Page 366 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

the formation of the black patina. Brasses of composition 60Cu40Zn and 50Cu50Zn do not
form a patina in the solution, confirming claims in the literature that the blackening process
does not affect the brass wire inlays. All available bidri analytical data show a copper content of
less than 12%, and the alloy therefore falls into the epsilon+nu phase field of the binary copper-
zinc diagram. Up to about 1% copper in the zinc forms the terminal solid solution, nu; at greater
than 1% copper, the epsilon phase precipitates at the grain boundaries. Apart from the work by
LaNiece and Martin, there has been surprisingly little interest in the analytical conundrum of
identifying this black coating, compared with the prolific number of papers that address the sub­
ject of black-patinated Chinese bronzes.

SOME F I N I S H E S AND P R E S E R V E D S T R U C T U R E S

Traditionalfinishes Birnie (1993) reviewed some of the traditional finishes used
on scientific instruments on scientific instruments made of brass. Aesthetic appearance,
corrosion resistance, and alteration of surface reflectivity are
potentially important aspects of the final color; brown, blue, black, gray, and green finishes are
used for particular purposes. Most instruments studied were not given a mirror finish; more fre­
quently a satin or machined finish was preferable. 22 For patination, dark gray or browns were
achieved using either platinum, arsenic, or mercury solutions (briefly discussed in CHAPTER 1
in relation to galvanic effects). Lacquer used to be a very important surface finish on scientific
instruments. The old lacquers were based on resinous concoctions used with methanol, ethanol,
turpentine, or white spirit. Many recipes used additives that included saffron, turmeric, sandal­
wood extract, and other dyes to give a colored surface finish. The technique used for applying
this lacquer coat over polished brass required great skill. A flat camel's-hair brush was used to
apply the lacquer in single long strokes on the metalwork, which had been heated to about 76 °C.
Fingerprints can be very damaging to the surface finish of these instruments. Perspiration con­
tains 2.17 g/1 of lactic acid and 2.5 g/1 of sodium chloride, in addition to amino acids and fatty
acids. For this reason it is essential that gloves are worn when handling instruments finished
with this kind of surface.

Tool marks Not only may recent patinas preserve inadvertent damage, such
preserved in patinas as fingerprints, that has etched into the metal surface, but
ancient patinas may also preserve the impression of tool marks
or other clues to how the surface was finished in antiquity. In waterlogged deposits, the absence
of a normal patina may allow selective etching of the metal surface, revealing details concern­
ing metallic structure that would normally be visible only after metallographic examination of
a cross section of the object concerned.

S O M E A S P E C T S O F B R O N Z E PATINAS
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