Page 44 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

requires the attachment of a wire to one edge or side of the dageurreotype, which is connected
to the positive, or zinc, couple of the Daniel or Bunsen cell, as the original technology would
have been. The back of the plate is covered with a beeswax coating to avoid copper deposition,
since the electrodeposited copper would otherwise coat every exposed surface. A cold, saturated
solution of copper sulfate was used as the electrolyte and the electrode of copper was connected
to the negative terminal; the plate was immersed in the solution and immediately became cov­
ered with a copper deposit. When this deposit was of sufficient thickness, that of stiff cardboard,
the plate was removed, rinsed in water, and dried. The separation of the electrotype from the
original dageurreotype was obviously the most difficult part of the process, and Hill (i854) cau­
tions that any copper sulfate solution that touched the surface of the copy would irretrievably
stain it. It was sometimes difficult to separate the two, and cutting shears would then have to be
employed to trim the edges and effect a separation.
Because of oxidation, caution had to be taken to protect the copper plate from air and dust
as soon as possible by placing it in a frame and never touching the surface with bare hands. It
is interesting that this early recipe calls for a cold, saturated solution of copper sulfate, since
this is not what we would employ today for this process; now additions of both sulfuric acid
and thiourea are commonly used, and the copper sulfate solution is not saturated. The very
subtle surface texture of the dageurreotype is reproduced perfectly in the electrotyped plate, as
a tinthotype by Fizeau in the collections of the J. Paul Getty Museum, shown in FIGURE 1.3,
illustrates.

Dezincification As mentioned earlier, brasses with substantial amounts of zinc
may become destabilized and lose the alloying element through
the process of dezincification. This is an interesting example of the galvanic corrosion of a binary
copper-zinc alloy during which zinc is preferentially lost from the brass, often with intergranu­
lar corrosion and severe penetration of corrosion into the metallic matrix. Brasses with over 15%
zinc are susceptible to dezincification, according to current theory, but this does not neces­
sarily pertain to ancient specimens because of casting segregation and because of the difficulty
of modeling the long-term effects associated with archaeological materials based on data from
modern short-term studies. Two-phased alpha+beta brasses, which contain more than 30%
i
zinc, are more prone to zinc loss, as would be expected, especially f the beta phase is continu­
ous. (Most ancient and historical brasses contain less than 30% zinc and are single-phased alpha
brasses.) Slow-moving or stagnant solutions in the environment—as in brass pipes, water heat­
ers, or ships' fittings —augment the corrosion process: uniform dezincification is more com­
mon in high chloride ion environments or where the pH of the solution is very low. Lucey
(1972) exposed brass samples to a saturated solution of copper (II) chloride with a small
addition of zinc (II) chloride and measured the potential between the sample and an electrode
of pure copper. A large potential difference was found to correlate with dezincification.

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