ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 29 examples in Fig. 1 show that the voids can vary in size and appearance. They also give clues to an alloy’s overall ductility and the influences of second phases, e.g., slag stringers in Fig. 1c. Sometimes the main types of loading may be deduced: tensile equiaxed dimples in Figs. 1a and 1b, and elongated voids due to local shear in Fig. 1c. Microstructural embrittlement is intergranular and very brittle (Fig. 2). The ring in Fig. 2a was likely embrittled by heating during a cremation burial; the Kantharos (Fig. 2b) underwent longterm age-embrittlement during burial; and the pile-shoe (Fig. 2c), derived from bog iron ore, was embrittled by localized decarburization during the forging process. Based on chemical analyses, the embrittlement was most probably due to grain boundary segregation of lead (Pb) in the ring and Kantharos, and phosphorus (P) in the pile-shoe. Only intergranular SCC has been reported for gold alloys, both heritage (Fig. 3a) and modern alloys[1]. However, intergranular and transgranular SCC has been observed in heritage silver[2] and bronze[3] alloys (Figs. 3b, 3c, and 4). Transgranular SCC occurs predom- inantly along slip planes, also in annealing twins of subsequently worked objects (Fig. 4a). It is important to note that the scales of intergranular and transgranular SCC are different. Figure 4c illustrates that intergranular SCC results in relatively large cracks, and therefore constitutes the main threat to an artifact’s integrity. IMPLICATIONS FOR RESTORING ARTIFACTS The most important result from fractographic analysis of these heritage alloys is the recognition of whether gold or high-silver artifacts have been microstructurally embrittled or subjected to SCC during long-term burial. This is important because microstructural embrittlement affects the entire artifact, or the decarburized volume in the case of phosphorus-containing wrought iron; but SCC results in localized attack and cracking. Of course, the susceptibility to further damage would be similar Fig. 2 — Microstructural embrittlement: (a) Scandinavian (Danish) gold ring, circa 300 A.D. (b) Roman Kantharos, 100 B.C.-100 A.D. (c) Roman pile-shoe, 340-400 A.D. Arrows show almost separated grains. (a) (b) (c) Fig. 3 — Intergranular SCC: (a) Runic (Germanic) low-karat gold coin, 500-700 A.D. The red arrows point to incompletely separated grain boundaries; the white arrows indicate adhesive trapped in larger cracks. (b) Romanesque Kaptorga, 10th century A.D. (c) Baba Jilan (Iran) button, 8th century B.C. The arrows show cracks in thin corrosion films covering the grain facets. (a) (b) (c) Fig. 4 — Transgranular SCC: (a) Egyptian vase, 300-200 B.C. (b) Sangtarashan (Iran) vessel, 8th-7th centuries B.C. (c) Baba Jilan (Iran) button, 8th century B.C. The arrow points to a transgranular block within a relatively large intergranular crack; the circle covers intense SCC + corrosion on slip planes. (a) (b) (c) LEARN MORE ASM Handbook Volume 12, Fractography, is in the process of being updated and revised. Updated and new articles will begin to be released in ASM’s Digital Library later this year. GET ENGAGED, GET INVOLVED, GET CONNECTED The ASM Archaeometallurgy Committee is an active group of ASM members with interest and experience in the study and characterization of historic metals and artifacts. Members with similar interests are welcome to join. For more information, contact committee chair Patricia Silvana Carrizo or staff liaison Scott Henry, scott.henry@asminternational.org. Volume 12 includes the study of this 13th century Khan cup shown here after restoration, which involved piecing together 154 fragments. Original image source: Gerhard Stawinoga, Archaeological Landesmuseum, University of Kiel, Schloß Gottorf, Schleswig, Germany.
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