"The late medieval French King Charles VI was one of the most notable sufferers of glass delusion. He was reported to have wrapped himself in blankets to prevent his buttocks from breaking.
Instances of the delusion cropped up in medical encyclopaedias from across Europe. There were references in fiction - most notably Cervantes' short story The Glass Graduate of 1613, in which the hero is poisoned by a quince intended as an aphrodisiac but which instead triggers a glass delusion.
Sufferers were seen to be normal in all ways bar the belief that they had turned to glass, and so could function, albeit anxious that other people shouldn't come too close and risk shattering fragile limbs.
But in the 1830s, cases disappear from the records.
It's easy to assume society and culture are so changed that mentally ill people would no longer manifest this particular delusion.
There are reasons why someone with mental illness in the Middle Ages - or indeed the 17th Century - might manifest glass delusion. That was a time when clear glass was a new material on the scene, seen as magical, alchemical even.
Why might glass delusion reappear at a time when glass is no longer new? What contemporary psychological resonance might it have?
Psychoanalyst Adam Phillips argues that the glass delusion has powerful contemporary resonance in a society in which anxieties about fragility, transparency and personal space are pertinent to many people's experience of, and anxieties about, living in the modern world.
The feeling of being made of glass could be a useful way of understanding how we negotiate society, a society that is increasingly crowded, in which modern technological advances isolate us and offer apparently boundary-less communication.
Novelist Ali Shaw, author of The Girl with Glass Feet, suggests that glass delusion might simply be at the extreme end of a scale of social anxiety which many of us experience to a lesser extent. The fear of tripping and breaking is really an exaggerated fear of social humiliation.
Prof Edward Shorter, a historian of psychiatry from the University of Toronto, suggests that it is the relative newness of clear glass as a material in 17th Century Europe which holds the key to understanding the disorder. Throughout history, Shorter argues, the inventive unconscious mind has pegged its delusions on to new materials and the technological advances of the age.
In the 19th Century cement delusions appeared at a time when cement emerged as a new building material, just as common delusions of recent decades include the false belief that the CIA or other security services can download thoughts through micro-transmitters, that people could "read your mind"."