“There are many forms of vessel described and depicted in the alchemical literature and emblematic engraving; a seeming multiplicity of forms of retort, pelicans, water baths, alembics, cucurbites, stills, etc.
However, in the interior work we will find that all these different outer manifestations reduce to three archetypal forms - which we can call the CRUCIBLE, the RETORT and the STILL.”
Adapted from http://www.levity.com/alchemy/vessel.html
The Crucible is essentially an open vessel, dish, mortar or cauldron, open to the outside world but capable of containing material…allowing impurities or unwanted facets to dissipate, as well as substances and forces to enter.