Buon fresco technique consists of painting in pigment mixed with water on a thin layer of wet, fresh, lime mortar or plaster, for which the Italian word for plaster, intonaco, is used. Because of the chemical makeup of the plaster, a binder is not required, as the pigment mixed solely with the water will sink into the intonaco, which itself becomes the medium holding the pigment. The pigment is absorbed by the wet plaster; after a number of hours, the plaster dries and reacts with the air: it is this chemical reaction which fixes the pigment particles in the plaster. One of the first painters in the post-classical period to use this technique was the Isaac Master in the Upper Basilica of Saint Francis in Assisi. A person who creates fresco is called a frescoist.
