Page 128 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

to one of the copper trihydroxychlorides. 10 This undesirable color change was observed on
the fourteenth- to fifteenth-century wall paintings depicting San Antonio Abate in the church
of San Pietro at Quaracchi, near Florence, Italy. Rosi (i987) reports the harmful effects that
occurred with this treatment during the restoration of a pietà by Florentine artist Masolino da
Panicale (i383-i440?), who painted this work around 1425 for the baptistry of the Collegiata at
Empoli. After the final barium hydroxide solution was applied, the green degradation products
of azurite changed to a dark blue; after two years, this blue changed to green again. This
demonstrates that the conservation treatment had created chemical alterations and that some of
these alterations were unstable.
At first it was thought that the treatment had changed the green degradation product (a cop­
per trihydroxychloride) back to blue azurite, but Dei and coworkers show that a different reac­
tion was occurring. The extremely high pH of the barium hydroxide treatment had transformed
the basic chloride to spertiniite, Cu(OH) 2 , a highly unstable compound that underwent further
reaction with chloride-containing water to become a basic chloride again. Dei's group attributes
this chloride to paratacamite, but it probably needs to be reassigned to clinoatacamite.
Azurite and malachite pigments are also sometimes altered by this conservation treatment
to black tenorite, CuO, again because of the high pH of the barium hydroxide solution.

F O R M A T I O N OF C O P P E R C A R B O N A T E S IN S O L U T I O N
Malachite and azurite are precipitated from copper (II) sulfate solutions—the most common
solution in which copper is simply dissolved—and complex salts of copper (II) by reaction with
bicarbonate ions (Lindgren 1933). They may also be produced directly from the reaction of cop­
I
per (I) oxide with carbon dioxide and water. f azurite is hydrated or exposed to an atmosphere
deficient in carbon dioxide, it is gradually converted to malachite (Garrels and Christ 1965).
Chalconatronite may be formed by direct precipitation from saturated solutions of the highly
soluble Na 2 Cu(C0 3 ) 2 complex. The presence of this sodium-copper carbonate prevents the pre­
cipitation of malachite, creating instead an alteration of the reaction process from malachite to
chalconatronite (Applebey and Lane 1918).
The Pourbaix diagrams for the copper-water-oxygen-carbon dioxide system illustrate
the conversion of azurite to malachite in carbon dioxide-poor atmospheres. Azurite does not
appear on these diagrams until the carbon dioxide level reaches 4400 ppm, whereas malachite
is present at a carbon dioxide level of 44 ppm. With increasing carbon dioxide concentration,
malachite stability increases at the expense of tenorite and cuprite at higher pH values. The cop­
I
per solubility in this system is limited by the precipitation of malachite. f chloride and sulfate
ions are absent from the solution, then malachite is the phase that limits copper solubility. In
solutions containing sulfate and carbonate ions, antlerite and brochantite limit the solubility of
copper at low pH values (Mann and Deutscher 1977).

BASI C C O P P E R CARBONATE S
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