Page 37 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Early history of When copper is immersed in a solution of a noble element such
electrochemical plating as gold or platinum, these elements will plate out and the cop­
per will go into solution. This technique is often referred to
as electrochemical replacement plating, which is thought to have been used during the early
centuries C E . by the Moche culture for the plating of gold onto copper objects (Lechtman 1979),
a technology unknown in Europe until many centuries later when strong mineral acids were
available. This remarkable achievement of the Moche relied on the fact that it is possible to
slowly dissolve native gold dust in corrosive natural minerals such as salt, alum, and potassium
nitrate. It is also possible that gold could be dissolved by cyanogenic glycosides derived from
plants, but in any event the evidence strongly suggests that gold was brought into solution and,
after mixing with soda or sodium bicarbonate, could be rubbed over cleaned copper.
For the plating of gold to take place, some of the copper must enter into solution in a pro­
cess of electrochemical exchange; gold ions from solution are reduced to metallic gold when in
contact with the copper while at the same time some copper atoms corrode as their part of the
anodic reaction and dissolve in the plating solution. The coating thus formed is only 1-2 μιη
thick, and microscopic imperfections will be present in this surface layer from the anodic dis­
solution of copper. Features of this extraordinary galvanic technique were fully replicated by
Lechtman (1979), confirming that it was indeed possible to produce such a coating without the
use of strong mineral acids, such as hydrochloric or nitric acid. A conceptual subtlety is the need
to raise the pH of the plating solution into the alkaline region before gold will deposit from the
solution at all. The solution of gold in potassium nitrate, potassium aluminium sulfate, and
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sodium chloride is very acidic, forming chloroauric acid in solution, H(AuCl 4 ) Η 2 0 . f copper
were immersed in this solution, it would be rapidly attacked, developing a layer of basic cop­
per chlorides and other salts on the surface, which would ruin an attempt to deposit gold gal-
vanically, since the optimum pH for deposition of gold is 9. In her replication experiments,
Lechtman (1979) achieved this more favorable pH by the addition of sodium bicarbonate fol­
lowed by immersion of cleaned copper sheet in the slurry for about fifteen minutes. Although
this electrochemical plating is poorly adherent to the copper, heating in charcoal between
500 °C and 800 °C for a few seconds may be enough to form a gold-copper diffusion bond, pro­
viding a metallurgical join between coating and substrate.
There are essentially two forms of galvanic deposition of another metal on copper, which
can be referred to as "electroless" plating, that are often used for chemical or industrial pro­
cesses and for "electrochemical replacement." In electroless plating the solution contains its
own reducing agent, and corrosion of the copper substrate is not essential for the reaction
to occur; in electrochemical replacement processes the copper surface must provide discrete
anodic and cathodic areas, for the plating of gold is a consequence of the corrosion of copper,

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