Page 23 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
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Extraordinary early brass objects have been found at Taxila in Pakistan. Brass coinage
began to appear in Bithynia and Phrygia in southwest Asia from the first century B.C.E. The
color of brass containing 10-20% zinc is very much like gold, and the presence of zinc confers
hardness and strength much as arsenic and tin provide to bronze. Brass was especially popular
in India, where it has been in continuous use for two thousand years for temple roofs, furniture,
and cooking and storage vessels (Lambert 1997). By the Renaissance, many objects identified as
bronze were, in fact, cast from leaded brass alloys, which continue to be used.
The alloys of copper and zinc were highly valued for their golden color and could be
mistaken for gold. Those alloys that most counterfeited gold were given particular names,
such as ormolu, prince's metal, Mannheim gold, or pinchbeck. Gold-colored brass alloys could
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be beaten into thin foil or leaf, which is often called by the misnomer "bronze powder." f
carefully lacquered and untarnished, such items could be distinguished from gold only by chem
ical analysis.
As early metalworkers discovered, the addition of arsenic, tin, zinc, or lead to copper can
dramatically alter the color of the metal as well as its physical and chemical properties; it can
also change the type and degree of corrosion of objects made from these alloys, whether the
objects are cast or wrought into shape.
C O P P E R M I N E R A L S AS P I G M E N T S
The natural copper minerals malachite and azurite were important pigments from the sixth
millennium B.C.E. onward, becoming gradually supplanted by synthetic copper compounds
because of the relative scarcity of these green- and blue-colored mineral deposits. Long before
the beginning of the Christian era, this scarcity led to attempts to make stable green and blue
pigments for use in wall paintings and tombs, and later in manuscripts and paintings. In the pro
cess, many interesting copper-containing compounds were discovered, and techniques for the
chemical synthesis of pigments, such as verdigris, were refined and transmitted from one gen
eration to another.
Pigments and colorants based on copper are very closely related to copper corrosion prod
ucts. Because of the varied and distinctly brilliant blues and greens that could be produced, the
deliberate corrosion of copper and copper alloys was often employed to make artificial pig
ments. In later periods, recipes for producing specific colors became an important part of the
corpus of painting materials. In many cases, these copper-based blue or green pigments are not
easy to identify. One aim of this book is to provide currently available background information
concerning these compounds, supplemented where possible with additional analytical, techni
cal, and historical information.
I N T R O D U C T I O N
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