The problems with painting "frescos" come from the plaster. It must be mixed up and put on the wall freshly every day and left to partly dry before it can be used. As the plaster begins to dry or "set", the artist can start the picture. The work must be very quick and careful. If there is a mistake, the plaster must be scraped off. While the plaster is setting, it becomes hot and gives off gases. This makes working on a fresco unpleasant. Because frescos are on walls, or ceilings, they can not be moved from place to place, or rearranged, like other paintings.