Page 26 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Why is it necessary to identify the corrosion products or colorants derived from copper
alloys, a subject that occupies much of this text? There are many possible reasons. First, it is very
useful to know what compounds are present in an object to determine its authenticity and to
i
decide f it is stable and suited to certain kinds of conservation treatments. The removal of cor­
rosion products without this knowledge could obliterate important historical information or
fine surface features, and might even destroy the original shape and detail of the object. Infor­
mation about corrosion products may also assist in choosing the best environments for objects
in storage, on open display, or in display cases. Second, corrosion products may be the only
available clue to the composition or origin of an object that may have become completely min­
eralized. Third, the corrosion products may preserve information about the past environment
of the object and the events that transformed it into a partially corroded matrix. This book is
designed to serve as a helpful resource for all of these purposes.

P R E S E N T A T I O N OF I N F O R M A T I O N AND O T H E R R E S O U R C E S

Information on copper compounds provided in this book includes the mineral name, common
chemical name (not necessarily following the International Union of Pure and Applied Chem­
istry, IUPAC, convention), chemical formula, density, color, mineral system, file number assigned
by the International Centre for Diffraction Data (ICDD), and relative molar volume. Not all of
this information is discussed in all instances, especially where standard references exist to
provide primary mineralogical data. Such references include the CRC Handbook of Chemistry
and Physics (Weast 1984); the ICDD Mineral Reference File (i989), Gmelins Handbuch der anor­
ganischen Chemie (Gmelin 1965), and A Comprehensive Treatise on Inorganic and Theoretical Chem­
istry (Mellor 1928). A listing of some of the compounds discussed in this book, together with
their ICDD reference file numbers, is provided in APPENDIXES C and D.
All percentages given in the chemical recipes, alloy compositions, and corrosion studies are
weight percent unless otherwise stated.

Notes s In some cases, corrosion products or pigments
1
ι Zhao Xigu (Kerr 990:70). The thirteenth-century may be poorly crystalline and give X-ray diffrac-
Chinese chemist, writing here in 1230, was evi- tion patterns that are unsatisfactory for identifi-
dently observing the darkening of cuprite, per- cation. In this event, F T I R is very useful, since
haps due to hydration, as it was boiled in water. the anions or chemical groups can be character-
2 Little attention has been paid to these isotopes ized, even if the precise composition of the corn-
in archaeometric studies. Gale and colleagues pound responsible for these groups cannot be
(1997), however, recently developed techniques determined. X-ray diffraction analysis, however,
for the relatively precise isotopic determination is preferable to F T I R . It is a more powerful tool
of copper and found isotopic anomalies in copper for mineral differentiation, and the database of
from certain minerals and ores. more than sixty-five thousand minerals main-
3 Pliny the Elder Natural History 34.2 (Pliny 1979). tained by the International Center for Diffraction
4 The alloy pinchbeck was named around 1725 after Data is not equaled by F T I R data for inorganic
the jeweler Christopher Pinchbeck and became compounds.
synonymous with any cheap gold-colored alloy
imitation.

I N T R O D U C T I O N
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