Page 167 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Precipitation studies show that after long wetting of a basic sulfate crust, the initial pH of
the aqueous phase is overwhelmed by the copper sulfate material, reaching intermediate pH lev­
els of 4-5. It is probable that ionic dissolution reactions rather than electrochemical reactions
predominate, since the dissolution of cuprite or copper is hindered by the thickness of the sul­
fate crust. In the corrosion of the metal itself, a different series of reactions occurs, modified by
the chloride ions that often are concentrated in this zone.
Previous models suggest that the bulk aqueous-phase reactions occur throughout the cor­
rosion crust, but Lins and Power (1994) show that the outer sulfate crust will supply ions to the
aqueous front and then serve as sites for solidification as the aqueous phase evaporates. Reac­
tions occurring during dry deposition of pollutants are also important; after pollutants are
deposited on a bronze patina, hydration and formation of very soluble chlorides and nitrates can
occur. This work shows that the transformation of brochantite with acid solutions to form
antlerite does not occur easily; the transformation of cuprite to antlerite is also not very likely.
FIGURE 5.1 shows the Pourbaix diagram for the system C u - S 0 3 - H 2 0 at 20 °C with an S0 2
level of 46 ppm. Lins and Power found that the zones of stability predicted for antlerite and
brochantite by this diagram did not apply for well-crystallized mineral specimens. The diagram
also suggests that there is a large field of stability for tenorite between areas of neutral to very
alkaline pH and in moderately oxidizing environments. As noted previously, tenorite is rare;
cuprite is the predominant oxide in outdoor exposure. It is likely that weathered corrosion films
in polluted atmospheres are not in equilibrium. For this reason, Lins and Power caution that
strict adherence to thermodynamic considerations may produce a misleading picture of the
sequence and nature of the precipitation and dissolution of these corrosion layers, in which a
number of complex hydrated species may exist.
This complexity discourages the usefulness of antlerite as an indicator of corrosivity,
although its existence cannot be denied, and it does have a high sulfate content. For ex­
ample, Selwyn and coworkers (i996) identified anderite in only 10% of the surface samples from
the statues they studied in Ottawa, Canada. The identification was made mostly from statues
unveiled between 1901 and 1940, and the surface samples containing antlerite were taken from
sheltered or only partially exposed areas. Robbiola, Fiaud, and Pennec (1993) also detected ant­
lerite in sheltered areas on outdoor bronzes. So did Strandberg (1997a) and Strandberg and
Johansson (1997c), who found antlerite principally in sheltered black patinas on bronzes that
had been exposed for several decades. The present consensus on antlerite is that some traces of
the mineral will form over time, even in protected areas.
Lins and Power (1994) used four comparative diagrams to evaluate the stability of basic cop­
per sulfate with that of the basic copper carbonates, chlorides, and nitrates. The suite of dia­
grams, shown in the Lins and Power report, reveal that the relative insolubility of the basic
sulfates is an important factor in their formation. The solubility of the copper nitrates, as well

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