Fresco is one of the oldest methods of painting and dates even back to Neolithic times, although it reached an aesthetic zenith in the golden days of the Renaissance in Europe. By its simplest definition, fresco is pigment on plaster, although there are many variations that have been experimented with and perfected throughout art history.
Of all the variations, the two most basic methods of fresco are secco fresco, or dry fresco, and buona fresco, or true fresco, also known as wet fresco. Secco fresco is where pigment is applied to dry plaster, whereas buona fresco is where pigment is applied to wet plaster. Buona fresco is the more permanent of the two since the pigment bonds throughout the plaster as it cures, rather than lying on top of the dry plaster.
I employ a secco fresco method of watercolor on dry plaster. There is a fresh, vibrant color that can be achieved with this method, but it is a much more controlled medium than painting with watercolor on its traditional support since water and pigment are immediately absorbed by the plaster, rather than the water flooding the support before letting pigment settle.