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.