Page 115 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

S P E R T I Ν U T E

The copper hydroxide mineral spertiniite, Cu(OH) 2 , rarely occurs as a stable mineral phase. It
is primarily a transitory, intermediate product that forms during corrosion. Spertiniite is an
amorphous or poorly crystalline solid or gel, colored a duck-egg blue, and is transformed to
other compounds with relative ease.
MacLeod (1991) reported finding spertiniite as a marine corrosion product of copper alloys
from shipwrecks. Once it was allowed to dry, the gel-like precipitate transformed into ataca-
mite. MacLeod, however, did not provide detailed analysis of the material he called spertiniite,
and there is the possibility that it was not copper hydroxide but the rare mineral georgeite, an
unstable basic copper carbonate (see CHAPTER 3).

C O N S E R V A T I O N I S S U E S

An old conservation treatment for cleaning corroded bronzes used sodium hydroxide, but this
method has been abandoned because of the damage it can do to an object. The treatment reduces
secondary copper corrosion products, such as malachite or cuprous chloride, to cuprite or
tenorite. The powdery, black product that forms was usually stripped from the surface of the
bronze, leaving bare metal without its original patina.
Dei and colleagues (i998) reported that copper hydroxide can form as an undesirable alter­
ation product during conservation treatments on wall paintings. Such reports are not uncom­
mon. Copper hydroxide formation has even been noted when historical recipes were used to
make light blue pigments.
Weisser (i987) suggested using a localized treatment for bronze disease (discussed in CHAP­
TER 4), based on the application of an aqueous sodium carbonate solution. One disadvantage of
this treatment noted by Weisser was the occasional formation of black spots on the surface fol­
lowing treatment. Pollard, Thomas, and Williams (199ob) investigated the reaction between the
cuprous chloride corrosion product and the aqueous sodium carbonate cleaning agent. They
found that adding Cu(II) chloride to the cleaning solution produced an initial blue, amorphous
precipitate of georgeite. 13 When cuprous chloride was added instead, the reaction proceeded
slowly with formation of cuprite, then tenorite. The formation of tenorite was responsible for
the black spots noted by Weisser.
The reaction of cuprous chloride with 5% w/v solutions of carbonate will often result in
tenorite formation as the end product of cuprous chloride removal. To avoid this, treatment
should be arrested following the conversion of cuprous chloride to cuprite, though this is
difficult to control in practice. The treatment is not so objectionable that it should be discarded,
however, since the light green pustules of copper trihydroxychlorides characteristic of bronze
disease are often so aesthetically displeasing that the possibility of tenorite formation may be
worth the risk.

C H A P T E R T W O
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