Use of mercury in hatmaking is thought to have been adopted by the Huguenots in 17th-century France,[1][16] at a time when the dangers of mercury exposure were already known. This process was initially kept a trade secret in France, where hatmaking rapidly became a hazardous occupation. At the end of the 17th century the Huguenots carried the secret to England, following the revocation of the Edict of Nantes.