Intergranular Failure of Roman Silver Artefacts

Abstract

Silver objects obtained from archaeological excavations often exhibit an extended embrittlement. The causes of this embrittlement were studied on silver artefacts buried for more than one thousand years in the soil using light and electron microscopy techniques and EDX microanalysis. Our investigation revealed presence of chlorine, sulphur and oxygen along the grain boundaries suggesting that most likely, the catastrophic failure of the grave objects made of Ag–Cu alloys is caused by an intercrystalline corrosion attack. The role of possible grain boundary segregation and/or precipitation of copper as a path of easy corrosion is discussed.

Case Histories of Ancient Silver Embrittlement

“…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][9][10][11].…”

“…Since 1994, the NLR has investigated four cases of ancient silver embrittlement [1][2][3][4][5][6][7], one as a re-assessment [7], and reviewed two cases investigated by others [8][9][10][11]. Experience and insight gained from the investigations enables presenting the most likely explanations of embrittlement for all six cases.…”

Copper surface enrichment of AgCu alloys

“…However, some ancient objects produced from Ag-Cu alloys exhibit surprisingly extended intergranular failure [1,2] if excavated from graves in which they were buried in contact with dead bodies. [3] It is very probable that this failure is a consequence of an extensive enrichment of the grain boundary regions in Ag-Cu alloys by copper during thermal treatment of the objects during manufacturing. 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.…”

“…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)].…”

Analysis of grain boundaries in an embrittled ancient silver necklace

“…coins from India, [3] Khan's cup [4] and roman kaptorga (a container for relicts or amulets). [5,6] This work describes the study of the embrittlement of a silver necklace which was buried for more than 1000 years in soil.…”