Embrittled Silver and Iron Conservation
extensive discontinuous precipitation of copper along the grain
boundaries. This is notable because the eminent metallurgist
Cyril Stanley Smith took the view that grain boundaries along
which discontinuous precipitation had occurred seemed to be
highly susceptible to corrosion [4]. Also, limited evidence for
the Byzantine paten [2, 3] suggested that corrosion occurred
along grain boundaries with discontinuous precipitation. On
the other hand, the Gundestrup Cauldron results are consistent
with the experience of Peter Northover [5] who observed
intergranular corrosion and cracking in ancient Bactrian silver
despite copper contents less than 1 percent, which is almost
certainly too low for discontinuous precipitation to occur [6].
Iron Embrittlement
A recent brittle fracture in a Roman pile-shoe recovered
from the Maas riverbed in 1992 was investigated in collab-
oration with three other institutes in the Netherlands. See Table 2.
Figure 5 shows a detail of the fracture, which was caused
by the pile-shoe falling onto a concrete floor during storage.
This obviously intergranular fracture was attributed to
Figure 2: Microstructurally induced brittle intergranular fracture in the Egyptian
local decarburisation during manufacture (a local carbon
vase [2].
content of only 33 ppm) and subsequent embrittlement by
Synergistic embrittlement combines corrosion-induced
high-temperature phosphorus segregation to the ferrite grain
and microstructurally induced embrittlement. For example,
boundaries. The decarburisation would have occurred owing
corrosion along slip lines, deformation twin boundaries,
to locally oxidizing conditions in the smithing hearth, and
and segregation bands can result in cracks. These cracks can
the virtual absence of carbon would then permit phosphorus
then initiate fracture along microstructurally embrittled
segregation to the ferrite grain boundaries [7]. It was also found
grain boundaries—which may fracture anyway, though less
that the pile-shoe was covered by a surface corrosion layer of
easily—under the action of external loads. In turn, the grain
akaganeite (an iron oxide) which would have formed after the
boundary fractures expose more slip lines, deformation twins,
pile-shoe was recovered [8].
and segregation bands to the environment and increase the
Remedial measures for restoration and
opportunities for corrosion.
conservation
SEM metallographic and fractographic examples of all
Modern restorations and conservation are concerned
three types of embrittlement are given in Figures 1-3. Retained with both technical and ethical aspects. Essentially, this means
cold-work is generally responsible for corrosion-induced respecting an object’s integrity and using reversible remedial
embrittlement. This can be seen from the EBSD Inverse Pole measures. However, reversibility is a controversial topic and not
figure (IPF) color-coded maps in Figure 4, which show annealed always practicable.
and cold-worked samples from the Gundestrup Cauldron. Remedial measures for silver. Table 3 summarizes how
The annealed sample is uncorroded even though there is the basic condition and type of embrittlement of ancient silver
Table 2: Diagnostic techniques for embrittlement of an iron pile-shoe [1, 7].
Techniques Specific Aspects Organization
Macrofractography Brittle fracture, corrosion Het Valkhof
SEM/FEG-SEM fractography Intergranular + cleavage fracture PR-MA/NLR
Optical metallography Microstructure, hardness CORUS, NLR
Chemical analysis
Bulk
X-Ray Diffraction (XRD) Iron and surface corrosion layer PR-MA
X-Ray Fluorescence (XRF) spectroscopy Iron composition CORUS
Combustion + InfraRed (IR) detection Carbon and sulphur content of iron CORUS
Metallographic surfaces
SEM+EDX, Electron Probe MicroAnalysis Iron and inclusion compositions
(EPMA)+WDX Phosphorus and oxygen line scans CORUS/PR-MA
Fracture surfaces
X-ray Photoelectron Spectroscopy (XPS) Grain boundary segregation PR-MA
Het Valkhof = Museum Het Valkhof, Nijmegen; PR-MA = Philips Research-Materials Analysis, Eindhoven; CORUS = Corus Research, Development and Technology, IJmuiden
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