Archaeological silver can become brittle as a long-term consequence of
corrosion and microstructural changes. Corrosion-induced embrittlement
and microstructurally-induced embrittlement may be independent,but can
act synergistically: R.J.H.Wanhill et.al.,Archaeometry,40,123-137(1998).
Corrosion-induced embrittlement can occur owing to (a) segregation of
copper to grain boundaries (discontinuous or cellular precipitation) at
low or even ambient temperatures;and (b) interdendritic and segregation
band corrosion,whereby copper segregated at high temperatures during
solidification of the metal.The segregation bands are the remains of
coring,after the cast metal has been worked and annealed,usually
repeatedly.Corrosion also occurs along slip lines and deformation twin
boundaries when the metal contains remanent deformation (cold-work) in
the finished state.
Microstructurally-induced embrittlement results in intergranular
fracture with clean grain boundary facets. The most likely embrittling
agent is lead,see the above reference and also: R.J.H.Wanhill,National
Aerospace Laboratory NLR Technical Publication TP 97647,Amsterdam,The
Netherlands.
The effect of grain size is secondary,i.e. a large grain size is not a
primary cause of embrittlement. This is explained in TP 97647,with the
aid of a classic dislocation model of grain boundary fracture in the
case of microstructurally-induced and synergistic embrittlement.
Going further,I have recently completed a report (provisional issue) on
microstructural embrittlement of gold and silver: R.J.H.Wanhill,National
Aerospace Laboratory NLR Technical Publication NLR-TP-2000-358. The
abstract follows:
Empirical and theoretical metallurgical knowledge enables specifying
which elements,in amounts less than about 5 at.%,could embrittle or
impair the mechanical properties of gold and silver. There are two
categories of this microstructurally-induced embrittlement: alloys most
probably embrittled as-cast,and alloys that could be embrittled by low
temperature ageing. From the chemical compositions and current evidence
for microstructural embrittlement of archaeological gold and silver,the
most likely embrittling elements are lead,bismuth and
antimony,especially lead. As yet there is no good evidence for
microstructural embrittlement of archaeological gold. But archaeological
silver can suffer age-embrittlement,most probably due to lead.
Suggestions for further investigation are made.
Russell Wanhill, National Aerospace Laboratory NLR, Structures and
Materials Division, P.O.Box 153, 8300 AD Emmeloord, The Netherlands.
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