Case Histories of Ancient Silver Embrittlement
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Case Histories of Ancient Silver Embrittlement R. J. H. Wanhill
Submitted: 2 November 2010 / Published online: 4 January 2011 ASM International 2011
Abstract This article discusses six cases of ancient silver embrittlement. The diagnostic techniques and results are surveyed, followed by the most likely explanations of embrittlement. There are two basic types of embrittlement: corrosion-induced, and microstructurally induced, which can act synergistically. Corrosion-induced embrittlement takes several forms, including what appears to be intergranular and transgranular stress corrosion cracking. The main intrinsic factor facilitating corrosion-induced embrittlement is retained cold-work in the silver. Microstructural embrittlement is characterized by intergranular fracture, most probably due to low-temperature segregation of lead to the grain boundaries. Knowledge of the details of embrittlement can be essential for optimizing the restoration and conservation of damaged artifacts. Keywords
Ancient silver Embrittlement Case histories
Introduction Ancient silver artifacts are rare and valuable exhibits in many museums. Some are apparently in excellent condition, but others may have been badly damaged and embrittled during many centuries of burial. Preservation of these artifacts depends, or should depend, on a detailed understanding of the types of damage and embrittlement. Since 1994, the NLR has investigated four cases of ancient silver embrittlement [1–7], one as a re-assessment
R. J. H. Wanhill (&) National Aerospace Laboratory NLR, P.O. Box 153, 8300 AD Emmeloord, The Netherlands e-mail: [email protected]
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[7], and reviewed two cases investigated by others [8–11]. Experience and insight gained from the investigations enables presenting the most likely explanations of embrittlement for all six cases. This is the main topic of this article.
The Artifacts Five of the ancient silver artifacts are illustrated in Figs. 1–5. The Gundestrup Cauldron, Egyptian Vase, and Roman Kantharos (drinking cup) are about 2000 years old. The Byzantine Paten (altar plate) and Romanesque Kaptorga (small container for relics and amulets) date from the 7th and 10th centuries AD, respectively. The sixth artifact is a Sasanian King’s Head [11], dating from the 4th century AD. The Kaptorga and Head are the subjects of the reviewed cases mentioned above [8–11]. The artifacts are very different in form and origin. This is advantageous for the consistency of some of the explanations of the types of embrittlement.
Diagnostic Techniques Table 1 lists the diagnostic techniques (besides visual observation) used for examining small (usually millimetersize) samples from the artifacts. Limitations were imposed by the available samples and techniques. Fractography could not be done on the metallographic samples from the Cauldron and Paten; chemical analyses other than EDX were not possible for the Vase and Kaptorga; and EBSD was available only for the Cauldron. Also, only a sample from the Kantharos inner cup was available.
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