An alkali-alumina-silica slag from the interior of a kiln used for the firing of refractories was found to contain recrystallized mullite, corundum, and beta-alumina. Isolated areas of the slag were also rich in iron oxides; in these areas beta-alumina crystallized from the melt as hexagonal plates, and at a later stage small hexagonal plates of bematite grew on the 0001 face of the beta-alumina plates in a regular fashion.