Does fire cause things to catch fire because it's an open flame or because it's hot?

If I had an open flame and piece of metal of the same temperature, would they catch things on fire just as easily?

For example, if I light a match and put it near paper, the paper will catch fire. But is that because of some property of it being ‘fire’ or just that it’s raising the temperature of the paper to a certain point that it burns? If I had a piece of metal the same temperature as the flame on the match, would the paper catch on fire from the hot metal just as easily?

If you heat a metal, let’s say magnesium because it catches alight at a relatively low temperature, it will “spontaneously” start to burn.

So I am pretty sure it’s the heat of the match that causes paper to burn, rather than the open flame.

Also… flames are not that hot: you can move your finger through a candle flame, for example.

The really hot bit is at the bottom, often invisible due to the visible flames surrounding it. The flame (while still hot) is substantially cooler as it is actively losing energy in both heat and light forms.

Also, you can set alight (flaming) a slight (i.e., low-mass) flammable solid material (such as a piece of paper) with a non-flaming heat source, like an car’s electric cigarette lighter (assuming you can find one) or orange-hot embers.

No open flames from the heat source, but flames plenty when the new fuel starts blazing.

You need 3 things to get fire: fuel (something flammable), oxygen, and heat to reach the “kindling point” temperature of the fuel.

An open flame usually facilitates the last 2, but it’s not hard to get enough oxygen to start a fire, so the kindling point is usually the important bit.

Certainly things will burn if they get hot even if there is no flame. But does a flame burn things easier than a hot object at that same temperature?

For example, a match will cause paper to have a flame almost immediately. But I can put a piece of paper on hot coals and the paper will smoulder and smoke but not necessarily burst into flame. Is it that the match is hotter than the coals? Or is it that the open flame of the match can cause the paper to catch fire easier than it can on hot coals?

The flame-ness is not the crucial difference.

What matters for heat transfer is temperature and size. The really, really hot invisible part of the gaseous flame is much hotter than mere embers. So it triggers more temperature rise in the paper more quickly than the embers do.

From the POV of a piece of paper of fixed size, touching it with a large piece of 500F metal will heat it faster than touching it with a small piece of the same metal at 500F. More size -> more heat transfer per unit time -> temperature rises to ignition point faster.

For radiant heat transfer, the size that matters is angular size. IOW how big does the flame look from the POV of the paper? A flame is bigger than a piece of solid material the same size as the match at the same temperature as the flame.

Last of all, there’s a thing called Pyrolysis - Wikipedia. Paper doesn’t burn. Surprise! What really happens is the paper begins to convert to a gas, to sort-of boil. Then the resulting gasses burn. That’s pyrolysis.

There *might *be a way in which a chemical reaction between the gasses in the flame interact with the chemicals in the paper to speed that up vice just simple heat transfer. That’s beyond my nil chemistry skills though.

That is new to me; thank you.

Pyrolysis is the other half of a common industrial activity: charring.

To make charcoal, you burn wood in a way that encourages pyrolysis but leaves behind non-burned but still burnable residue: charcoal.

Pyrolizing wood forces such chemicals as wood alcohol out (and, in rick-based charring, burns it to sustain the heat). What’s left after pyrolysis is almost pure carbon, because that vaporizes at a far higher temperature.

So thank pyrolysis the next time you eat barbecue. (I mean real barbecue, not gas grilling outside.)

You get charcoal out of pyrolizing paper, too. It’s the flimsy black char that glows at the edges as it burns into white ash. (I see this all the time because I start my backyard barbecue charcoals in a starter chimney, with a starting charge of crumpled paper.)

All things have a flash point. Whatever means for it to reach that temperature will cause it to combust.

That’s true, but it’s not the whole story. Flames are hot, but they’re also full of reactive compounds that will help start combustion. The gas is hot enough that some of it ionizes and becomes a plasma, plus there are plenty of radicals floating around that are super reactive. That’s why some of the most common forms of flame retardants are halogenated organic compounds. When they get hot, the halogens form radicals pretty easily, which will quench the more reactive radicals in the flame and slow it down (they also tend to have health concerns, which isn’t great when they’re used in lots of upholstery and stuff).

Yes. Thank you.

Pyrolysis is also responsible for the fireman’s nightmare: Flashover - Wikipedia

There are some amazing vids on Youtube *et al *in addition to the specific ones referenced in that wiki.

I understand the OP’s question because fire seems like a thing that can spread to another object it comes into contact with. Like touching a greasy rag to a clean car leaves a streak of grease on it, making it also greasy. That’s how much of the physical world works and it’s a natural way for a person to interpret what they’re seeing.

But that’s not what’s happening. Fire isn’t a “thing” that’s on top of one object and spreading to another object that’s it’s in contact with. Fire is being produced by one thing via combustion, and it won’t be produced by a second thing until it is also experiencing the right conditions to also combust.

Just think of all the times you’ve tried to light something on fire from a match that won’t light up at first, like firewood. Nothing happens at first, then it starts to blacken and smoke, then finally it combusts if you’re lucky. The match’s heat has conducted to the wood sufficiently to cause it to also combust. You don’t see fire immediately “rubbing off” into the wood.

That is not what the flash point is. The flash point of petrol is -42C. At its flash point all it will do is evaporate. To make it burn, you need a means of ignition (like a spark in a car engine).

Chem102 - Any real chemists, correct my 40-year-old memory. “Fire” or any similar chemical reaction - oxidation - is oxygen combining with the fuel, the flammable atoms. This could be carbon, hydrogen, magnesium, etc. When an oxygen atom hits, let’s say, a carbon atom hard enough, the two react to combine and give off heat- they are electrically attracted, and combine to fall to a lower energy state and give off their “free range” energy. A hard enough reaction - collision of molecules - strips the oxygen from it’s partner (it’s O2, after all) and ditto for whatever the carbon is bound to; different materials take more or less energy to disassemble.

And heat is basically atoms in motion, so the hotter something is, the more likely reactive collisions occur. After all, not all the molecules move at exactly the same speed in a certain temperature; it’s a statistical distribution, some faster, some slower. the faster ones cause reactions. The hotter the environment, the more molecules going fast enough to react. Now, most oxidizing reactions give off heat - the oxygen and carbon combining releases heat. If things are hot enough, and there’s enough reactants present, most of the fuel material will combine with oxygen causing a release of heat good enough to ignite the neighbouring part of the fuel material, and so on.

Fire is simply a self-sustaining chain reaction - the heat being given off by something burning is enough to ignite what’s nearby until either fuel or oxygen gives out or something (doused with water?) takes away the heat. (water also removes access to oxygen in the air; water is very stable, an open flame does not usually strip the oxygen from the hydrogen because - it’s already burned hydrogen, gave off a lot of heat when it burned, takes a lot of energy to pull it apart.)

So basically - heat material enough, it starts to react - heat paper, it chars. Heat it beyond a certain point, and it bursts into flame - enough material burns - the charcoal and the gases - to create a self-sustaining reaction.

But consider the warning - why don’t throw oily rags in a box in the garage? Oil will oxidize. At room temperate, hardly at all. An open pot of oil, the surface molecules will occasionally bond, just as other materials do. (After all, rust is just iron oxidizing in slow motion). Get a rag oily, now the surface area of the oil is immensely larger than a bucket’s surface. More oxidizing, each reaction giving off a bit of heat. Crumple these rags in a confined space, and this heat does not easily escape - instead, it raises the general temperature of the confined space.

Oops… the rags are getting warmer - what does that mean? Even more oxidation follows, the air and oil in the confined space is getting hotter, molecules move faster, more strong collisions and even more oxidation, hotter still. If the container is too well insulated, the temperature can raise enough to cause a fire, a self-sustaining reaction. (I bet there’s a youtube safety video of this process somewhere)

So the flame is just a manifestation of the high temperature. As others point out, it’s the gasses evaporating then burning. If you ever watch a strip of magnesium burn (do they allow that sort of stuff in the modern helicopter parent world?) it’s not so much a dancing flame shooting inches or feet up, as it is the surface of the metal giving off luminescent glowing gas (magnesium oxide) which stops glowing once it gets away from the burning metal surface.

Similarly, you can start a material charring or make it burst into flames by the appropriate application of concentrated sunlight, if you have lens.

So short answer - all it takes is enough heat to dissociate the fuel molecules in whatever form they take.

Also may happen with the occasional mummified Egyptian god-king.

Have you ever tried to start a fire with a magnifying glass and paper?
It isn’t always as easy as you might think. You have enough heat, and fuel, and oxygen, too. You can see the paper charring, and smoke, but no flame. Why?

There are few free radicals, which are important to self-sustaining flame in the gases driven out by heat from the paper. It takes careful blowing to direct those gases back to the glowing carbonised edge of the focal area to get them to ignite, and the flame front takes hold.

Matches are easier to use to light paper/tinder because they have a flame full of free radicals to assist the process. The free radicals are not necessary for fire, but they do catalyse the formation of a self-sustaining flame, allowing it to happen faster and easier.

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Those flashover videos are horrifying, but explain to me again in light [heh] of this thread: furniture gets hot, leads to pyrolysis of furniture, leads to pyrolysis of the gaseous, secondary products (i.e., “the couch is on fire and no it’s not smoldering”).

Heat rises throughout the room locally by the usual three manners (I don’t know which is the “most effective” to spread the conflagration), one of which is convective, primarily by the increasing smoke running along the ceiling.

We now have three relatively distinct vapor types in play: the coolest original air of the room, the hottest immediately produced vapor-pyrolysized-flame (is that a verb?), and the in-between “black smoke” vapor/mist of physical and gaseous secondary products.

Heat rises to flashover: is it
a) when that previous (blackish) smoke–the vapor that is not being consumed (yet) into visible flame–is pyrolysized or
b) when the whole damn gaseous environment (the remaining original air and the products of the initial pyrolysis) lights up?

Good post, but I would have swapped the two final paragraphs to clinch your point, and in that aspect I might continue it by noting that the pyrolysis of materials to produce “a flame” (luminescence) does not require an obvious oxygen environment–H2O being a good example, used as a source in underwater flares. Nor O2 (or H), for that matter: calcium phosphide and H2O produces phosphine, which is pyrolysized for underwater flares. They tell me. (:))

Let me amend that: …the hottest immediately produced vapor just before it becomes pyrolysized into flame…
And, md2000, you do of course note (it’s your point) that it’s the magnesium that is luminescent, and flame doesn’t need O2.

Leo: It appears you’re using “pyrolysis” as meaning “burst into flame”. That’s wrong.

Pyrolysis is the process of converting a solid or liquid that has potentially flammable chemical constituents into a gas that is directly combustible. IOW, using heat (the pyro) part as the energy source to spilt (the lysis part) the constituents into flammable gases.

Flashover occurs when the total quantity of flammable gas (fuel gas) created by pyrolysis increases enough to obtain a burnable mixture versus the air (oxygen) supply, *and *the overall temperature has risen to the auto-ignition point for whatever fuel gas mixture is at hand.

When those two conditions are met, substantially all the fuel gas catches fire simultaneously or nearly so. A flame starts someplace essentially at random, then a flame front courses across the entire room in the space of a second or at most a few seconds.

As all that fuel gas ignites, the heat output skyrockets. Which in turn speeds the process of propagating the flame to the rest of the room. Accompanied by a lot of expansion of the room air + fuel + smoke. So there’s a FOOM!! that knocks stuff over, including people followed by a massive increase in the radiant heat which quickly cooks anything and everything in its path.