Page 345 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Lead and patinas Some bronze and brass alloys contain lead, which may influ­
ence the patina and the kinds of corrosion products that can be
formed. Dews (1930) concluded that lead has no great effect on the color of new bronze but it
does increase the rate of tarnishing. A lead-free bronze with 8-10% tin content will remain fairly
bright in dry air for a long time, but a leaded bronze will tarnish quickly and become darker
than an unleaded one; for this reason ornamental bronzes that are required to develop a deep
patina usually contain 10-18% lead.
These conclusions about new bronze, however, do not necessarily apply to ancient bronzes
containing lead; here the situation is much more complex than that outlined by Dews. For
example, during casting it is possible to get some segregation of the lead to the outer surface of
the mold, with the result that there may be a prevalence of lead-containing corrosion products,
such as the commonly encountered hydrocerrusite, in the patina. Similar segregation events
may occur when the bronze contains discrete globules of lead. Because of lead's particular sus­
ceptibility to corrosion in the presence of organic acids, patches of white lead acetates and basic
carbonates, in particular, can result from storing lead-containing bronze objects in unsuitable
wooden drawers or displaying them where organic-acid components may be emanating from
fiber composites, glues, rubbers, and so on. The lead may also be slowly corroded along with
other bronze components during burial and be replaced with lead corrosion products or leached
away and replaced with cuprite. The leached lead salts are often relatively insoluble and may
form part of the overall patina of the bronze.

Black patina in the Zenghelis (1930) studied the ephebe of Marathon 9 and the
aqueous environment beard from a statue of Zeus that was found in the sea at Cape
Artemision. 10 Zenghelis reported that both bronzes were cov­
ered with the same black patina, which was considered to be original in both cases. Analysis of
the hoof of a horse from the Marathon sculpture showed a composition of about 64.8% copper,
1.1% iron, 13.7% sulfur, and 18% oxygen, with 2.4% insoluble matter. It is possible, but not proved,
that these patinas are original and not formed by sea burial. This cannot be assumed, however,
since the production of sulfidic patinas is expected as the principal corrosion event in oxygen-
deficient waters because of the prevalence of sulfate-reducing bacteria.
X-ray diffraction analysis of the outer part of the concretion covering the Riace bronzes, 11
which were submerged for many centuries at a depth of 8 m at the bottom of the Ionian Sea,
showed the presence of alpha quartz, atacamite, cuprite, and chalconatronite. The blackish sur­
face below the sandy encrustation layer consists of cuprite, atacamite, tenorite, and chalcocite.
Formigli (1991) maintains that this layer is not to be confused with the original patina, which is
also black. Energy dispersive analysis of a sample of the patina about 70 μιη thick showed a

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