Buon’, or “true,” fresco is the most durable technique and consists of the following process. Three successive coats of specially prepared plaster, sand, and sometimes marble dust are troweled onto a wall. Each of the first two rough coats is applied and then allowed to set (dry and harden). In the meantime, the artist, who has made a full-scale cartoon (preparatory drawing) of the image that he intends to paint, transfers the outlines of the design onto the wall from a tracing made of the cartoon. The final, smooth coat (intonaco) of plaster is then troweled onto as much of the wall as can be painted in one session. The boundaries of this area are confined carefully along contour lines, so that the edges, or joints, of each successive section of fresh plastering are imperceptible. The tracing is then held against the fresh intonaco and lined up carefully with the adjacent sections of painted wall, and its pertinent contours and interior lines are traced onto the fresh plaster; this faint but accurate drawing serves as a guide for painting the image in colour.