James Hillman, "Therapeutic Value of Alchemical Language"
Dragonflies: Studies in Imaginal Psychology 1/1 (1978): 33-42.
I've been thinking about alchemical vessels made of glass (clear), such as they are in many of the most elaborately illustrated books, such as the Splendor Solis or the Mutus Liber. Glass give one the ability to monitor the processes as well as control the "heat" being applied, as glass isn't as good a conductor of heat as metal. Glass is also quite moldable into the various and sometimes complicated shapes needed for some processes, such as distillation. But maybe more importantly, glass is fragile and it's fragility warns the alchemist/practitioner that the processes are dangerous, capable of real injury, and care need to be taken at all times.