Page 268 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

TABLE 7.2 CHARACTERISTICS OF SOME COPPER NITRATE MINERALS

MINERAL FORMULA CRYSTAL COLOR MOHS
NAME SYSTEM HARDNESS

gerhardtite Cu 2 (N0 3 )(OH) 3 orthorhombic transparent green 2
likasite Cu 3 (N0 3 )(OH) 5 -2H 2 0 orthorhombic translucent blue ?
buttgenbachite Cu (NO 3 ) 2 (OH) 3 2 · CI 3 H 2 O hexagonal vitreous blue 3
1 8

Aoyama (i960) identified another basic copper nitrate, Cu(N0 3 ) 2 Cu(OH) 2 , in green corro­
sion products on copper power lines in Japan, although the reported stoichiometry of this salt
looks a little odd; perhaps gerhardtite was found. Gettens (i963b) mentions green crystalline
corrosion on bronze vessels from a royal tomb at Gordion in Anatolia that was excavated by the
University of Pennsylvania. Preliminary studies showed that these crystals have X-ray diffrac­
tion patterns identical with those of monoclinic, synthetic basic copper nitrates. Florentino and
coworkers (i982) determined that basic copper nitrate is present on Ghiberti's Gates of Paradise,
the gilded bronze doors of the Baptistery in Florence. Fabrizi and Scott (i987) found the same
compound, identified as gerhardtite, on a set of doors to the marble-faced mausoleum of the
Dalziel family in Highgate Cemetery, London; these doors, with iron-stained facade from ill-
chosen iron locks, are shown in PLATE 48.
In a very unusual discovery, Banik (i989) identified gerhardtite as a green copper pigment
in a study of medieval illuminated manuscripts, one of which dates to the fifteenth to sixteenth
century. It is surprising that a basic copper nitrate should have been found at all on an illu­
minated manuscript. Because it is difficult to obtain a sufficient sample from these manuscripts,
identification of the compound becomes even more intractable.

Notes
ι Because of some earlier confusion about the exact Their appearance can be improved by burying
stoichiometry of these copper phosphates, the them in damp earth or by immersing them in
name "tagilite" had been used for some time a dilute ammonium hydroxide solution, which
before it became evident that taglite is actually complexes with the copper ions and forms the
pseudomalachite in most, if not all, instances. colored cupra-ammonium ion complex.
2 The type sample is from Chuquicamata, Chile. 4 For information concerning the history, ethnol­
Sampleite is thought to be isostructural with ogy, archaeology, and folklore of turquoise, see
freirinite, which is a hydrated copper-sodium- Pogue 1915.
calcium arsenate. A blue-green mineral from the 5 Pliny the Elder Natural History 37.33 (Pliny 1979).
department of Freirina, Chile, it is similar to 6 See Foord and Taggart 1998 for additional chemi­
lavendulan, NaCaCu 5(As0 4) 4Cl-5H 20. cal analyses of these minerals.
3 Because turquoise is opaque with a waxy luster, 7 Pliny 37.33.
it is cut en cabochon (in convex form but not 8 Marco Polo The Most Noble and Famous Travels of
faceted) for jewelry. The mineral tends to absorb Marco Polo 1.14 (Polo [1579] 1937).
oil or grease, which may discolor it, so it is often 9 S. Ν. Campbell, personal communication, letter to
protected today with a plastic film. Some deeply the author 9.22.1997.
colored stones lose their intensity in sunlight.

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