Mass‐Produced Mullite Crucibles in Medieval Europe: Manufacture and Material Properties

Abstract

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.