2
absorption of water into the wall, evaporation of water from the surface, and the
carbonation of the slaked line by carbon dioxide, CO2.
Ca(OH)2(s)
+
CO2(g)
CaCO3(s)
+ H2O(l)
(3)
On the dried arriccio, the artist sketches the sinopia, usually first with charcoal and then
with a lime-compatible pigment mixed in water. The sinopia is then traced onto paper to
serve as a guide for continued work. A final layer of plaster, the intonaco (0.5-1 cm
thick) contains less sand than the underlying arriccio layer(s). In large fresco paintings,
the artist applies the intonaco plaster over a portion to complete in a short time, the
giornata. The paper sinopia is then placed over the fresh intonaco and the image
transferred to it in one of two ways: (1) by gently incising the lines of the drawing
through the paper into the plaster or (2) by dusting dry, dark pigments through
perforations in the paper along the lines of the drawing ("pouncing"). To ensure that the
pigment is incorporated into the plaster layer, the painting must be complete in the first
hours of drying of the intonaco. Pigments are mixed with water, brushed onto the wet
plaster, and fixed to the painting by carbonation of the slaked lime (equation 3). Overly
wet plaster yields cloudy colors; whereas, a too dry plaster produces pale and dull hues.
The integration of pigment into the dried plaster itself makes the fresco technique
especially unforgiving.
Degradation and Conservation of Frescoes. Deterioration of fresco paintings results from
the open, porous nature of their support--walls and ceilings of buildings--and their
interaction with the surrounding microclimates. The porous mortar backing provides an
easy route for the movement of dilute salt solutions. Salts contained in the building
materials or the surrounding area can be readily transported to the plaster underlying the
painting. Volumetric expansion associated with crystallization of these salts disrupts the
plaster-pigment adhesion and leads to disintegration of the surface. Such crystallization
depends on the identity of the salts and the moisture content of masonry which is subject
to seasonal variations in atmospheric humidity and rainfall.
.
Conservation methods have been developed to remove salts formed from reactions of
sulfate ion traveling through the mortar. When the sulfate ion reacts with calcium, it
forms calcium sulfate, gypsum. Since gypsum is more soluble than calcium carbonate, it
dissolves and re-crystallizes with changes in humidity. These cycles lead to severe
damage when the gypsum crystallizes just beneath or throughout the paint layer.
Florentine floods of 1966 spurred the development of a conservation treatment, the
"Barium Method," in which a series of poultices are applied to such sulfate-damaged
fresco paintings to form the less soluble barium sulfate from calcium sulfate The first step
in this procedure solubilizes calcium sulfate. A poultice of rice paper pulp containing a
supersaturated solution of ammonium carbonate [(NH4)2CO3] is applied to the surface of
the fresco so that the solution is absorbed by the wall. Any calcium carbonate forming on
the surface of painting is swabbed away; whereas, that formed within the plaster helps re-
establish cohesion.
CaSO4) + (NH4)2CO3(aq)
[(NH4)2SO4(aq) + CaCO3(s)
(4)
Application of a second poultice containing barium hydroxide converts any remaining
calcium sulfate present into the less soluble barium sulfate:
