to give it a richer appearance. Julius, when the heat of his enthusiasm had subsided, really wanted Michelangelo to furnish these touches; but when Michelangelo thought about the trouble it would give him to reassemble the scaffolding, he answered that what was lacking was nothing of importance. ‘It really ought to be retouched with gold,’ answered the Pope, to whom Michelangelo responded with the familiarity which was his way with His Holiness, ‘I do not see that men wear gold.’
The Pope said, ‘It will look poor.’
Michelangelo rejoined, ‘Those who are depicted there, they were poor too.’
So he remarked in jest, and so the work has remained.”