Page 258 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

blue, has also been reported as a corrosion product, although rarely from the same contexts in
which libethenite occurs.
Pseudomalachite, Cu 5 (P0 4 ) 2 (OH) 4 , a vitreous green mineral, has been used as a pigment
but has not been reported to date as a corrosion product on bronze objects. The mineral tagilite,
Cu 2 P0 4 (OH) Η 2 0 , named in 844 from the type location of Nizhne Tagilsk in the Ural Moun­
1
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tains, is apparently identical to pseudomalachite. 1 In addition, ludjibaite, Cu 5 (P0 4 ) 2 (OH) 4 ,
1
which was named for the city of Ludjiba, Zaire, in 988, is an isomorphous variety of pseudo-
malachite. Like pseudomalachite, neither tagilite nor ludjibaite has been reported to date as a
corrosion product.
Two complex copper phosphates have been identified as corrosion products. One
is a pearly, light-blue, sodium-calcium-copper-phosphate chloride known as sampleite,
NaCaCu 5 (P0 4 ) 4 Cl-5H 2 0. The other is an aluminum-copper phosphate called zapatalite,
Cu 3 Al 4 (P0 4 ) 3 (OH) 9 -4H 2 0, a translucent, pale blue mineral with a Mohs hardness of 1.5. The
incorporation of aluminum in this formula is rather unexpected because ancient copper alloys
do not contain aluminum; therefore, the element must have been introduced on the surface of
the object by pollution. One report of a hydrated copper phosphate, Cu 3 (P0 4 ) 2 -xH 2 0, has been
published, although insufficient evidence is available to assign a precise formula to this phase
(Mattsson et al. 1996) .
Turquoise, CuAl 6 (P0 4 )(OH) 8 -4H 2 0, is commonly a vitreous blue or blue green, with
2
a Mohs hardness of 5 - 6 and a density of .84. Turquoise is isomorphous with chalco-
siderite, CuFe 6 (P0 4 )(OH) 8 -4H 2 0, which is a vitreous, light green mineral with a hardness
4
of .5. Some of the copper content of turquoise can be replaced with zinc to form faustite,
(Zn,Cu)Al 6 (P0 4 ) 4 (OH) 8 -4H 2 0. This mineral, which has been identified to date only from New
World contexts (ancient South America), is waxy with a dull, apple-green color.
Some of these minerals, particularly turquoise, are subject to blanching when weathered.
Characteristics of the copper phosphate minerals discussed here are summarized in T A B L E 7.1 .

Copper phosphate chemistry The stoichiometrics of libethenite, pseudomalachite, and cor-
netite suggest that these minerals form in this order under
conditions of increasing basicity. This is a general trend for the crystallization of these three
phases. Pseudomalachite is the most common cupric phosphate in terms of natural occurrences
(Magalhaes, Pedrosa de Jesus, and Williams 1986) , which is anomalous in the sense that the
mineral is also the rarest in terms of corrosion products. Cornetite is rare in most environments;
the stability diagram in F I G U R E 7.1 shows that cornetite is stable only under conditions of rela­
tively high pH and high cupric ion activity. As previously discussed, however, high cupric ion
activity is quite possible on bronzes in burial environments, which accounts for the existence of
cornetite in this context.

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