Painting Against the Clock

Wet lime plaster is the most demanding support a painter has ever worked on, and for four centuries it carried the most ambitious painting in Europe.

The chemistry is why. Lime plaster is calcium hydroxide; as it dries it reacts with carbon dioxide from the air and turns to calcium carbonate, the same substance as limestone, forming a crystalline surface. Pigment laid into the wet plaster is caught inside that crystal as it forms. The colour is not a film sitting on the wall. It is the wall.

That is why frescoes survive centuries of damp, smoke and neglect, and why they can be cleaned relatively safely when a panel painting cannot.

The price is a deadline. The plaster stays workable for a matter of hours, and once it has set it takes no more colour. So the painter has the mason lay only as much fresh plaster as can be finished that day (a giornata, a day's work) and cuts the edge of each patch along a contour where the join will be invisible. On the Sistine ceiling the giornate are countable, and from them you can reconstruct the order Michelangelo worked in, where he sped up, and where he stopped using assistants.

Everything therefore has to be decided in advance. Full-scale cartoons are drawn on paper, then transferred to the fresh plaster by pricking the outlines and dusting charcoal through the holes, or by scoring the lines with a stylus. You can see both on the wall if you look with raking light. Underneath, on the rough coat, there is often a sinopia: a red-earth underdrawing laid out before the final plaster went on.

The medium rewards decisiveness, broad handling and clear design, and punishes fussing. A large part of what we recognise as the monumental Italian manner is a working method dictated by how fast lime sets.

When a painter wanted something the method forbade (very fine detail, or pigments that lime destroys, such as azurite and ultramarine) they worked a secco, on dry plaster, with pigment in a binder sitting on the surface. It allows revision, it allows delicacy, and it does not bond. Secco is almost always the first thing to be lost, which is why medieval walls that look bare frequently carried a layer that has flaked away, and why blue skies are missing from murals whose other colours are intact.

Which brings us to the most famous mural in the world and the reason it is a ruin.

Leonardo, painting the Last Supper in Milan between 1495 and 1498, would not accept the deadline. He worked slowly, returned to passages, changed his mind, and reportedly stood in front of the wall for hours without touching it. Fresco cannot accommodate any of that. So he painted on dry plaster, in a mixture of oil and tempera over a sealing ground.

It never bonded. Flaking was reported within about twenty years, and Vasari, seeing it in 1556, described it as a blur. It has been restored repeatedly for four hundred years, most recently in a twenty-one-year campaign that stripped earlier repaintings and left a fragmentary original. The freedom that let Leonardo compose the picture is precisely what destroyed the object.

The other great fresco story is about dirt.

Between 1980 and 1994 the Sistine ceiling was cleaned, and the result caused one of the fiercest disputes in the history of conservation. Under four and a half centuries of candle soot, lamp black and animal glue applied by earlier restorers were colours nobody expected: acid greens, shot pinks, oranges against blues. Critics argued that the restorers had removed a final layer of glazing in which Michelangelo had unified the tones, and that what emerged was a stripped picture.

The consensus settled the other way. Fresco does not take glazing well, the tonal handling is consistent with the cleaned passages, and comparisons with protected areas support the bright reading. The sombre, sculptural Michelangelo that art history had been built on was largely soot, and the painter of those colours is a slightly different artist from the one in the older books.

That is a recurring lesson in this part of the subject. What you are looking at is the current state of a physical object, and technique determines how far that state has drifted from the decision.