The properties of metals and alloys are dependent on their atomic structure. Metals are an aggregation of atoms that, apart from mercury, are solid at room temperature. These atoms are held together by “metallic bonds” that result from sharing available electrons. A negative electron bond pervades the structure, and heat and electricity can be conducted through the metal by the free movement of electrons. The atoms are regularly arranged, forming a symmetrical three-dimensional group, which repeats itself periodically and which is called a unit cell. Unit cells of identical characteristics form a crystal with regular spatial distribution of atoms, which can be defined by a space lattice. Atoms can be stacked in different ways to create a variety of simple lattice types. There are three common types of lattice structure that most metals belong to (Fig. 4.1): face-centred cubic (fcc), body-centred cubic (bcc) and close-packed hexagonal (cph).

The Structure of Metals and Alloys

Since the 18th century many analytical methods have been used to identify the chemical composition of archaeological artefacts, the provenance of the raw materials and the technical aspects of the manufacturing process. These techniques generally probe the bulk chemical and structural features of the objects and overlook surface characteristics often essential for understanding the manufacturing techniques and the ageing processes. This work describes the contribution of x-ray photoelectron spectroscopy (XPS) combined with bulk analytical techniques such as scanning electron microscopy + energy − dispersive spectrometry (SEM + EDS), inductively coupled plasma mass spectrometry (ICPMS) and x-ray diffraction (XRD) by discussing the origin of the embrittlement of some selected series of silver Republican Roman coins and the study of the microchemical nature of black and coral red glosses on Attic figured vases. The results show that XPS, if allowance is made for contamination or unrepresentative or non-homogeneous samples, complements the existing bulk analytical techniques and can provide detailed information about the surface chemical nature of the archaeological objects, as well as about their present state, and lead to recommendations for restoration and preservation. Copyright © 2002 John Wiley & Sons, Ltd.

Contribution of surface analytical techniques for the microchemical study of archaeological artefacts

Ancient silver may become brittle and damaged owing to long-term corrosion and changes in the microstructure. Recognition and determination of corrosion-induced and microstructurally-induced embrittlement, and also their synergy, are important for restoration and conservation of ancient and historic silver. The types of embrittlement are described and illustrated, using examples of ancient and historic silver artefacts, including the famous Gundestrup Cauldron, a masterpiece of European Iron Age silverwork. In particular, the use of automated Electron BackScatter Diffraction (EBSD) enables improved analysis and assessment of corrosion-induced embrittlement. The knowledge obtained from detailed investigations is helpful not only in determining the best ways to restore and conserve embrittled silver objects, but also in defining the possible extent of the embrittlement problem. This is illustrated by a straightforward statistical analysis.

Embrittlement of Ancient Silver

Some archaeological silver objects are found to be brittle. The brittleness is a consequence of long-term corrosion and microstructural changes. The types and mechanisms of embrittlement are defined and explained. The severity of embrittlement is increased by a larger grain size and by synergistic action of corrosion-induced and microstructurally induced embrittlement. Micromechanical models of fracture that incorporate the grain size provide an insight into these effects and help to explain why severely embrittled objects are frangible or even friable.

Brittle archaeological silver: a fracture mechanisms and mechanics assessment*

Indoor environmental corrosion of Ag-based alloys in the Egyptian Museum (Cairo, Egypt)

An Egyptian metal vase from the Ptolemaic period was investigated metallurgically and fractographically. The metal is nearly pure silver, and chemical analysis indicates it was obtained from the cupellation of lead. Despite its high purity (in archaeological terms) the metal is severely cracked and embrittled, with fractures mainly along grain boundaries. Factors contributing to this damage are work-hardening and residual deformation in the microstructure (slip and twinning), corrosion along slip lines and twin boundaries, corrosion in bands that are the remains of coring (solute element segregation during solidification), large equiaxed grains, externally applied forces and internal residual stresses. The metal is now easily broken into small pieces, i.e. it is friable. This must be taken into account during any further restoration of the vase. A procedure to better preserve the vase is suggested.
A joint publication of the National Aerospace Laboratory NLR and the Allard Pierson Archaeological Museum of the University of Amsterdam.

Damage assessment and preservation of an Egyptian silver vase (300 - 200 B.C.)

An Egyptian metal vase from the Ptolemaic period was investigated metallurgically and fractographically. The metal is nearly pure silver. Despite its high purity (in archaeological terms) it is severely embrittled and cracked, mainly along grain boundaries. Factors contributing to this damage are work-hardening and residual microstructural deformation (slip and twinning), corrosion along slip lines and twin boundaries, corrosion in bands that are the remains of coring, large equiaxed grains, externally applied forces and internal residual stresses. The metal is now friable. This must be taken into account during any further restoration of the vase: a procedure is suggested.

Damage assessment and preservation of an egyptian silver vase (300–200 bc)*

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Damage assessment and preservation of an Egyptian silver vase (300 - 200 B.C.)

Damage assessment and preservation of an egyptian silver vase (300–200 bc)*