Fresco is a very beautiful but unforgiving medium. Because of the process of creation it will not allow for mistakes. There is no erasing or overpainting. Preparing for a fresco requires prepatory drawings to be produced. Sometimes these can be sketches where composition, detail, light and shadow are worked out and there are times when preliminary paintings are done to help solidify color and design.  These sketches and paintings are used to produce the cartoons that are instrumental in the final frescoes.