Page 97 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

from touching the copper. The placement of often in a mixed formulation. This results in a
asphalt would have been of practical use if this shiny, opaque, very dark green, brown, or black
object had been used as a battery. A number of patina. Harris (1994) claims that many treatments
similar finds have been made in Iraq, though in the United Kingdom involve restoration prac­
none with an iron rod present. tices that can cause extensive damage or are not
6 A typical recipe for arsenic plating, given by Fish- supportable on aesthetic grounds. Clearly more
lock (1976), consists of 370 g of arsenic trioxide, information is required regarding what exactly
284 ml of hydrochloric acid, 71 ml of sulfuric acid, has been done to these bronzes since the early
and 4550 ml of water. The surface is usually pro­ part of the twentieth century.
tected with lacquer or wax after patination. 15 Deposition is the process by which a pollutant
7 Hiorns (i892) gives the following recipe for mer­ reacts with the surface of a material. This can vary
cury plating: 9.7 g of mercuric chloride, 9.7 g of greatly, depending on how the material is finished.
ammonium nitrate, 99 ml of water, and enough 16 The so-called original surface is the discrete,
ammonium hydroxide added until the precipitate recognizable boundary often preserved at the
that forms just redissolves. interface between the corrosion material that has
8 To make electrotyped duplicates for study, developed on the object and the corrosion that has
the objects were first cleaned in acetone and occurred within it. Many objects now in museum
coated with a nitrocellulose lacquer. A system storage were cleaned in the past down to their
of fiberglass-polyester resin pads and tubes was metallic remnants, resulting in significant loss
constructed to hold the metallic fragments in posi­ of shape of the original artifact. This cleaning
tion on a carved wooden base. The shape of the was undertaken in the mistaken belief that any
objects was completed with Plasticine, and miss­ remaining corrosion implied long-term instabil­
ing decorations were remodeled. A mold was then ity. It is now known that this is not necessarily
taken from the assembly, and an electrotyped the case.
copy was made. 17 Helena Strandberg, letter to the author, 8 Sep­
9 An impression in the soil left by a completely tember 1998.
decomposed body. 18 iso standards, as well as those of the American
ίο It is slightly surprising that so many of the Viking- Society for Testing and Materials (ASTM), are of
age Birka finds proved to be copper-zinc alloys. interest in helping to evaluate the effects of cor­
This could be explained by the reuse of Roman rosion, particularly in outdoor exposure trials, on
scrap, as was done, for example, during the metallic materials, or on organic coatings. The use
Anglo-Saxon period in England. This does result of these standard protocols enables different labo­
in the presence of zinc in some of these alloys ratories to achieve some degree of comparability
because of the earlier Roman cementation tech­ in the evaluation of description of the sample or
nique for making brass. corrosive events that may have occurred.
11 In a related article by Meijers et al. (i997), the 19 The data from previous studies were incorporated
term "conservability" is introduced. It is not here to give a composite indoor distribution func­
exactly clear what this word encompasses; one tion, and a lognormal approximation was found
possibility is that it may refer to the ability of the to be an appropriate fit.
object to be "conserved/' or to the measurement 20 Witcamine RAD 1100, manufactured by Witco
of the benign or aggressive tendencies of the soil Corp., Ontario, Canada.
in which the object was buried, or to the com­ 21 Hostacor SI, manufactured by Hoechst AG,
K
bined effects of deterioration on the treatment Frankfurt, Germany.
options available for a particular class of mate­ 22 Yuan Hung-tao, I60 5 (Kerr 1990:68) .
rials being considered. 23 See Scott 1991 for additional information on the
12 Plutarch Moralia, the Oracles at Delphi no longer preparation and metallography of ancient metals.
given in verse 5.35ic-438e (Plutarch 1984).
13 The Clean Air Act was passed by the U.S. Con­
gress in 1970.
14 The formation of these dark patinas influenced
London's aesthetic approach to the maintenance
of the city's exposed bronzes beginning in about
the 1950s, at least. In this process, corrosion
excrescences are removed, and the bronze surface
is treated with lanolin, beeswax, or paraffin wax,

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