Christophe Vischi and Ariane Lalande presented a talk on practical tests they conducted to determine how to clean brass mats of photographs from the collection of the Colby Curtis Museum, Stanstead, Quebec. These mats suffered from multiple corroded spots (which I know as pitting corrosion from industry), containing stable Cu2O cuprite and active atacamite Cu2Cl(OH)3. They compared two electrochemical methods and the use of an ion exchange resin to treat the mats:
– spot electrolysis using an EDTA electrolyte and at 9 Volts,
– brushing an glycerine electrolyte on the object, wrapping it in aluminum foil and laying it a humid chamber
– use of the ion exchange resin, Amberlite IR 120 HM. This comes in the form of beads, which were applied locally on the object as a poultice using a fine brush.
The authors did not obtain good results with the electrochemical methods, with staining being a problem for the spot electrolysis, and a gray patina a result of the wrapping method. They chose the method using the ion exchange resin. It was found that grinding the beads before application improved the results. Very local cleaning was possible, and the solution could be rinsed off with a mixture of ethanol and water. Still, there were flecks left where the pitting corrosion was, but these could be retouched to match the finish. Care had to be taken to avoid staining uncorroded areas. The result appeared to be satisfactory, and they want to continue work on optimizing this method.
I would like to note that electrochemical methods should not be ruled out based on this paper. When performed properly, electrochemical cleaning can be used to clean most metals found in museums. C. Degrigny, among others, has demonstrated that local electrochemical cleaning can work under properly controlled conditions (cathodic potential/voltage), using the proper equipment, and electrolytes. The aluminum foil method might have worked if the objects were not wrapped (traps the reaction products), and the proper electrolyte was used. If I heard correctly, glycerin was used as the electrolyte, but it is non-conducting, so it could not have worked.