Redox reaction at metal-matrix interface. (a) BSE image showing close-up of Location f.8 (Fig. 2b). Metallic Fe beads occur mostly along interface between Al-Cu-Fe metal and glass. Spinel crust surrounds the Al-Cu-Fe metal grains. The Fe beads and spinel crust are the products of a redox reaction between Al-Cu-Fe metal and silicate melt. (b) Schematic illustration showing a proposed sequence of how the interaction unfolded between Al-Cu-Fe metal and silicate melt. The Start panel represents the initial contact between metal and silicate melt. In Step 1, the metal is being partially assimilated into the melt, which leads to an enrichment of Al, Cu, and Fe in the melt. The metal-matrix interface retreats into the metal. In Step 2, the interface continues to retreat, while the redox reaction occurs in the reaction zone, producing oxidized Al and metallic Fe and Si. In Step 3, the interface stops retreating and the redox reaction stops, as various phases begin to crystallize, including a layer of Al-oxide (“Al-O”), spinel crystals, and Fe and Fe-Si beads. In Step 4, the metal grains are still being heated. Some, like the one represented here, begin to melt, beginning at the metal-matrix interface. In Step 5, the system cools, and the newly melted metal regions re-crystallize. This final panel reflects what is observed in (a).