Page 96 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

C O R R O S I O N P R O D U C T S A N D P I G M E N T S

Corrosion products and pigments can be characterized as minerals only f they correspond to
i
naturally formed substances previously identified in nature by mineralogists and geologists.
Some products may closely resemble synthetic substances produced in the laboratory rather
than the natural mineral version. In some cases, there are very distinct differences between
these synthetic and natural minerals; in others, the differences are so slight that the distinction
is meaningless. Although there are some exceptions to this generalization, unidentified corro­
sion products will remain as such unless an equivalent in nature or the laboratory is found.
In the following chapters, corrosion products and pigments are discussed, along with any
relevant environmental distinctions, in the following general order:
oxides and hydroxides
basic carbonates
chlorides and basic chlorides
basic sulfates
sulfides
• phosphates
nitrates
silicates
organic salts
bronze patinas and corrosion processes

Notes
ι Pliny the Elder Natural History 34.21 (Pliny 1979). AG value is negative, the reaction will proceed,
2 Oxidation-reduction power, or redox potential, unless the rate of reaction is so slow that no prac-
abbreviated as Eh. tical change occurs.
3 The following spot test for the presence of 5 The neck of the jar had been removed, and a cir-
copper(II) ions provides a useful example of a cular break in the clay showed traces of asphalt.
chemical corrosion reaction. To the unknown, A copper cylinder, 98 mm in height and 26 mm in
one drop of a 5% solution of potassium ferro- diameter, was found inside the jar with a round
cyanide, K 4Fe(CN) 6, is added followed by one copper sheet soldered to the bottom. A heavily
drop of a 10% aqueous solution of hydrochloric oxidized iron rod, 77 mm in length, was found
acid. The released copper (II) cations react with inside the copper cylinder, held firmly in place
the ferrocyanide to form cupric ferrocyanide, by an asphalt stopper that had been fitted into
which has a brick-red color, confirming the près- the top of the cylinder. When discovered, the
ence of copper. This is described by rod extended 10 mm above the stopper, and the
cylinder protruded slightly from the asphalt seal,
K 4Fe(CN) 6 + 2Cu = Cu-Fe(CN) e +-4K+ !, . , f /
++
allowing access to both the copper and iron sur-
4 A Gibbs free energy of reaction is the difference faces at the top of the jar. An asphalt layer about
in free energy between the products and reac- 3 mm thick covered the base of the cylinder, prê-
tants. It is expressed as AG = AH - TAS, where AH venting the rod from coming into contact with the
is the change in enthalpy, Τ is the absolute tern- base of the cylinder. To make a working battery,
perature, and AS is the change in entropy. If the it would have been necessary to prevent the iron

C O R R O S I O N AN D E N V I R O N M E N T
79