Page 384 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

In general cleaning operations, sodium tripolyphosphate, which is an effective chelat­
ing agent particularly for calcium ions, has been recommended by Sharma and Kharbade (1994)
for cleaning bronze surfaces. They found that sodium polyphosphate drastically altered the
infrared spectra of malachite, indicating that the hydroxyl stretch bands disappeared as the
compound was complexed. On the other hand, no change was noted when sodium tripolyphos-
phate was used, indicating that the tripolyphosphate may be a safe complexing reagent for the
removal of calcareous accretions without disturbing the copper patina minerals. The authors
had safely cleaned an eleventh-century bronze from Thanjavur, in southern India, that had been
covered with a tough whitish concretion. They used a carboxymethyl cellulose gel containing a
0.5%-2% solution of sodium tripolyphosphate. No further evaluations of this cleaning agent
have been published to date.
MacLeod (i987a) examined the efficacy of chloride extraction from bronzes using a variety
of solutions, including sodium sesquicarbonate, benzotriazole, acetonitrile, citric acid, and alka­
line sodium dithionite. The rate of chloride ion extraction was fastest with the alkaline dithion­
ite; thiourea-inhibited citric acid also showed good extraction rates, but both solutions created
substantial patina alteration. With alkaline dithionite, the patina changed within a few minutes
from a blue-green copper (II) hydroxychloride, through a yellow-orange transient copper (I)
hydroxide, to a chocolate-brown patina containing finely divided metallic copper (a reaction
discussed later in this chapter under "Cleaning Marine Finds"). After two hours, the thiourea-
inhibited citric acid had altered the patina color to a dull brown attributed to a mixture of
cuprite and the adsorption of thiourea on the corrosion interface. f there were any carbonates
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or hydroxychlorides in the patina, they would have been dissolved by the highly acidic condi­
tions (pH 0.95) and the complexing ability of the citric acid.
Weisser (i987) suggested the localized application of sodium carbonate solution as a
treatment for bronze disease. One disadvantage of the treatment noted by Weisser was the
occasional formation of black spots on the surface (see CHAPTER 2). This method is only one
of many different treatment options for bronze disease that have been published over the last
hundred years.
In 1917 Rosenberg published the results of his research into the problem of bronze disease
and recommended the use of an electrochemical reduction method. In one of the several vari­
ants to this technique, the object to be treated is wrapped in aluminium foil and exposed to high
humidity, typically higher than 90% RH. To effect an electrical connection between the bronze
surface and the foil, Rosenberg used a poultice of 6 parts agar-agar, so parts water and 6 parts
glycerol. The object was coated with the jelly, wrapped in foil, and exposed for two to four days
in the humid atmosphere. After completion of the reduction, the poultice was removed by wash­
ing in hot water. With this method, any active chlorides in the object will react with the alu­
minium foil, forming localized corrosion of aluminium chloride spots on the foil and reducing
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the copper chlorides to cuprite or tenorite. f exposure of the treated object to high humidity

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