What alchemist’s den would be complete without a crucible? The tough little vessels used for mixing all those odd ingredients, goat urine, cow’s blood, sweat, philosopher’s wool, saltpeter etc etc…
Now, the 500-year old mystery of how crucibles could survive all that chemical punishment and high temperatures has been revealed by archaeologists at University College London and Cardiff University.
Earlier research had demonstrated that the crucibles are found in archaeological sites across the world, including Scandinavia, Central Europe, Spain, Portugal, the UK, and even colonial America. Many researchers have tried to reproduce these vessels but have always failed.
Now, writing in Nature, the researchers reveal using petrographic, chemical and X-ray diffraction analysis that Hessian crucible makers made use of an advanced material only properly identified and named in the 20th century.
Marcos Martinón-Torres explains, “Our analysis of 50 Hessian and non-Hessian crucibles revealed that the secret component in their manufacture is an aluminium silicate known as mullite (Al6Si2O13). Today mullite (not to be confused with mullet) is used in a wide range of modern conventional and advanced ceramics, such as building materials, electronic packaging devices, optical materials and catalytic converters, as well as in ceramic matrix composites such as thermal protection systems and liners for aircraft and stationary gas turbine engines.
Mullite was only described in the 20th century although the makers of crucibles were exploiting its properties almost half a millennium ago. It was produced by firing a crucible made from kaolinitic clay to above 1100 degrees.
Mullite is extremely resistant to heat, chemical and mechanical stresses, making it perfect for the Alchemist’s den. It is thanks to the availability of Hessian crucibles that the discoveries of several chemical elements and their thermochemical behaviour took place.
‘Crucible makers were not aware of mullite, but they mastered a very successful recipe, and that’s why they kept it constant, and secret, for centuries.’