Fresco Painting: How Artists Paint Into Wet Plaster

Fresco is painting with pigment mixed in water onto wet lime plaster, so the colour is locked into the wall as the plaster dries and hardens, making it durable.

Category: Visual Arts · Created: 2026-10-15 · Updated: 2026-10-15 · 3 min read

Fresco is a wall-painting method in which pigment mixed with water is applied to fresh, wet lime plaster. As the plaster dries, it reacts with carbon dioxide from the air and forms a hard crust of calcium carbonate, which traps the pigment inside the surface. The painting becomes part of the wall rather than a layer sitting on top of it.

How is a fresco made?

The wall is built up in two coats of plaster. The first, rough coat is called the arricio. The second, thin finishing coat is the intonaco, and it is the surface the painter works on. Only as much intonaco is spread as can be painted before it sets. That area is called a giornata, an Italian word meaning a day's work.

Before the plaster went on, the design was usually drawn full size on paper, a sheet called a cartoon. Its outlines were pricked with small holes and dusted with charcoal, so the drawing transferred to the damp intonaco as a row of dots. The painter then worked along those lines.

The painter must work quickly. Pigment mixed with water sinks into the damp plaster as it is brushed on, and the colours look different once they dry. Painters judged how wet the surface was by its look and feel, and they planned each giornata so that it could be finished in one session.

What happens in the plaster?

The plaster is the moving part of the process. Slaked lime is calcium hydroxide, and as the water evaporates it reacts with carbon dioxide from the air: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The product is calcium carbonate, the same mineral as limestone. Tiny crystals of it grow between the pigment particles and lock them in place.

This is why the painting is permanent. The pigment is not glued on. It is trapped in a mineral crust that grows as the wall dries, and the reaction can take many days to finish. Fresh lime plaster is strongly alkaline, and it is the high pH that lets it take up carbon dioxide and set.

Why is fresco hard to do?

Fresco leaves little room for error. A mistake cannot easily be painted over, so the painter cuts out the damaged piece of wet plaster and replaces it with fresh plaster. Each giornata has to be finished before the surface sets, which turns a large wall into a sequence of separate small jobs.

The choice of pigment is also limited. Earth colours such as ochres, and lime white, are stable in lime. Many blues and greens made from copper, and some other pigments, react with lime and cannot be used this way. Painters often added those colours after the plaster had dried, using a method called fresco secco. The result is less durable, because the colour sits on the surface rather than inside it. Blue was the most expensive colour on the palette, and its best-known source is described in the article on ultramarine blue.

Where can you see famous frescoes?

Giotto painted the Scrovegni Chapel in Padua about 1305, and its wall and ceiling scenes are among the best-preserved early fresco cycles. Michelangelo painted the ceiling of the Sistine Chapel between 1508 and 1512. Most of it is buon fresco, though some areas were finished secco.

Older examples survive too. Roman painters decorated walls in Pompeii and Herculaneum with plaster paintings, and many were preserved when Mount Vesuvius buried both towns in 79 CE. Layers of plaster on a wall can be read in much the same way as archaeological layers, showing which coats were applied first.

Why does fresco still matter?

Fresco shows that a painting does not have to sit on a surface to last. In fresco the binder is the wall itself, so the picture and the structure age together. Conservators today study the lime chemistry of fresco to decide how to clean and stabilise walls that have survived for centuries.