Fahrenheit 451 is apparently the temperature at which paper spontaneously catches fire and burns. It was used by Bradbury as the title of his dystopian novel about a society in which reading is illegal.
The concept began life in a series of short stories on the theme of book-burning, including ‘Bright Phoenix’ and ‘Bonfire’, and developed into a 1951 novella ‘The Fireman’, about a municipal employee paid to burn books, before finding final form in Fahrenheit 451.
But Bradbury said that another short story, ‘The Pedestrian’ (1950), was also an important staging-post on the way to Fahrenheit 451. It was based on a real incident. Bradbury and a friend were taking an after-dinner walk when they were stopped and questioned by police. Indignant, Bradbury wrote a story about a future in which policemen arrest pedestrians instead of protecting them; this finds obvious parallels in a story about a future in which firemen start fires instead of stopping them. An evening stroll thus led to a critique of McCarthyist America. Bradbury later said: ‘When the wind is right, a faint odour of kerosene is exhaled from Senator McCarthy.’
This still doesn’t quite explain the title, though. Perhaps Bradbury had been reading a precursor to the Handbook of Physical Testing of Paper By Jens Borch (2001). This states:
The ignition temperature of paper is about 450 degrees C, but it is somewhat dependent upon the paper quality. The ignition temperature is 450 degrees C for rayon fibers, 475 degrees C for cotton, and 550 degrees C for flame-resistant cotton (treated with N-methyl-dimethyl-phosphonopropionamide). From the data published the ignition temperature of paper treated with fire retardants seems to be about 100 degrees C higher than that of an untreated sample.
What seems to have happened is that Bradbury mixed up his Fahrenheit with his Celsius. 450 degrees C is correct for paper – only one off from 451 – but this is Celsius (or Centigrade), not Fahrenheit. The equivalent in Fahrenheit would be about 843 degrees. The famous formulation ‘Fahrenheit 451: The Temperature at which Book Paper Catches Fire, and Burns’, should perhaps be changed: I would suggest something such as: ‘Fahrenheit 843: The Approximate Temperature at which Rayon Fiber Untreated with N-methyl-dimethyl-phosphonopropionamide Catches Fire, and Burns’.
Bradbury, Ray: Match to Flame: The Fictional Paths to Fahrenheit 451 (2006)
Borch, Jens: Handbook of Physical Testing of Paper (2001)