Fresco: Painting Into Wet Plaster

Buon fresco means true fresco: pigment mixed with water only, applied straight onto wet plaster. The wall gets built up in layers. First arriccio, a rough coat of lime plaster that gives the surface shape. Over that the painter often sketches a sinopia, a reddish underdrawing showing the composition. Then comes intonaco, the thin final layer of wet lime plaster, and pigment goes onto it immediately while it is still damp. This forces the painter to plan the whole wall in advance and then execute it in sections, since there is no going back once a patch sets.

The chemistry is why the deadline exists. Wet lime plaster is calcium hydroxide. As it dries, it reacts with carbon dioxide in the air and carbonates into calcium carbonate, essentially turning back into stone. Pigment applied while the plaster is still wet gets trapped inside that crystal structure as it hardens, so the color becomes part of the wall rather than sitting on top of it. Once the plaster fully sets, it can no longer absorb pigment this way. Painters divide the work into giornate, one day's patch of fresh intonaco, sized to how much detail they can finish before the chemistry locks the wall shut. Michelangelo's Sistine Chapel ceiling is made of hundreds of these patches, each one a separate day's painting session fitted into the larger design.

Not every mark on a fresco survives that process. Fine details, gold leaf, or certain pigments that react badly with lime get added a secco, onto plaster that has already dried, using a binder like egg or glue instead of letting the wall itself hold the color. A secco work sits on the surface and flakes or fades faster than true fresco. This technique shaped how painters told stories across whole rooms: Giotto's Arena Chapel in Padua wraps three registers of giornate around all four walls, from Joachim and Anna at the top down to the Last Judgment, turning the room into one continuous narrative a visitor walks through.