Page 161 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Notes
ι Davy 1826: 57. 7 Clinoatacamite was identified too recently to have
2 As a result of the work of Jambor and colleagues appeared in the pigment literature to date.
(i996) , it now appears that the mineral anarakite 8 Historical recipes for copper verdigris pigments
is, in fact, simply another name for zincian para­ are discussed at greater length in C H A P T E R 9.
tacamite. Because of the complexity of the data 9 Norman H. Tennent, letter to the author,
for this group of minerals, however, there is still 16 March 1996.
some difficulty in deciding exactly what mineral 10 The Persian twenty-seventh dynasty of Egypt was
species is present, especially since anarakite is founded by Cambyses II of Persia and named after
mentioned in the I C D D files, though its separate his family of the Achaemenids.
existence is now considered doubtful. These 11 Sue Stock, e-mail to the author, 22 September 1999.
doubts have been substantiated by Jambor and
coworkers' discovery that small amounts of zinc
are often present in regular paratacamite speci­
mens. Holotype paratacamite was found to con­
tain about 2% zinc. It appears that replacement of
copper by small amounts of other cations, such as
nickel or zinc, is either favorable or essential to
stabilize the paratacamite structure.
3 I C D D file 25-1427.
4 The remaining metallic core or body of the object,
now often preserved within the corrosion prod­
ucts, may have little resemblance to the original
appearance of the object.
5 Atacamite would have been part of the original
corrosion that had formed naturally over time, and
Berthelot suggested that reaction with the sodium
chloride would have taken place on excavation.
6 Observations concerning the hydrolysis of cuprous
chloride have been made by Knight (conversation
with the author, 19 June 2000) . A group of axes
with well-preserved patina showed some warty
corrosion with CuCl under the warts. The axes
were treated by soaking in changes of warm dis­
tilled water, which slowly hydrolyzed the cuprous
chloride. Cuprite was deposited on the glass dish,
away from the pits, and no oxidation to Cu 2 +
was observed. A possible explanation is that the
cuprous chloride dissolved as a complex salt and
that away from the warts these complex anions
decomposed with formation of cuprite. The over­
all reaction would then, in fact, be
2CuCl + H 2 0 = Cu 2 0 + 2HC1
Cuprous chloride can undergo disproportionation
into cupric chloride and metallic copper, although
that reaction did not occur here. Redeposited
copper does occur within the corrosion crusts
of some objects, however, either from exposure
during corrosion to reducing conditions or from
the disproportionation of cuprous chloride.

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