Embrittled Silver and Iron Conservation
Table 3: Potential remedial measures for restoration and conservation of ancient silver [2, 3].
method for corrosion-embrittled
artifacts is warranted.
Nominally intact artifacts and coins
Remedial measures for
• undeformed: corrosion protection
iron pile-shoes. Many more
• deformed: corrosion protection; heat treatment of coins to remove microstructural
pile-shoes were recovered besides
embrittlement, followed by corrosion protection
the accidentally fractured one.
Restored artifacts Investigation of this pile-shoe
• old restoration: corrosion protection; disassembly, reassembly, corrosion protection indicated two aspects for
• modern restoration: corrosion protection conserving the others. Firstly,
Fragmented artifacts and coins
these must be expected to
• assembly and corrosion protection
have surface corrosion layers
• heat treatment, assembly, and corrosion protection
containing akaganeite. Because
akaganeite is hygroscopic,
point to potentially appropriate remedies. Corrosion protection
corrosion will continue unless actively prevented by drying
can be a generally applicable measure, but heat treatments are
out the corrosion layers and either storing the pile-shoes in
not. Heat treatments obviously are irreversible and should be
a low-humidity environment [8] or applying a protective
allowed only if preceded by thorough diagnostic investigations
(organic) coating. Secondly, some of these pile-shoes could also
and if judged essential by expert technical staff .
have local carbon contents that are extremely low. Th us if any
Th e most likely and feasible corrosion protection measure
are to be removed from storage, and whether or not they are
is cleaning, outgassing to dry crack surfaces and any entrapped
still attached to piles, they should be handled and transported
corrosion products, and application of a protective coating. Th e
with care to avoid breakages.
choice of cleaning methods and protective coatings requires
References
much forethought and care [3, 9].
An innovative cleaning
[1] RJH Wanhill, “Embrittlement of archaeological silver
method is hydrogen plasma reduction. Th is requires no more
and iron, National Aerospace Laboratory Report NLR-TP-
than an hour at temperatures of 40-100°C, which minimizes
2009-105,” Amsterdam, the Netherlands: to be published in
or avoids signifi cant alterations of an artifact’s microstructure,
Structural Integrity and Life 9 (2009).
especially at the lower end of the temperature range. Th e
[2] RJH Wanhill, Journal of Failure Analysis and Prevention 5
hydrogen plasma reduces surface corrosion products to
(2005) 41-54.
metallic silver and off ers a possible alternative to heat treatment
[3] RJH Wanhill, A Lecture Course on Metallurgy, Embrittle-
of artifacts that are penetrated and severely embrittled by ment and Conservation of Ancient Silver, National Aerospace
corrosion. Such heat treatments require temperatures of Laboratory NLR, Amsterdam, the Netherlands, 2008.
700°C, or more, to remove the corrosion and are generally [4] CS Smith, Application of Science in Examination of Works
unacceptable because of a high risk of damaging the artifacts. of Art, WJ Young, ed., Boston Museum of Fine Arts, Boston,
Hence further investigation of the hydrogen plasma reduction 1965, 20-52.
Figure 3: Synergistic embrittlement example: Microstructurally induced intergranular fracture and corrosion-induced damage and cracking along segregation bands in
the Egyptian vase [1].
2009 September •
www.microscopy-today.com 37
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