Page 36 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

containing copper as a preliminary to gilding, which would have deposited a coating of copper
over the iron. A similar process is also mentioned in the ninth-century Mappae Clavicula (Smith
and Hawthorne 1974).
A solution of copper sulfate in sulfuric acid was found to produce a coating of copper quite
rapidly, although the plated copper surface did not have good adhesion. The galvanic replace­
ment of iron with copper was used historically for the recovery of copper from waste mine
waters, and, as is well known, the knowledge was also used for thaumaturgical purposes, the
process of transmutation of the elements —from iron to copper or from copper to gold—being
very much a part of the alchemical tradition. In the primary extraction of copper from waste,
the galvanic deposition of copper from aeruginous mine waters even produced a minor indus­
try in the form of Herrengrund ware, made in the town of that name in Erzgebirge in Bohemia.
Some examples are illustrated in Smith (1971).
Ingenious metallurgical techniques have been applied to the coating of iron with either cop­
per or copper alloys, some of which are simply based on mechanical cladding or dipping the
iron object into molten copper or copper alloy. The earliest technique was simply to wrap cop­
per sheet over the shaped iron object, carefully hammering together any seams or joins in the
copper. Corfield (1993), for example, illustrates an Early Iron Age bridle bit from a hoard dis­
covered at Llyn Cerrig Bach, Anglesey, Wales, originally excavated by Fox (i946), which was
fabricated by this technique.
The fusion plating of copper onto iron was also common and is described by the German
monkTheophilus (i96i) in his De diversis artibus (ca. 1110 - 40) as part of the manufacture of bar­
rel padlocks. Theophilus tells of using an alloy of 66% copper and 33% tin, which was crushed,
mixed with flux, and heated in charcoal. A related technique was reported by Arwidsson and
Berg (i983) for coating iron bells from Viking Scandinavia. The bells would be covered with
strips of copper inside and out and heated in dung to fuse them together. The electrolytic plat­
ing of iron with brass alloys was already well known in the late nineteenth century. Phillips
(1911) describes the galvanic deposition of brass from an electrolyte of copper and zinc salts in
cyanide solution, and producing variations in the color of the brass plating by adjusting the
exposed brass anode area in the solution. An alternative technique was to first plate a coating
of copper, followed by one of zinc, then sandwich it with another layer of copper before heat­
ing the plated metals to alloy them together. Copper alloys are still used in industry today for
plating iron and steel; for example, mild steel can be plated with a binary brass by using an
electrolyte of copper and zinc salts in the ratio 6:4 dissolved in aqueous sodium cyanide. It is
surprising that bronze and high-tin bronze alloys can also be plated from solution (Silman and
Averill 1978), because one element will usually be plated out before the other, and specially
designed solutions must be employed to achieve satisfactory alloy electrodeposits.

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