Mass-Produced Mullite Crucibles in Medieval Europe: Manufacture and Material Properties
Crucibles from the German region of Hesse have been famous since the Middle Ages due to their exceptional quality, regarded by many as a mystery. We analyzed 50 Hessian and non-Hessian archeological crucibles using SEM-EDS, FESEM, and XRD to investigate their technology and material properties. It was revealed that Hessian crucibles were systematically made of kaolinitic clay with a low flux content, mixed with quartz sand, and fired to temperatures in excess of 1300°C. Primary mullite developed in most of the glass matrix, with secondary mullite in some regions of clay–feldspar relict mixtures. Consequently, the vessels showed superior creep and thermal shock resistance, high-temperature strength, and thermal and chemical refractoriness. These crucibles represent the earliest industrial exploitation of mullite in Europe, which explains their historical success.
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