Hillary and Monica

Moll C says:

It’s a fresco:

In painting a fresco, the surface of a plastered wall is divided into areas roughly corresponding to the contours of the figures or the landscape, generally drawn on a rough underlayer of lime mortar, called the arriccio. Many artists sketched their compositions on this underlayer, which would never be seen, in a red pigment called sinopia. From this pigment, the underdrawing acquired its name, the sinopia. On top of this first, rough layer of plaster, a second layer is added, called the intonaco. This is the final layer, and would be smoothed and perfected as the painting surface.

Buon fresco technique consists of painting in pigment mixed with water on wet, fresh, lime mortar or plaster (intonaco). Due to the chemical makeup of the plaster, a binder is not required, as the pigment mixed solely with the water will be enough to bind the pigment to the wall. 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 to the plaster.

http://en.wikipedia.org/wiki/Fresco

The fresco technique requires that the artist paint a freshly plastered wall which is still sufficiently humid to allow the paint to bond chemically so that when the plaster dries, the paint is completely a part of the wall. In order to paint the plaster which dries very quickly, the artist must have a very rapid and precise techniques of painting. He must clearly know how much he can paint during the course of the day (‘giornata’). It is possible to identify the extent of the various daily paintings from the plaster on the borders of the frescoes. From these it is clear that Michelangelo painted at a remarkably high speed. He completed this immense monumental work of art in only four years between 1508 and 1512.

http://www.italianfrescoes.com/michelangelo.asp