In 1392, the King of France became convinced that his body was made of glass. He had iron rods sewn into the lining of his clothes so that he would not shatter if anyone bumped into him. He sat motionless in his chambers for days. He spent the next thirty years on the throne. The kingdom collapsed around him.

His name was Charles VI. Born in 1368, crowned at eleven, known to his subjects as the Beloved for the first decade of his reign. On the fifth of August, 1392, riding through the Forest of Le Mans with his royal retinue, a page boy dropped a lance and the sudden clang of metal spooked the king. Charles drew his sword and attacked his own knights. He killed five of them before being pulled from his horse and restrained. The episodes never stopped.

He went on to have more than forty documented attacks over the next thirty years. He forgot his own name. He forgot that he had a wife. He refused to bathe for months at a time. During one of those episodes, Pope Pius II later recorded in his Commentaries, the king became convinced his body was made of glass. His tailors sewed iron rods into the lining of his clothes. He refused to be touched. The condition has a name. The Glass Delusion. After Charles VI, it spread. Across the next three hundred years, European aristocrats reported the same fear. They believed they had glass hearts, glass feet, glass heads. Princess Alexandra of Bavaria believed she had swallowed a glass piano. The Glass Delusion was treated by physicians as a recognised medical condition through the seventeenth century.

Then it vanished. The literary historian Gill Speak, whose 1990 study remains the authoritative account, traced the disappearance to the rise of cheap industrial glass. Glass stopped being a rare luxury material associated with the wealth and fragility of the upper classes. It became ordinary window panes and drinking tumblers in working-class homes. The metaphor died. In the nineteenth century, a new delusion began appearing in European asylums in its place. Patients reported their bodies were made of concrete. The new fragile material of the Industrial Revolution. Charles VI died in 1422, still believing on his bad days that he was made of glass. His son was disinherited in favour of the King of England. Joan of Arc was seven years away from saving him.

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