This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Fresco painting depends on a narrow and unforgiving window of time. In buon fresco, a mason applies a thin, smooth top layer of lime plaster, called the intonaco, over a rougher base coat, and a painter must complete all work on that section before the plaster sets, typically within a single working day. This is why fresco painters historically divided a wall into sections known as giornate, literally days, and planned their compositions so that the boundary between one day's plaster and the next would fall along a contour, a fold of drapery or an architectural line where the seam would be least visible. If mistakes were made the entire day's layer of plaster had to be removed, which meant a fresco painter worked with almost no room for revision, unlike a painter working in oil or tempera. The technical foundation of the method is a chemical process rather than a purely mechanical one. Lime plaster is made from limestone that has been burned and slaked with water, and as it dries it absorbs carbon dioxide from the air and gradually converts back into calcium carbonate, the same mineral compound found in limestone. Because the pigment, ground in plain water with no binding medium, is brushed on while this reaction is still taking place, the color becomes physically locked into the crystalline structure of the wall as it hardens. This is what gives true fresco its extraordinary durability compared with painting on a dry surface, where pigment sits as a film that can flake, fade or be wiped away. Fresco secco, by contrast, offered painters far more control at the cost of permanence. Pigment was applied to plaster that had already dried and was often re-wetted with limewater, so the paint layer never fused with the wall and instead behaved more like ordinary paint on any other surface, allowing corrections and fine detail that pure buon fresco could not accommodate. Many historical fresco cycles combine both methods, with large fields of color and figures established in true fresco and finer details added afterward in secco, a hybrid practice that explains why some frescoes have lost certain details over centuries while the bulk of the composition survives intact.