Intonaco is an Italian term for the final, very thin layer of plaster on which a fresco is painted. The plaster is painted while still wet, in order to allow the pigment to penetrate into the intonaco itself. An earlier layer, called arriccio, is laid slightly coarsely to provide a key for the intonaco, and must be allowed to dry, usually for some days, before the final very thin layer is applied and painted on. In Italian the term intonaco is also used much more generally for normal plaster or mortar wall-coatings in buildings.
Intonaco is traditionally a mixture of sand (with granular dimensions less than two millimeters) and a binding substance.
Read more about Intonaco: Types of Intonaco, Types of Stabilizers