Page 383 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Cleaning reagents A wide range of chemical cleaning reagents have been tradi­
tionally used on bronze objects. Over the past fifty years or so
of practice, the most common ones have been alkaline glycerol, alkaline Rochelle salt, Calgon,
citric acid, buffered citric acid, formic acid, ammonium hydroxide, ammonium tricitrate, diso-
dium or tetrasodium ethylene diamine tetra-ace tic acid (EDTA), dilute sodium hydroxide, and
dilute sulfuric acid. Commonly available reagents used today for surface cleaning include alka­
g
g
line Rochelle salt (so /1 of sodium hydroxide and 150 /1 of sodium potassium tartrate); alka­
line glycerol (150 /1 of sodium hydroxide and 40 ml/1 of glycerol); Calgon (used as iso g/1);
g
citric acid (as 40 g/1); and citric acid buffered with ammonium hydroxide (25 /1 citric acid and
g
14 ml/1 ammonium hydroxide). All of these reagents are capable of etching the metallic surface
of a typical tin bronze (Merk 1978). Alkaline glycerol and alkaline Rochelle salt will attack cupric
salts preferentially and cuprite only slowly, whereas Calgon and citric acid both attack the
cuprite layer rather severely. With reagents such as citric acid and some others there is always
the danger of dissolved copper salts precipitating as redeposited copper on the surface of the
object. This vitiates the purpose of the cleaning, and severe chemical or mechanical methods
may be needed to remove the redeposited copper from the pores of the object.
In alloys with a higher percentage of anodic metal, Merk found, for example, that leaded
tin bronzes were etched more rapidly. Buffered citric acid solution showed the least severe etch­
ing of the bronze metal, but as mentioned previously, citric acid tends to be a harsh cleaner
because it readily dissolves cuprite. In a related study, Merk (i98i) found that the addition of a
1% (w/v) solution of benzotriazole to the stripping solutions resulted in substantially less corro­
sion of the bronze surface. Citric acid, however, was the only reagent that still showed substan­
tial attack on the bronze substrate even with the added benefit of benzotriazole.
Cleaning reagents for outdoor bronzes are similar to those used on indoor bronzes,
although some of the reagent concentrations are much stronger. For example, Jack (1951) some­
times used 0.88 S G ammonia, diluted to half this strength with water, to clean grime-encrusted
13
London statues. The solution was rubbed into the surface with a wire brush or natural pumice
stone. Jack made a paste to clean badly corroded outdoor bronze surfaces with a mixture by
volume of one part powdered soda, two parts slaked lime, and two parts sawdust. The high pH
of the slaked lime would be a cause for concern today because of the likelihood of patina alter­
ations. Matteini and Moles, as recently as i98i, repeated the old alkaline, Rochelle-salt method
to remove corrosion, and apparently carried out treatment without attacking the substrate
metal. Ternbach 14 (1972) used a dilute solution of ammonia, while rubbing the surface with
bronze wool, to reduce the amount of loose corrosion products on bronzes under treatment
prior to repatination. This is a similar approach to Jack's, in that the old patina was attacked and
abraded prior to repatination, but it probably did not have as severe an effect.

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