Ray Bradbury’s famous dystopian novel, Fahrenheit 451, takes its title from the alleged temperature at which paper burns.
Written in 1953, the book is set in a futuristic America where books are banned and are actually set on fire by ‘firemen’ in order to censure citizens, destroy culture and suppress knowledge.
The novel was a huge success for Bradbury, and it was praised for its interrogation of oppressive regimes, and highlighting the importance of literature and art. Bradbury was said to have drawn inspiration from the McCarthy era of American history, and from Nazi Germany when creating his dystopian world.
But how accurate is the science in the book, and does paper really burn at 451 degrees Fahrenheit? Read on for a closer look at where this figure came from, and what the many variables are that cause this number to change.
What Do We Mean By ‘Burn’?
Before we can answer the question: at what temperature does paper burn, we first need to establish exactly what is meant by the word ‘burn’ and exactly what is meant by the word ‘paper’.
Both of these terms have multiple interpretations, and scientists have to be very specific when drawing their conclusions (unlike novelists).
There are in fact three key interpretations of the question ‘at what temperature does paper burn? And these are as follows:
- At what temperature does paper catch alight when exposed to a flame / fire?
- At what temperature does paper spontaneously combust on its own?
- What temperature (how much heat) does paper produce whilst it is burning?
These are three quite distinctly different questions, and in this article we will try to answer them each individually.
What Do We Mean By ‘Paper’?
Paper is generally acknowledged to be a thin sheet of material made from the compressed fibers of grasses, trees, vegetables or rags. These fibers are mixed with water to create ‘pulp’, flattened and then dried.
However, these days there are many, many different types of paper with vastly differing qualities of thickness, texture, and color.
Writing paper can be glossy or matte, thin or thick, and then there is tissue paper, toilet paper, tracing paper, sugar paper, cardboard, card, crate paper and many more. Each type of paper will burn at a minutely different temperature depending on its individual properties.
What is more, by ‘paper’ do we mean a whole book or a single sheet? And if we mean a book, do we mean something as thick as the Holy Bible, or as thin as a tourist leaflet?
In general, thicker books will require a higher temperature to catch fire and burn because they have a greater density. This is clear when you consider how easily a thin cigarette paper catches alight compared to how long it takes to make a block of wood catch fire.
Technically they are the same material, yet their auto ignition temperature is very different.
As paper is an insulator, it draws heat into its own fibers. A thick book will take longer to catch fire because it will draw heat into the center of the book where that heat is then starved of oxygen and extinguished.
Only when the outer pages (that are exposed to oxygen in the atmosphere) catch fire will the rest of the pages take flame. This will happen more quickly with a single sheet or thin booklet than it will with a massive tome.
What Other Variables?
Let us not forget that the atmospheric and environmental conditions will affect the temperature at which paper will burn as well.
Moisture level and air pressure play a part in auto ignition and ignition temperatures, however, they would have to be pretty extreme to create much of a difference to the flashpoint of paper.
If you were up incredibly high, like on top of mount Everest, or down incredibly deep, like the bottom of the ocean, then the atmospheric changes would cause the combustion temperature of paper to be different.
Within the controlled environment of a science laboratory, conditions can be kept steady and constant in a way that they can’t in real life.
At What Temperature Does Paper Ignite?
The temperature at which paper will ‘ignite’ refers to the temperature at which it will set fire when exposed to a spark or flame.
As we all know from having thrown newspaper and other types of paper on a campfire, paper lights up very easily when exposed to a flame. This is because flames are very hot indeed, even little flames like cigarette lighters and candles.
They do not always seem like they are particularly hot, but this is because the heat of a flame is very localized. This means that you can place your hands on either side of a flame, just inches away, and not be burned.
The hottest part of a flame is not actually the bright orangey, yellow center but the blue aura that hovers just above the flame.
This blue aura can sometimes be seen by the naked eye and sometimes not, but it is always there and it is known as the point of ‘absolute combustion’. This is the point at which all of the material is being burned and therefore the maximum amount of energy (or heat) is being produced.
- A candle burns at approximately 1670 degrees Fahrenheit
- An alcohol or ethanol flame burns at approximately 3448 degrees Fahrenheit
- A butane cigarette lighter burns at approximately 4074 degrees Fahrenheit
As you can see from these flame temperatures, flames are far hotter than 451 degrees Fahrenheit, which is the famous flashpoint temperature at which paper is said to set fire by itself. It is for this reason that paper is so easy to ignite with a naked flame or spark.
At What Temperature Does Paper Auto-Ignite?
Auto-ignition or combustion relates to the temperature at which a material will set on fire all by itself. This is the temperature to which Bradbury was referring in his book title Fahrenheit 451.
It is essentially the temperature a room or compartment would have to reach for a book or piece of paper to get so hot that it generated enough energy to produce a flame. This figure is also known as a ‘flashpoint’ or ‘combustion temperature’.
At the time of Bradbury’s writing (in 1953), 451 degrees Fahrenheit was the scientifically acknowledged flashpoint of paper. So - Bradbury was right… sort of.
These days scientists suggest that paper has a flashpoint of 481 degrees Fahrenheit. This may seem like a small difference but in scientific terms it is significant.
However, Bradbury’s book has been so successful and become so culturally influential that it has actually engrained the figure 451 in people’s minds, and most people would still cite 451 degrees Fahrenheit as the combustion temperature of paper.
We don’t suggest you do this, but if you were to turn your oven up to 481 degrees Fahrenheit (that is 249 degrees celsius) and then toss a book inside and shut the door, it would not immediately burst into flames like some kind of fire bomb.
This is because of the insulating quality of paper that we mentioned earlier. Whilst the outer papes would feel the heat first, they would not heat up immediately as that heat would be absorbed into the center pages where it would be smothered without access to oxygen.
Eventually however, the pages would become so hot that the heat would be absorbed back out to the external pages and these pages would then catch flame due to the oxygen surrounding them.
Once these pages catch flame, the temperature would increase dramatically and cause the inner pages to ignite because (as we saw earlier) flames are very hot indeed.
Even if you switched the oven off, the book itself, once set on fire, would continue to burn due to the heat produced by its own flames. It would burn until those flames cooled and died out, or until the book was reduced to ash.
How Hot Is Paper When it Burns?
This brings us on to the third and final interpretation of that original question: at what temperature does paper burn? The final interpretation considers how much heat paper itself gives off as it burns, and this is in fact far more than 451 degrees Fahrenheit.
Flames are incredibly hot because they produce so much energy. Although a book might auto-ignite at 481 degrees Fahrenheit, it would quickly produce over double that amount of energy and heat when it converts to flames.
In fact, it is estimated that the hottest part of a paper fire would be around 1600 degrees Fahrenheit!
What Is The Flashpoint Of Cardboard?
Corrugated board (otherwise known as cardboard) is made from the same source materials as paper but is much thicker and stronger.
Though it is made from paper pulp, wood chips and timber shavings, it is heavier than paper, harder and more densely structured. It is therefore a better heat insulator than paper, meaning that it can funnel heat away and store it inside itself very successfully.
This is why many people use cardboard to line their animal boxes or even their homes to protect against the cold.
Due to its great heat absorbing properties, cardboard has a much higher flashpoint than paper. In fact, the auto-ignition temperature for cardboard is 800 degrees Fahrenheit - a significant increase from 481 degrees Fahrenheit!
This means that you would have to whack your oven up to Fahrenheit 800 before your cardboard would combust. This is because it is so effective at funnelling heat away from its outer layers where there is oxygen, and into its central layers where there is not.
Of course, we have already established that the average naked flame is far hotter than 800 degrees Fahrenheit, which is why cardboard still catches on fire quickly when thrown on the campfire or bonfire.
In the same way, once cardboard has combust, the energy produced by its own flames are far greater than 800 degrees Fahrenheit and this is why cardboard will continue to burn itself to ash.
Can Paper Melt?
Because paper ignites at 451 degrees Fahrenheit, it does not technically ‘melt’. Melting is the process of changing from solid into liquid, usually when heat is applied, however this does not occur in paper.
Instead, paper decomposes over time as it composts, and some of the chemicals in its make up will become liquified as they mulch.
Paper dissolves when a mixture of acid and heat are applied. It can take approximately 20 minutes for liquid acid and heat to dissolve the fibers in paper.
Paper is a fascinating and highly useful material. It has a flashpoint of 481 degrees Fahrenheit, it will combust and burn when exposed to temperatures of this heat or higher.
Because naked flames are actually much hotter than this, paper will set on fire very easily when it comes into contact with sparks or flames.
In the same way, when paper begins to burn and produce flames of its own, the energy it releases is far greater than 481 degrees Fahrenheit. In fact, a paper fire will reach a temperature of around 1600 degrees Fahrenheit - that’s hot!
Because cardboard is thicker and more dense than paper, it is a better heat insulator and has a higher flashpoint of 800 degrees Fahrenheit. Neither cardboard nor paper will melt when exposed to heat, however they can be dissolved if acid and heat are applied.
With all this being said, it seems like Ray Bradbury was not far wrong when he named his novel about burning books Fahrenheit 451.
Although the figure has changed slightly thanks to the accuracy of modern science, the principle that Bradbury used to form the basis of his novel remains - books will technically catch fire if they experience temperatures of 481 degrees Fahrenheit or over.