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

plaster serves as ground, support and binder. This ancient method of painting on
masonry walls is deemed fresco (Italian for fresh), appropriate since a pigment-
water solution is applied to damp plaster.
The plaster is a mixture of slaked lime (calcium oxide in solution with water,
forming calcium hydroxide), and an aggregate (sand, marble dust, or a volcanic ash
called pozzolana). The wall surface is built up of layers of different formulations
of lime, water and aggregate as shown is Figure 2.16. Each successive layer has a
greater percentage of lime, gradually increasing the binding strength of the plaster.
Layering insures even, slow drying, which reduces the likelihood of cracking.
Figure 2.16: As layers of a fresco dry, water migrates through the surface
layer or intonacco (A), and arriccio (B) toward the masonry wall (C)
while the rest evaporates into the air at the surface. As carbon dioxide
is drawn into the plaster, pigments are locked into the surface. Adapted
from [TM00].
Each layer is kept damp until after the succeeding layer is applied, and the
colors are applied to the final layer before it dries, usually within about eight
hours. As the composite of layers dries, water migrates out of the plaster layers.
The carbon dioxide from the air transforms the calcium hydroxide in the lime into
calcium carbonate. During this process, platelets of calcium carbonate lock the
particles of pigment into the plaster. This process acts as the equivalent of an
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