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Chemistry 103 Lab: The Fresco Technique
Introduction. The fresco technique is one of the most ancient and widespread art forms in
world. The raw materials--lime, sand, and colored clays--are geographically common,
easy to collect, and process. In contrast to other painting techniques, the pigment used in
frescoes is applied directly to the surface--wet plaster--without a binder. The wet slaked
lime [Ca(OH)2] plaster reacts with carbon dioxide, CO2, in the air to become insoluble
calcium carbonate, CaCO3, which acts as the binder for the applied pigment. This
consolidation of the pigment into the plastered wall itself makes fresco paintings
extremely durable, but, at the same time, vulnerable to any deleterious environmental
conditions to which the wall is subjected. In this lab, student will create small fresco
paintings on tiles and explore the chemistry involved in fresco creation, degradation, and
restoration. This lab is an adaptation and expansion of a similar one developed by
Professor Sue Roper of Sacramento City College in California for a course on chemistry
and art.
Terms used in fresco paintings.
fresco
fresh
giornata
one days work, usually 3-5 m2 (a saints head)
arriccio
underlying coat of plaster applied directly to the wall
intonaco
last coat of plaster applied the day of painting
a secco
" when dry," painting the dry fresco pigments in organic binders such as
egg, oils, or waxes
pentimenti
corrections added to the painting after the days work
sinopia
stencil of the major shapes of the painting transferred to the wall before
painting begins, sometimes called the cartoon
Preparing a Fresco. Preparation of the painting surfaces involves the construction of a
wall coated with layers of plaster of increasingly finer texture. The first step in the
preparation is the heating (calcination) limestone (calcium carbonate, CaCO3) at 800-
900ßC to make porous lime (calcium oxide, CaO).
HEAT +
CaCO3(s)
CaO(s) +
CO2(g)
(1)
To form the plaster for fresco work, the lime is "slaked." The slaking process, which
requires the addition of 2 or 3 moles of water for each mole of lime, yields calcium paste
or lime putty, an aqueous gel of thin crystals of calcium hydroxide.
CaO(s) +
H2O(l)
Ca(OH)2(s) + HEAT
(2)
Excess water acts as a lubricant so that the crystals slide easily over one another.
Historically, lime was slaked in pits or troughs over a period of at least six months to
obtain lime putty of the desired consistency. Artisans in Michelangelos time use plaster
aged for as long as ten years. Fresco plaster itself is made from the slaked lime and
varying portions of sand or marble dust. Generally, walls are plastered with several
layers of such fresco plaster in order of decreasing proportions and particle size of sand.
The first layer of plaster, the arriccio (1-2 cm thick), consists of one part slaked lime to
two parts of sand; some fresco techniques use several layers of arriccio. Hardening of the
arriccio on the wall includes several simultaneous physical and chemical process: the