Analysis of grain boundaries in an embrittled ancient silver necklace

Abstract

Archaeological silver objects are often found to be embrittled to some extent. Using light and scanning electron microscopy techniques, X-ray diffraction and AES, the causes of this embrittlement were studied on a silver artefact buried for more than 1000 years in soil. This investigation revealed the presence of chlorine, sulphur and carbon at grain boundaries of the Ag-Cu based object suggesting that most likely; the substantial embrittlement is caused by an inter-crystalline corrosion attack. The role of copper enrichment at the grain boundaries as a possible path of preferential corrosion attack is discussed.

Crystallographic aspects of intergranular failure of archaeological silver artefacts

“…[1] The fragments of the chain links were longitudinally fine ground. Ar + ion milling was used to remove the damaged surface layer caused by metallographic operations.…”

“…Our previous investigation of the links of a necklace dated to the 10th century A.D. made of a silver-copper base alloy unambiguously proved that this failure is exclusively intergranular. [1] This phenomenon is evidently a consequence of the processes occurring both during the manufacture of the object and during the long-term burial. It is assumed that copper segregates at the grain boundaries.…”

“…[4] In agreement with the literature, [5] it is accepted that these intergranular layers, which are less noble than the surrounding volume with a lower content of copper, are prone to predominant corrosion attack in soil electrolyte under the conditions of decomposing human body, thereby causing extended intergranular damage. [1] Due to the different structure of individual grain boundaries, the solute segregation is principally anisotropic. Let us briefly recollect that there are two basic groups of grain boundaries -general and singular (special) -which differ in both the atomic structure and the interfacial energy.…”

Copper surface enrichment of AgCu alloys

“…[6] However, there exists indirect evidence of the above hypothesis: (i) computer simulations of grain boundary composition in Ag-Cu system suggest the existence of sites for preferential copper segregation, [7,8] and (ii) the elements forming the corrosion products -chlorine, sulphur and oxygen -were detected at failed grain boundaries. [4] The aim of this article is to summarise the XPS study of copper surface enrichment in the Ag-2.2 mass% Cu and Ag-5.1 mass% Cu alloys that can indirectly prove the grain boundary segregation in this system.…”

“…Copperenriched regions become less noble than the grain volume and can be preferentially attacked by localised corrosion in the soil electrolyte containing the products of the decomposing body. [3,4] Due to the fact that copper segregation should enhance interfacial cohesion of silver, [5] it cannot be determined by direct measurement of interfacial composition by techniques of surface analysis [e.g. Auger or X-ray electron spectroscopy (XPS)].…”