5
8. Paint the tile. Shading with dark pigments is done first and then over-painted with
lighter pigments. Since the colors are transparent, several thin layers of paint are needed
to obtain bold colors. Wait 10 or 14 minutes between color layers to ensure that they do
not mix.
9. When the painting has been completed, set the finished tile in a plastic bag, but leave
the top open. (This procedure will ensure that the tile drives slowly and will minimize or
eliminate crack formation.) Let the tile cure so for several days to consolidate the
pigment with the newly formed calcium carbonate matrix of the fresco, a permanent and
colorfast art object for generations to come.
Chemistry of Frescoes (Week 1 or 2).
A. Acid-base Chemistry. Use pH test papers as demonstrated by the instructor.
1. Chip a piece of the intonaco/arriccio from a sample fresco tile or from your hardened
excess arriccio plaster and put it in a disposable test tube. Carefully add a drop of 3 M
HCl to it. Repeat with a sample of fresh lime plaster.
2. Determine the pH of the slaked lime and pure CaCO3 suspended in water.
3. Add a pinch of calcium oxide to a disposable test tube. Add about 1 ml to the test
tube and test it for heat evolution. Determine the pH of the resultant mixture.
4. Add about 20 ml of water to a 100-ml plastic beaker. Measure the pH of the water.
Carefully add crushed dry ice, solid CO2, to the beaker. After most of the solid has
disappeared, measure the pH again.
Write equations for any reactions that occurred above and discuss the results of the pH
tests.
B. (Week 1) Evaluation of the relative solubilities of CaCO3, CaSO4, and BaSO4.
Dissolution of these salts can be represented generically as follows:
MX(s)
M2+(aq) +
X2-+(aq)
(7)
The calcium and barium ions are represented by M2+ and the carbonate and sulfate ions,
by X2-. In this experiment, students will prepared saturated solutions of each of the three
salts and determine the amount dissolved by measuring the concentration of the cation,
M2+(aq), in this solution. This measurement will be done by following the reaction of the
cations with the reagent EDTA whose structure is shown below.
N
CH2
CH2
N
CH2
CH2
H2C
H2C
C
C
C
O
O-
C
O
O-
O
-O
O
-O
EDTA
Although EDTA appears to have a complicated structure, it can be represented by Y4- as
shown in the following equation for its reaction with cations:
M2+
+
Y4-
MY2-
(7)