"something on his head that shone as if it were made of gold"

A barber's bleeding basin
Permission Granted by Copyright Owner for Use on Book DrumA barber's bleeding basin - Credit: The Royal College of Physicians and Surgeons of Glasgow
   The golden helmet of Mambrino was an enchanted item which could bestow immortality on its wearer, coveted by various parties in several chivalric romances.  Quixote probably came across it in Ludovico Ariosto's epic Orlando Furioso (1516), one of his favourites, and is dismayed to see that it appears to have been melted down and reformed into the shape of a barber's basin.  'Barbers' at this time not only cut hair but were employed to perform minor surgery such as bloodletting, for which basins of this type were commonly used. 

   Quixote's delusions are here well defended against reason, as he does not simply argue that fantasy is reality, but instead insists that a fantastical reality has been obscured by a convincing illusion of the mundane, which his greater knowledge allows him to dismiss as such.  The existential difficulty this presents is shown by a later scene (Chapter XVIII) in which a group of nobles amuse themselves by holding a vote to determine whether Quixote or the bewildered barber he stole it from are seeing the basin for what it really is.

Online edition of 'Orlando Furioso' as translated by William Stewart Rose (Serialized 1823-1831)