Iron slag is formed as a byproduct of iron smelting. Here a large slag nodule has broken in half, revealing its interior texture. At the archaeological site of Ngre Kataa, large chunks of 'bubbly' slag like this were occasionally found in household and other contexts, away from areas otherwise associated with metal-working activities. Potters at the time of the site's occupation had begun to use crushed iron slag as a tempering agent in their potting clays, which may explain why large nodules were being carried and cached in areas away from metal-working locations. Ngre Kataa, June, 2008.