Page 343 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

records instructions for the restoration of a bronze statue of a tyrannicide (Pernice 1910). The
clerks of the market are told to ensure that the statue is kept free of rust and that it is provided
with a garland and kept bright.
Also of interest are some papyri recording the accounts of the Roman temple of Jupiter
Capitolinus in the ancient Egyptian settlement of Arsinoe for the year 215 C E . Three of
the accounts record the treatment of some bronze statues and utensils. The treatment is referred
to as an "anointment" and was entrusted to a worker specially trained for this vocation.
Here again, special steps were taken to ensure that the bronze surface was maintained and not
allowed to develop an unsightly patina. Pliny notes that "in early days people used to stain stat­
ues with bitumen." In a subsequent passage, he remarks that burnt bitumen, "among other
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uses," is applied as a coating to copper and bronze vessels to make them fireproof and that "it
used to be the practice to employ it [bitumen] for staining copper and bronze and coating stat­
ues." 6 Here Pliny appears to be referring to the practice, in vogue in his own time, of gilding
statues rather than giving them a protective lacquer with bitumen, as was apparently done
before his time; this could explain his use of the past tense. Clearly the application of a thick or
burnt bitumen coating would provide good protection for bronzes designed to be used as caul­
drons and other utensils. For statues, however, Pliny implies that a shiny bronze surface was
maintained by the use of a protective lacquer of bitumen rather than an opaque finish.
Pernice attempted to replicate an opaque finish on a bronze by applying a solution of bitu­
men in turpentine, which enhanced the color of the bronze and gave it a slightly yellowed tint.
Not only may this finish have been more attractive than an opaque one, but it also provided some
corrosion protection to the bare metallic surface. The Renaissance practice of applying a trans­
lucent, reflective coating to enhance the beauty of a finished bronze or brass surface of a sculp­
ture served a similar purpose.
During the medieval period, the treatment know as émail brun was used to patínate copper
alloys and to impart a rich, dark brown color. The practice is discussed by Theophilus in De
diversis artibus of 1110-40. According to the translator's commentary (Dodwell i96i), this tech­
nique of coloring copper was known in the eleventh century but used only sporadically until the
twelfth century, when it became established practice. Theophilus's text instructs the metalsmith
to first engrave the copper object with the desired design and then to coat it with linseed oil
using a goose feather. The copper object is then heated over red-hot embers. When the oil has
melted, another coat is applied and the heating repeated. When the coating is evenly dried, the
copper object is placed on a strong fire and left until all fumes have dissipated and the émail brun
has reached the desired shade. Theophilus continues: "When it has been cooled—not in water
but by itself — carefully scrape the small flowers with very sharp scraping tools, but leaving
the backgrounds dark." The use of organic lacquers or finishes — as, for example, bitumen dis­
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solved in a suitable solvent such as turpentine or linseed oil—to color copper can therefore be
traced from ancient Roman times to the medieval period.

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