Page 24 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

A N C I E N T B R O N Z E S AND PATINA

Art historians, scholars, collectors, and scientists have long been drawn to the aesthetic qualities
of ancient bronzes, with their varied green patinas, and intrigued by the technical skill shown
by the unknown artisans who made them. Two bronzes from the collections of the J. Paul Getty
Museum illustrate some aspects of patina: the finely cast Greek bronze Statuette of a Dead Youth
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(PLATE ) that preserves a patina of cuprite and malachite spotted with reddish blisters from
past eruptions of pitting corrosion, probably from "bronze disease" (discussed in CHAPTER 4),
and the well-preserved Herrn (PLATE 2) with a patina containing both malachite and azurite
over a finely developed cuprite layer. FIGURE 3 shows some possible variations in copper alloy
surfaces due to corrosion.

Early scientific analyses The scientific examination of ancient bronzes includes studies
by many eminent chemists, such as the research done by the
French scientist Balthazar-George Sage in 1779 and by John Davy (brother of the noted English
chemist Sir Humphry Davy), who in 1826 described his examination of a bronze helmet recov­
ered from the sea near the Greek island of Corfu. Using wet chemical analysis, Davy determined
that the metal was an alloy of copper and tin. He also identified the corrosion products: the ruby
red suboxide of copper (cuprous oxide, or cuprite); the green rust of the carbonate (basic cupric
carbonate, or malachite) ; submuriate of copper (one of the copper trihydroxychlorides, proba­
bly paratacamite or atacamite); crystals of metallic copper (which can be redeposited from solu­
tion during the corrosion of bronze objects); and a dirty white material, tin oxide (stannic oxide,
or cassiterite). This was an impressive study for the time. Davy was also aware that some of his
work was potentially important for the authentication of ancient bronzes, as evidenced by his
presentation to the Royal Society of London.

Permit me to lay before the Royal Society, the results of some experiments and observations
on the incrustations of certain ancient alloys of copper, which I trust may not be undeserving
of notice, whether considered in connexion with the arts of Ancient Greece, or in relation
to the slow play of chemical affinities acting during a long period of time. (Davy 1826:55)

Modern analytical techniques A range of physical methods are used today to analyze the
composition of metals and their corrosion products. These
techniques include X-ray diffraction (XRD), inductively coupled plasma-mass spectrometry
(ICP-MS), scanning electron microscopy (SEM), electron microprobe analysis (EPMA), Fourier
transform infrared spectroscopy (FTIR), 5 X-ray fluorescence spectroscopy (XRF), and optical
metallography, particularly polarized-light microscopy (PLM) .

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