3

Ba(OH)2(aq + CaSO4(s) BaSO4(s) + Ca(OH-)2       (5) The excess barium hydroxide is homogeneously absorbed into the intonaco.  It reacts with any sulfate ion present to form barium sulfate and arrests further migration of this ion or with atmospheric carbon dioxide to act as a binding agent to consolidate the plaster and pigments: Ba(OH)2(aq + CO2 BaCO3(s) +   H2O (6) The scientific basis of this conservation rests on differences in the solubilities of several salts. In this lab, students will determine the relative solubilities of calcium carbonate, calcium sulfate, and barium sulfate. The barium method was applied to restoration of the frescoes of Brancacci Chapel in Florence in which calcium sulfate had been shown to be the major agent of destruction. The  second  most  common  cause  of  deterioration  of  frescoes  comes  from  previous conservation efforts in which early conservationists attempted to protect the fresco or brighten colors with glues, oils, egg white and/or waxes.  In time, these organic materials darken, shrink, and lift off paint layers.  In addition, they provide nutrients for the growth of microorganisms and prevent the exchange of moisture and air between the fresco and atmosphere. Recent restoration of the Sistine Chapel frescoes focused on removing the residues of early conservation efforts by treating the frescoes with a cleaning solution to solubilize glues and waxes.  The cleaning solution contained ammonium bicarbonate, (NH4)2CO3(aq to remove calcium sulfate (equation 4).  Consolidation of loose paint and intonaco was, however, done with polymeric injections as the paintings in the Sistine Chapel contained low levels of sulfates, but high levels of nitrates, a counter indication to barium hydroxide treatment Speculate on the reasons why the barium method would be inappropriate under these conditions.