Fresco is the technique of wall painting on wet, freshly laid plaster, known about since Ancient times. It is called fresco because the wall that is to be painted is plastered with various layers of lime plaster and while the final layer is still fresh and wet it is painted on. When the lime plaster dries the pigments are chemically integrated into the wall itself, making frescos very durable. A disadvantage is that the intensity of the colours is reduced somewhat. This process is called "buon fresco" (or true fresco) and it required very careful planning, as reworking was impossible. The master traced the general plan of the work, and day-by-day the part that was to be painted was plastered. The painting had to be done on the same day and all the details of the fragment in question had to be completed so that work could begin on the adjacent section the following day. This required the construction of scaffolding systems so that teams of painters could work in difficult positions, directly on the ceiling, on vertical or even curved walls. It was very unusual for a painter to work alone on a fresco, because of the speed the technique demanded. It was rather a case of a master directing team of specialists, reserving the most important parts for himself: the design of the composition and the painting of the most important figures. There is another variety of fresco painting called "fresco secco" or "retoque alla secca". In this case the painting is done on lime plaster which has set. With this technique there is always the risk that dampening the layer of plaster again with paint will make it crack or bulge as it dries for a second time, meaning a deterioration in the quality of the frescos. Leonardo da Vinci practised variations of this technique. They were not always successful such as in the case of his Last Supper, which deteriorated only a few months after it was painted. However, the fresco secco technique allows details to be done with greater precision, it gave the option of reworking images and offered a range of soft, delicate colours, which is why it was widely exploited during the Baroque.