Page 169 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

as the high concentration of anions required for them to precipitate, is evident. The range of val­
ues reflects those that might be found in average rain or fog for each anionic species.
McNeil and Mor (1992) attempted to rationalize thermodynamic data on the basis of sta­
bility diagrams for four variables: Eh, pH, activity of sulfate ions, and activity of copper ions.
The resulting diagram is four-dimensional and difficult to apply to particular cases. Projec­
tion into a Pourbaix diagram format allows more than one stable sulfate mineral to be plotted;
chalcanthite, antlerite, and brochantite can form depending on the conditions. These diagrams
are of interest because they show a more restricted field of stability for antlerite than for
brochantite occurring between Eh +0.4 and +1.0, and between pH 3.5 and 4.5. Brochantite can
form under a variety of conditions, including those in which water is the most prevalent phase.
I ARTIFICIAL BROCHANTITE AND ANTLERITE PATINAS Because
brochantite and antlerite are the most stable phases in the outer corrosion crust under many
atmospheric conditions, Vendl (1999) has been evaluating the possibility of using these miner­
als in an artificial patina as a protective finish for outdoor bronzes. Synthetic brochantite or a
mixture of synthetic brochantite and antlerite should be much more stable than organic coat­
ings to withstand exposure to the elements; f sufficiendy compact, they can provide good pro­
i
tection for the underlying bronze. Since 1996, Vendl has been conducting exposure trials of
copper and bronze samples coated with brochantite in Freemantle, Western Australia; in Bang­
kok, Thailand; in Göteborg, Sweden; and in New York, San Francisco, and Honolulu in the
United States. Preliminary results suggest that the brochantite patinas are performing well.
They are aesthetically pleasing and show no appreciable dissolution or change in appearance.
As would be expected, there are differences in the surface color of the patinas depending on the
pollutants in the different exposure environments.

Posnjakite Posnjakite, which is monoclinic like brochantite, is a vitreous
green or dark blue mineral with a low Mohs hardness, between
2 and 3. It is not particularly common as a patina component. Essentially a hydrated brochan­
tite, posnjakite was identified as a mineral by Komkov and Nefedov (i967) from the oxidation
zone of tungsten mineral deposits in the central regions of Kazakhstan. It was first observed in
copper patinas by Biestek and Drys (1974).
Posnjakite is isomorphic with langite, Cu 4 (S0 4 )(OH) 6 -2H 2 0, which has not yet been
reported as a corrosion product of bronzes, although it has been identified as a painting pigment
(discussed later, under "Basic Sulfates as Pigments"). The existence of a hydrated basic sulfate
of copper was first mentioned by Vernon and Whitby (1930), but since posnjakite and langite
were unknown at the time, this phase remained unidentified. Posnjakite is identical to brochan­
tite in stoichiometry, except for the water of hydration. Graedel (i987a) thought that posnjakite

C H A P T E R F I V E
152