The technique of wall painting known as Fresco called for the
application of colour onto plaster. Two methods are available,
and buon fresco (good fresco). and 'fresco secco' (dry fresco).
Below is an example by Giotto an artist of the Florentine
School. With this approach, colour is applied onto fresh, still
damp lime plaster. As the colour is applied it immediatly sinks
into the surface and is locked in as the plaster dries.Two coats
of plaster were applied (I say 'were' because this teqnique is
seldom practiced nowadays). The drawing was carried out after
the first layer, a lime and sand mix, had fully dried. Charcoal
followed by a red ochre set out the design. The day piece A
second layer of plaster was then applied, a small area at a
time. each large enough for one days work, At the end of the day
the edges of the plaster were undercut ready for the next days
application. Wheras oil is the binder in oil paints, the
'binder' in fresco is the damp plaster. The paint being a simple
mix of pigment and water.