ADVANCED MATERIALS & PROCESSES | JULY 2026 15 In the late Roman and early Byzantine Empires, ornately decorated silver dishes and plates, usually round and some partially gilded, were made as ceremonial and prestige artifacts. These typically had silver contents of about 95 wt% or more, with copper as the main alloying metal. Many were hidden in burial hoards for safety during difficult times in the 5th-6th centuries AD. When accidentally found and excavated, usually more than a thousand years later, some were damaged by dents and buckling. However, others were deliberately damaged before burial or as a consequence of long-term burial. Figure 1 provides examples from a variety of locations. The missorium was originally 29 cm in diameter and was deliberately folded and broken into three large pieces, presumably to facilitate hiding it. The paten would have been about 25 cm in diameter before major loss of the outer ring. Other dishes and plates varied in size up to as much as 71 cm, like the largest from the Kaiseraugst Treasure. These examples were chosen to show annular cracks and losses of material from the cracks. The most probable cause is stress corrosion cracking (SCC) during many centuries in the burial environment[1-4]. Specifi- cally, the use of a chasing tool (punch) to obtain annular decorating grooves on the front surfaces of thin-walled objects (such as the dishes and plates) can result in significant residual tensile stresses at and near rear surface locations, and hence driving forces for SCC that eventually could penetrate through the thickness[1,2]. Other decorative features produced by engraving and repoussé work (the rear surface opposite of chasing) appear to have been less harmful or innocuous. This may be because the front surface annular grooves were finishing touches that were not followed by an annealing heat-treatment[2]. Ancient silversmiths would have known the need for intermediate annealing for further cold working, but not the concept of final annealing for stress relief. See the study of a high- status ancient silver object called the Enkomi cup[5]. DETAILS OF DAMAGE TO THE MISSORIUM The missorium is part of a hoard found on the Caelian Hill in Rome in the 18th century AD (exact date unknown). It is kept in the Vatican Museums, Department of Christian Antiquities. Figure 1 exhibits clear evidence of damage to the missorium; it is separated into three pieces and is missing an annular strip from the middle piece. However, there is other damage, better visible at higher magnifications. This damage is labeled and described in Fig. 2 and its caption. More details are provided in Figs. 3-5: • Figure 3: One of several partial and hardly visible repairs of cracks using a silver-painted adhesive. This figure also shows the locally undecorated plate thickness to be about 1 mm along the deliberate left-hand breakage. • Figure 4: The micro-facets visible along part of the lower crack B suggest a brittle fracture mode, most probably intergranular SCC. • Figure 5: The intersecting pattern of cracks, at point C, also suggests a brittle fracture mode, again probably SCC. Note that the fragments are too large to represent individual grains. This region was skillfully repaired using a silver-painted filler material. Fig. 1 — Examples of damaged silver dishes and plates: (1) Caelian Hill missorium, 5th century AD, with arrows indicating narrow, open, and repaired annular cracks; (2) Casena plate and (3) Byzantine paten, 4th and 7th century AD, with large annular cracks and material losses; (4) Several Kaiseraugst Treasure dishes and plates, 4th century AD, the three larger ones with evident annular cracks. Note: The Casena plate and the partly shown dish from the Kaiseraugst Treasure included areas of silver gilt. Courtesy of A. Alemi (1); J.P. Northover (3); Wikipedia/Wikimedia Commons (2,4).
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