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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)