Crucible Secrets

As early practitioners of the pyrotechnical arts prepared stronger acids and cranked their furnaces to higher temperatures, they were pushing the envelope for their labware. "They were constantly bumping against the limits of their materials," Johns Hopkins University historian of science Lawrence M. Principe said at last month's International Conference on the History of Alchemy & Chymistry in Philadelphia.

That's why a particularly tough type of triangular crucible mass manufactured in the Hesse region of what is now Germany was so coveted by alchemists, early chemists, metal assayers, glassworkers, mint officials, and metallurgists throughout Europe and even in the Americas. Working in late medieval and early modern times, only the makers of these crucibles knew what made their labware so superior. To uncover what the 17th-century English chemist Robert Plot once described as "the mystery of the Hessian wares," archaeologists have now applied modern analytic techniques to samples obtained from early laboratories.

Manufactured on potters' wheels, the Hessian triangular crucibles were made by pushing in on the upper walls of beaker-shaped vessels, creating three easy-pour spouts. The heat and fracture resistance, toughness, and overall quality of these orange crucibles distinguished them from the second-rate crucibles also available to early laboratory workers, archaeologist Marcos Martinón-Torres of University College London said at the Philadelphia meeting.

To determine what made the Hessian crucibles so attractive to early lab workers, he and colleague Thilo Rehren have been examining crucible samples with optical and electron microscopes and chemical techniques, including X-ray diffraction, X-ray fluorescence, and electron microprobe analysis. The scientists found that the Hessian crucibles were invariably made with high-alumina clays, heavily infiltrated with fine grains of quartz sand. They concluded that the crucibles were fired at temperatures exceeding 1,200 °C, as evidenced by the glassy character of the clay matrix. Most notable, Martinón-Torres observed, is the presence in the matrix of the mineral mullite (3Al₂O₃·2SiO₂), which helped give the Hessian crucibles tensile strength, chemical and thermal refractoriness, and imperviousness. "Mullite is the key to the mystery of Hessian crucibles," Martinón-Torres proclaimed.