A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 1 2 6 Cupellation was a multistage pro- cess employing three separate hearths. Figure 5 is a schematic of a first stage hearth for enriching smelted lead bul- lion. This was remelted to a high temperature using wood fuel. Bellows- powered tuyères oxidized the lead to litharge (PbO), which melts at 880°C, hence the need for a high temperature. The litharge drained via a surface groove and was discarded. More bullion was added until sufficient silver-enriched lead was obtained for the second stage. Then the enriched lead was transferred to a second hearth and again oxidized, but here the litharge was removed by dipping iron rods into it (before 1000 B.C., wooden poles) to form layered litharge cones on the rods. These rods were repeatedly removed, the litharge cones discarded, and the rods re-dipped. Eventually this second stage left a silver globule on the hearth. In the third stage, a number of globules were melted and further refined in another hearth to obtain ingots, the remaining PbO being absorbed by pores in the cupel wall. Cupellation is very effective in producing silver above 95% purity. It usually contains minor-to-trace amounts of copper, gold, bismuth, and lead (generally below 1 wt% for each), and traces of antimony, arsenic, tellurium, zinc, and nickel. Several studies have shown that copper contents above 0.5‒1 wt% indicate deliberate additions, most probably to increase the strength and wear resistance in high-silver alloys, and also in larger amounts to make lower-quality artifacts and coins. Copper additions appear to have been done since about 3000 B.C.[14]. The artifacts themselves were commonly made from ingots by cold working with intermittent annealing, although cast silver objects were also produced. Many artifacts were high-quality thin-walled vessels with exquisite craftsmanship. POST-PROCESSING PROBLEMS: CORROSION AND EMBRITTLEMENT Unfortunately, many ancient bronze and silver artifacts have suffered corrosion and embrittlement damage owing to millennia of burial before recovery. An example from the famous high-silver Gundestrup Cauldron, dated to the 1st or 2nd century B.C., is given in Fig. 6. There are numerous publications on the burial damage, and they usually concentrate on conservation and restoration techniques but not on details of the damage. Basically, both ancient bronzes and silver may undergo both general corrosion and stress corrosion cracking (SCC), which is promoted by retained cold work and also external forces on thin-walled hollow artifacts (e.g., vessels and cups) during burial. The SCC damage is both intergranular and transgranular (along slip planes). Also, some silver artifacts show evidence of intergranular microstructural embrittlement, most probably due Fig. 3 — Metallographs of two binary Cu-As alloy artifacts from Iran. (a) An EBA as-cast axe head, 2.17 wt% As. (b) An LBA worked and annealed bowl, 2.10 wt% As. (a) (b) Fig. 4 — Metallographs of two binary Cu-Sn alloy artifacts from Iran. (a) An EBA worked and annealed vessel, 8.67 wt% Sn. (b) An Iron Age I as-cast tool, 10.83 wt% Sn. Note that (a) shows some retained cold-work, and (b) shows interdendritic (α + δ) eutectoid, shrinkage porosity and coring. (a) (b) Fig. 5 — Schematic of Stage I cupellation about 500 B.C., Laurion, Greece. Adapted from Conophagos[13].
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