pigmented colorants: dependence on media and time - Cornell ...

organic binder found in more conventional paints.
Figure 2.17: Cross sections of two ancient fresco fragments, showing
coarse aggregates below the paint layer. Adapted from [Weh75].
Fresco serves well as a medium for mural painting due to its permanence, for
it maintains the same longevity as the architecture. However, the difficulties for
the painter reside primarily in the fact that the work must be executed quickly
and directly. Extensive preparatory work must be done in advance, usually in the
form of sketches. Large-scale cartoons, or full-sized drawings, are made to work
out the composition beforehand. As covered previously, most fresco paintings
are large in scale; hence the intonacco is sectioned into giornate that are more
manageable. Each giornata must be completed in under approximately eight hours,
after which the plaster has dried too much and pigment refuses to adhere. The
painting cannot be revived after the plaster has set, and alterations can only be
made by chipping away the plaster and starting anew. For small alterations,
sometimes paint is applied secco (Italian for dry), on top of the dried plaster with
tempera or distemper. Secco is also used for pigments chemically incompatible
with lime (pigments containing copper, for example).
46