Page 42 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

amperage per square centimeter must also be kept within certain limits, otherwise the deposit
is too coarse because it is laid down too fast, altering the grain morphology; such deposition
does not reproduce good detail.
In the majority of museum laboratory processes, copper anodes are used with copper sul­
fate and sulfuric acid, with small additions of ethanol as a wetting agent and thiourea as a com-
plexing agent. The principal ions in solution are Cu , H , and S0 4 ~. On the surface of the
2
+
2+
cathodic object, the positive charge of the copper (II) cations is discharged by the electrons sup­
plied by the current at a potential of +0.34 V for the reaction (see equation 1.4).
It should be noted that hydrogen ions are also attracted to the cathode, but these are dis­
charged at a different potential (0.0 V), and it is the copper cations that are discharged first,
although the whole process is very complex and much more involved than equation 1.4 would
indicate. In aqueous solutions, the ion Cu(H 2 0) 6 ] 2 + is present. In electrolytes containing
[
hydrated metallic cations, the concentration of dischargeable metal ions is usually high, and
for the precipitation of copper from hydrated ions, dehydration is involved as a preliminary
reaction. This removal of water molecules occurs mainly in the outer diffusion zone of the
Helmholtz double layer and is composed of several individual steps. The final stage of this pro­
cess occurs when the dehydrated copper ion passes through the double layer to the cathode
where it is neutralized and absorbed as soon as it touches the surface as an adatom. This neutral
adatom diffuses to become a part of the crystal lattice, the field strength within the double layer
having no effect on this part of the process. The hydroxide and sulfate anions travel to the anode,
where they are discharged f the voltage is sufficient. The copper anode is gradually dissolved
i
and delivers to the electrolyte the same quantity of copper ions as are deposited at the cathode;
I
this is the principal anode reaction. f the potential is raised, the reaction of water can result in
the production of oxygen, as follows:

2 H 2 0 - 4e" = 4 H + + 0 2 1.6
Normally, the sulfate anions are not discharged, since this reaction would require a much
greater potential than that normally used in museum electrotyping work. These technical
aspects are often glossed over, making it appear that electrotyping is a completely simplistic
operation, but this is not the case, and the practical details of successful laboratory practice are
well described in the handbook by Larsen (1984).
A good example of the traditional craft of electrotyping is provided by the conservation
work on a Romano-British bronze parade helmet carried out by Shorer (1979). Work by Larsen
8
(1984) has also led to a revival of interest in electrotyping for tool marks, fabrication studies,
and copies. In another modern application of electrotyping, Bertholon and coworkers (1995)
reported on their conservation re-treatment of the copper Dead Sea Scroll from Qumran,
Jordan, that was first conserved in England in 1955. The re-restoration work consisted pri­
marily of additional consolidation and the construction of a new support. X-radiography of

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