How is fresco secco different from buon fresco?

In the context of art conservation and restoration, the primary difference between buon fresco and fresco secco lies in the state of the plaster surface during application and how the pigments bind to that surface.

Buon Fresco ("True" Fresco)

Buon fresco is considered the most durable technique. In this process, water-based pigments are applied directly onto a freshly prepared, damp lime-plaster surface (intonaco). As the plaster dries, a chemical reaction occurs: the lime absorbs carbonic acid from the air and converts into calcium carbonate. This process causes the pigment particles to become permanently fused and cemented within the wall itself, making them an integral part of the structure rather than a superimposed layer.123

Fresco Secco ("Dry" Fresco)

Fresco secco involves painting on plaster that has already dried. Because the chemical binding process of buon fresco cannot occur, the artist must mix the pigments with a binding medium—such as egg, oil, gum, or glue—to ensure they adhere to the wall. Unlike buon fresco, the paint forms a surface film rather than penetrating the plaster, which makes it more susceptible to flaking over time. This method is often used for detailed work or for retouching areas of a buon fresco mural.3412