Page 34 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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F I G U R E . i Components of
what may be an ancient bat­
tery. Iraq, first century . E .
C
At left is the clay jar in
which the iron rod (center)
and copper cylinder (right)
would have been placed.
The dark fragments are
bitumen, which would have
been used to hold the three
components together.
Department of Antiquities,
Iraq.

The first published report of these finds was by Waterman (i93i), although he was at a loss to
explain what they were. The objects consisted of a number of common, unglazed jars contain­
ing a bronze or copper cylinder 7.62 cm (3 in.) long and 3.18 cm (1.25 in.) in diameter. Asphalt
seals and iron rods were also found with these jars. Similar but larger and more elaborate finds
were unearthed during excavations by König (i938) at Khuyut Rabbou'a, southeast of Baghdad.
Among the finds made by König was a clay jar fitted with a copper cylinder containing an iron
rod that was held in place with an asphalt stopper. Such objects may have once functioned as
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batteries long before the work of Volta and Galvani, but their purpose remains unknown. Elec­
trical insulation of the iron rod from the copper would ensure that anode and cathode were kept
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apart, and, f a salt or acidic solution were introduced into the jar, the battery would theoreti­
cally produce a potential on the Standard Hydrogen Electrode (SHE) scale from the sum of

Cu 2 + + 2e" = Cu +0.34 V (SHE) 1.4

Fe 2 + + 2e~ = Fe -0.44 V (SHE) 1.5

This would represent a maximum voltage of 0.777, but only in 1 molar solutions of each ionic
type, and a fairly weak current of about 1 mA. Because of difficulties with the electrolyte, it is
doubtful f the voltage of 0.777 could be achieved in practice. FIGURE 1.1 shows one of these
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objects disassembled.
A replication experiment by Keyser (1993) found that a sodium chloride solution rapidly
corroded the iron, polarizing the cell and resulting in a voltage drop to 0.4 V within less than
one minute. Citric acid from freshly squeezed grapefruit, or strong vinegar, was preferable, pro­
ducing 0.49 V with slow polarization. The current flowing is readily detectable on the tongue or

C O R R O S I O N A N D E N V I R O N M E N T
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