Since at least the 16th century, the wootz ultra high carbon white cast iron ingot was produced in India by melting or carburizing iron in a crucible. This ingot was forged into sword blades of so-called Damascus steel. The charged crucible was fired in a long (24 hour) single cycle at high temperature (1150-1250°C) in a strongly reducing atmosphere. Raw materials for the refractory vessel are clay and coked rice husks. At high temperatures, two phases reinforce the glassy matrix: cristobalite relics of rice husks and a network of mullite crystals. The microstructure and chemistry of the mullite network in the glassy matrix are characterized using a combination of techniques: optical microscopy, x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) with EDS, and high resolution electron microscopy (HREM).