How Fresco Painting Bonds Colour to Plaster

Fresco begins with a stable wall and layers of plaster. A rough lower layer evens the surface and provides support. Artists can draw the main composition on this preparation before applying the final smooth coat used for painting.

The final wet layer is called the intonaco. Only as much plaster as the artist expects to finish in one working session is applied. This daily section is sometimes called a giornata. Boundaries may follow figures, folds or architectural lines so joins are less noticeable.

A full design can be transferred from a large drawing called a cartoon. Artists may trace lines, press them into the surface or dust pigment through small holes. Planning matters because wet plaster waits for nobody and has never respected a creative block.

In true fresco, pigments mixed mainly with water are brushed onto fresh lime plaster. As the plaster reacts with carbon dioxide in the air, calcium carbonate forms. The pigment particles become locked into the hardening surface.

Because the colour is integrated with the wall rather than sitting only on top, a well-made fresco can be durable. Mineral pigments often work best because they can survive the alkaline wet plaster. Some colours change or fail under those conditions and must be added later.

Painting on dry plaster is called fresco secco. In that method, pigment needs a binder to stick to the wall. It allows details and colours unsuitable for wet plaster, but the paint layer may flake more easily because it does not become part of the wall in the same way. Conservators can often identify later dry-paint additions because they age differently from the surrounding true fresco.

Wet plaster absorbs paint quickly, so broad areas must be handled with confidence. Corrections are difficult once the surface begins to set. A badly painted section may need to be removed and replastered.

Fresco artists plan colour, sequence and working size before each session:

• •Prepare a clear full-scale design.
• •Choose a manageable area of wet plaster.
• •Use pigments stable in lime.
• •Paint broad forms before small details.
• •Hide joins along natural visual edges.

Leonardo da Vinci's Last Supper in Milan is often called a fresco, but it is not true fresco. Leonardo liked to work slowly, revisiting details and building subtle shading, which wet plaster does not allow. He experimented instead with paint applied to a dry wall. The result let him work as he wished, but the paint did not bond into the plaster. It began flaking and deteriorating within decades, and the painting has needed repeated, difficult restoration ever since.

Michelangelo's ceiling in the Sistine Chapel, by contrast, was painted mainly in true fresco. Over centuries it darkened under candle smoke, grime and layers applied by earlier restorers. A major cleaning in the late twentieth century revealed surprisingly bright, strong colours, which changed how many people understood his painting. The work was also controversial. Some critics argued that final touches Michelangelo may have added on dry plaster were removed with the grime, and that debate has never been fully settled.

Because a fresco is part of its wall, saving it from a damp or damaged building is difficult. Conservators developed ways to detach paintings. In one method, called strappo, cloth is glued to the painted surface with a soluble adhesive and then pulled away, bringing only the thin layer of paint and some plaster with it. In another, called stacco, a thicker layer of plaster is removed as well. The detached painting is then backed with a new support and the facing glue is dissolved away.

Detachment sometimes reveals the artist's underdrawing on the rough layer beneath. This drawing is called a sinopia, after a reddish-brown earth pigment named for the town of Sinope on the Black Sea. When fire damaged the Camposanto, a monumental cemetery in Pisa, during the Second World War, surviving frescoes were removed for conservation, exposing large sinopie that are now displayed in a museum of their own. They let you see the free, confident drawing beneath the finished painting, and sometimes show places where the artist changed the design.

The chemistry of fresco is a loop that starts and ends with limestone. Limestone, which is mostly calcium carbonate, is heated strongly in a kiln, driving off carbon dioxide and leaving quicklime, calcium oxide. Adding water to quicklime, a process called slaking, releases a great deal of heat and produces calcium hydroxide, the slaked lime used in the plaster. Once spread on the wall, that lime slowly absorbs carbon dioxide from the air and turns back into calcium carbonate. In effect, the painter's pigments are locked into a thin skin of newly made limestone.

That alkaline, slowly hardening surface decides the palette. Earth pigments such as yellow and red ochres, umbers and green earth are stable in lime and were the backbone of fresco painting, while lime itself served as the white. Blues were the great problem. Azurite, a copper mineral, could change colour in wet lime and was often added a secco with a binder, and ultramarine, ground from costly lapis lazuli, was usually applied dry as well. That is one reason the blue areas of some old frescoes have flaked or changed colour more than the rest.

True fresco did not end with the Renaissance. After the Mexican Revolution, the government of the early 1920s commissioned artists to paint large murals on public buildings, telling the nation's history to people who might never visit a gallery. Diego Rivera, who had studied Renaissance frescoes during a stay in Italy, became the best-known figure of the movement, alongside José Clemente Orozco and David Alfaro Siqueiros. Fresco suited the muralists' aims: it was durable, it became part of the building itself, and it could not easily be bought, moved or hidden away by private collectors.

In 1932 and 1933 Rivera painted the Detroit Industry murals at the Detroit Institute of Arts, 27 panels in true fresco showing workers at Ford's River Rouge plant. Assistants prepared fresh plaster each day, and Rivera often painted for long stretches to finish each section before it set. Fresco's permanence also had a cost. In 1934 his mural for the Rockefeller Center in New York was destroyed after he refused to remove a portrait of Lenin, because a painting bonded into a wall could not simply be taken down and stored. Rivera later repainted a version in Mexico City.

True fresco bonds water-based pigment into fresh lime plaster as the wall hardens. Daily sections, limited working time and pigment chemistry shape every decision the artist makes. Once you notice the joins, planning and confident brushwork, a fresco becomes not only an image but also a visible record of how it was built day by day.