Page 38 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

which occurs at the same time. When the anodic regions are covered in gold the reaction comes
to a standstill, and the gold coating is not satisfactory unless the anodic regions slowly shrink in
size while general cathodic deposition of gold occurs over the rest of the cleaned surface.
Some evidence exists to suggest that small anodic pits formed in the copper under the sur­
face layer are the remnants of anodic sites in these gold-plated Moche objects. The existence of
these extraordinary gold (and silver) coatings over copper does not seem to have generated great
interest among scholars dealing with Old World technology, and the whole subject deserves
further research and attention. An interesting application of electrochemical plating from a
completely different period and culture is that used on the cast sculptural group of Medea Reju­
venating Aeson in the collections of the J. Paul Getty Museum. This group, shown in PLATE 3,
exemplifies a common Neoclassical theme of vestal virgins and rejuvenating scenes much
loved by French artists in the eighteenth century. In the course of the examination of this
work, surface analysis of the metal composition by X-ray fluorescence spectroscopy showed
that the sculpture was cast in a leaded brass alloy, rather than a bronze, and that the thin, glossy
surface, which was dark brown or steel gray in color, consisted of a platinum coating (Bewer
and Scott 1996).
This platinum patina could have been applied by an electrolytic technique, by the use of a
mercury-platinum amalgam, or by application of a chemical solution; in this case, a platinum
salt solution was used. The platinum coating process for the surface treatment of bronzes was
greatly facilitated by the research carried out in France by Alfred Roseleur (d. 1879) (Roseleur
1872). Prior to this, attempts to coat bronze objects with platinum had resulted in the deposi­
tion of a very thin film that was unsatisfactory as a surface deposit. The first historical report
of a deposition technique for platinum without the use of electric current is that of Stodart
(ca. 1760-1823), which dates from 1805. Stodart dissolved platinum in aqua regia and extracted
the salt (platinum chloride) in ether. f polished iron, steel, or brass is immersed in this solution,
I
it becomes coated with platinum, a technique Stodart probably used on surgical instruments
that he manufactured (Smeaton 1978).
Hiorns (i892) records that successful and attractive surface coatings could be achieved with
platinum salt solutions, and the recipe employed was successfully reproduced in the laboratory:

[T]here is one chloride, however, which is not subject to variation by the action of light,
but the shade is blackish, and altogether different to the beautiful light browns produced
by certain other soluble chlorides. This difference is due to the deposition of a thin film of
metal on the copper instead of a metallic compound. (Hiorns 1892:87- 88)

The process is one of electrochemical replacement plating; as copper or zinc atoms are attacked
by the platinum salt solution, they enter into solution, and platinum is simultaneously deposited
on the surface of the copper alloy as a coherent steel gray - colored film.

C O R R O S I O N AN D E N V I R O N M E N T
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