Embrittled Ancient Silver and Iron Objects
and Their Conservation
Russell Wanhill
National Aerospace Laboratory NLR, Amsterdam, the Netherlands, P.O. Box 90502, 1006 BM Amsterdam
wanhill@nlr.nl
Introduction
Embrittlement of ancient metals is often complex. The
details of embrittlement should be investigated in order to select
the best remedial measures for restoration and conservation.
This article surveys the techniques used for investigating several
ancient embrittled silver artifacts and an iron pile-shoe from a
Roman bridge. The possibilities for preserving such objects are
also discussed.
Silver Embrittlement
The investigated artifacts were an Egyptian vase, the
Gundestrup Cauldron, and a Byzantine paten (a plate used
during celebration of the Eucharist). Table 1 lists the techniques
and their purposes: SEM = Scanning Electron Microscope; SE,
BSE = Secondary and BackScattered Electron imaging; EBSD
= Electron BackScatter Diffraction; EDX = Energy Dispersive
analysis of X-rays. The choice of techniques depended on
Figure 1: Corrosion-induced embrittlement along deformation twins, slip lines,
and copper-rich segregation bands in the Egyptian vase [1].
the accessibility and condition of samples from the artifacts.
For example, the vase samples were initially unembedded stamping. Inside the metal these kinds of corrosion can lead
fragments that could be examined fractographically. The other to additional corrosion along segregation bands. These bands
samples had already been prepared for metallography. are the remains, modified by working and annealing, of solute
There are three types of embrittlement likely in these element segregation (coring) and interdendritic segregation
objects: corrosion-induced, microstructurally induced, and that occurred during solidification of an ingot or cupelled
synergistic. button.
Corrosion-induced embrittlement is due to several Microstructurally induced embrittlement is
forms of selective corrosion. Intergranular corrosion is the characterized by brittle intergranular fracture, with sharply
most commonly reported. This can occur in mechanically defined cracks and grain boundary facets. The embrittlement
worked and annealed objects, which constitute the majority. is most probably a consequence of long-term low-temperature
Interdendritic corrosion can occur in castings, which are aging, whereby an impurity element, or elements, segregates
uncommon, especially in the Old World. Corrosion along slip to grain boundaries. The available evidence indicates lead to
lines and deformation twin boundaries can occur in objects be the most likely perpetrator [2, 3], though this has yet to be
that have not been annealed after (final) mechanical working, verified directly. Other impurity elements might be involved,
which includes striking a coin and decorating by chasing and notably bismuth [3].
Table 1: Diagnostic techniques for ancient silver embrittlement [1].
Egyptian Gudestrup Byzantine
Purposes Techniques
Vase Cauldron Paten
Determine basic Visual inspection • •
condition and “hidden”
•
damage
X-ray radiography
Final manufactured Light optical metallography
condition; grain size; SEM metallography and analysis
chemical composition; -SE andf BSE images • •
internal damage and -EBSD •
cracking; types of -EDX • •
embrittlement Microhardness
•
Types of embrittlement SEM fractography
•
34 doi: 10.1017/S1551929509000352
www.microscopy-today.com • 2009 September
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