King Charles VI of France was prone to bouts of insanity where, among other things, he didn’t recognize his family or thought he was made of glass. Modern doctors think he may have suffered from schizophrenia or bipolar disorder.
Charles VI was king of France from 1380-1422. He was known as Charles the Beloved at the beginning of his reign but soon became known as Charles the Mad. His first fit of insanity was in 1392. The king had come down with a fever while in the forest with his army, and when he was startled by the clang of a dropped lance he drew his sword, swinging at his knights in a fit of paranoia. Charles killed four knights and almost killed his brother before he was subdued.
King Charles’ bouts of insanity then became frequent and could last for months at a time. He would sometimes be unable to recognize his wife or his children, or forget that he was king altogether. He would often run and scream through the halls of his residences; other times he would sit perfectly still for hours. When questioned about the latter occasions, Charles claimed that he was made of glass and could not move for fear that he would shatter.
This condition became known as the “glass delusion.” This delusion started with Charles VI and became more and more common until the 1600s when it became part of the cultural climate. It was written about in books on illness as well as in plays, poems, stories and more. The disorder was written about by Miguel de Cervantes and Rene Descartes, among others. The delusion then died out after the 1600s.