# What exactly is fire? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [March 15, 2013, 9:15am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991 "2013-03-15T09:15:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ChrisEvilgenius](https://avatars.discourse-cdn.com/v4/letter/c/7ab992/32.png) [@ChrisEvilgenius](https://boards.straightdope.com/u/ChrisEvilgenius)\ **Post date:** [March 15, 2013, 9:15am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/1 "2013-03-15T09:15:27Z")
While the Encyclopaedia Britannica definition is pretty poor and Cecils version is a clear improvement (apart from the “in the presence of heat” bit), surely the real point is what do you see when you’re looking at the flames? For the classical yellow/orange flame the answer is mainly solid particles (soot) at ± 900 degrees C. The colour of light emitted from a body is a function of it’s temperature and is commonly referred to as black body radiation, interestingly it was this observation that lead Max Planck to come up with the concept of Quanta and kicked off the whole Quantum Mechanics malarkey. There is an excellent video on this here: [![](https://img.youtube.com/vi/1pfqIcSydgE/hqdefault.jpg "What is fire?") ](https://www.youtube.com/watch?v=1pfqIcSydgE) ---
**Author:** ![aldiboronti](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@aldiboronti](https://boards.straightdope.com/u/aldiboronti)\ **Post date:** [March 15, 2013, 9:36am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/2 "2013-03-15T09:36:44Z")
I’m guessing that this is the column in question. [Why don’t we consider fire alive?](http://www.straightdope.com/columns/read/1255/why-dont-we-consider-fire-alive-what-is-life) ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [March 15, 2013, 11:58am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/3 "2013-03-15T11:58:22Z")
> [@ChrisEvilgenius](#): > > While the Encyclopaedia Britannica definition is pretty poor and Cecils version is a clear improvement (apart from the “in the presence of heat” bit), surely the real point is what do you see when you’re looking at the flames? For the classical yellow/orange flame the answer is mainly solid particles (soot) at ± 900 degrees C. The colour of light emitted from a body is a function of it’s temperature and is commonly referred to as black body radiation, interestingly it was this observation that lead Max Planck to come up with the concept of Quanta and kicked off the whole Quantum Mechanics malarkey. There is an excellent video on this here: [![](https://img.youtube.com/vi/1pfqIcSydgE/hqdefault.jpg "What is fire?") ](https://www.youtube.com/watch?v=1pfqIcSydgE) ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [March 15, 2013, 8:25pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/4 "2013-03-15T20:25:07Z")
> [@aldiboronti](#): > > I’m guessing that this is the column in question. > > [Why don’t we consider fire alive?](http://www.straightdope.com/columns/read/1255/why-dont-we-consider-fire-alive-what-is-life) I bet it is this one. > **[What exactly is fire?](https://www.straightdope.com/21343561/what-exactly-is-fire)** ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 15, 2013, 8:46pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/5 "2013-03-15T20:46:31Z")
> [@ChrisEvilgenius](#): > > … the concept of Quanta and kicked off the whole Quantum Mechanics malarkey. What is malarkey about Quantum Mechanics? **moriah** , something happened to your post. ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [March 15, 2013, 9:25pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/6 "2013-03-15T21:25:46Z")
Not really. The OP asked and answered their own question. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 15, 2013, 9:51pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/7 "2013-03-15T21:51:49Z")
I believe that’s called a “rhetorical question”. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 16, 2013, 6:13pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/8 "2013-03-16T18:13:10Z")
> [@](#): > > For the classical yellow/orange flame the answer is mainly solid particles (soot) at ± 900 degrees C. Particles at +900C are straightforward enough, but I’d like to hear more about those particles which are at -900C. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [March 20, 2013, 4:22pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/9 "2013-03-20T16:22:34Z")
> [@Chronos](#): > > Particles at +900C are straightforward enough, but I’d like to hear more about those particles which are at -900C. Especially since that’s well “below” absolute zero (−273.15° on the Celsius scale). ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [March 20, 2013, 7:30pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/10 "2013-03-20T19:30:48Z")
> [@ChrisEvilgenius](#): > > Cecils version is a clear improvement (apart from the “in the presence of heat” bit) What’s wrong with “in the presence of heat”? One part of Cecil’s column I didn’t follow was this: > [@Cecil Adams](#): > > I suspect charcoal fires do create flame — you just can’t see it due to the lack of impurities or incompletely burned fuel in the plume. (You can’t see a fuel fire at an Indy or CART race either, because the cars run on clean-burning methanol.) So far as I know, the color of the incandescent glow is a function of temperature. Is he saying that an Indy car fuel fire burns too hot to be in the visible spectrum? And if that’s it, surely charcoal would, if anything, burn too cold. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 20, 2013, 7:45pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/11 "2013-03-20T19:45:01Z")
No, the color of the visible flame is really the color of the particles in the flame that are glowing based upon their temperature. Clear flames are flames that do not have unburned fuel or soot in them. The energy itself does not glow. ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [March 20, 2013, 7:55pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/12 "2013-03-20T19:55:53Z")
> [@Irishman](#): > > No, the color of the visible flame is really the color of the particles in the flame that are glowing based upon their temperature. Clear flames are flames that do not have unburned fuel or soot in them. The energy itself does not glow. Why don’t the air particles glow? (oxygen molecules, nitrogen molecules, etc.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 20, 2013, 9:57pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/13 "2013-03-20T21:57:51Z")
The air particles _can_ glow, but if so, it’ll be in discrete spectral lines. Gas makes a lousy blackbody. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [March 20, 2013, 10:43pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/14 "2013-03-20T22:43:13Z")
> [@Chronos](#): > > The air particles _can_ glow, but if so, it’ll be in discrete spectral lines. Gas makes a lousy blackbody. Isn’t the blue part of the flame discrete spectral lines? ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [March 20, 2013, 10:58pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/15 "2013-03-20T22:58:29Z")
> [@Chronos](#): > > The air particles _can_ glow, but if so, it’ll be in discrete spectral lines. Gas makes a lousy blackbody. Ah, good point. Somewhere in the recesses of my brain, I knew that about spectral lines. ---
**Author:** ![Derc](https://avatars.discourse-cdn.com/v4/letter/d/dfb087/32.png) [@Derc](https://boards.straightdope.com/u/Derc)\ **Post date:** [March 21, 2013, 1:12pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/16 "2013-03-21T13:12:22Z")
If a flame is the result of the chemical reaction of heat, fuel and oxygen and is termed ‘rapid’, compared with oxidation and digestion, is an explosion even more rapid or even instantaneous combustion? Hello members, looking forward to spontaneous chats. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 21, 2013, 2:27pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/17 "2013-03-21T14:27:18Z")
> [@Derc](#): > > If a flame is the result of the chemical reaction of heat, fuel and oxygen and is termed ‘rapid’, compared with oxidation and digestion, is an explosion even more rapid or even instantaneous combustion? An explosion is a rapid expansion of pressure due to rapid combustion. Typically the combustion for an explosion is of the type that produces large volumes of gas from small volumes of fuel. Thus the gas expands, pushing outward, causing the outgoing pressure wave we call an explosion. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [March 21, 2013, 4:08pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/18 "2013-03-21T16:08:15Z")
Something I’ve wanted to ask but was too small to start a thread on: does the material affect the radiation/colours produced? Why use thorium in mantles and lime in limelight? Is this also why some metals melt before glowing, and some glow before melting? ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 21, 2013, 7:06pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/19 "2013-03-21T19:06:47Z")
Yes, flame color is affected by the material being burned. I had a Jr High science experiment where we burned different substances in a bunsen burner flame and then identified the substance based upon the color of the flame. [aside]That is trickier than you might think if the two partners are both color blind.[/aside] For thorium in [lantern mantles](http://en.wikipedia.org/wiki/Gas_mantle), > [@](#): > > A mantle will glow brightly in the visible spectrum while emitting little infrared radiation. The rare earth oxides (cerium) and actinide (thorium) in the mantle have a low emissivity in the infrared (in comparison with an ideal black body), but have high emissivity in the visible spectrum. There is also some evidence that the emission is enhanced by candoluminescence, the emission of light from the combustion products before they reach thermal equilibrium.[2] The combination of these properties yields a mantle that, when heated by a kerosene or liquified petroleum gas flame, emits intense radiation that is mostly visible light, with relatively little energy in the unwanted infrared, increasing the luminous efficiency. Thorium is used because when heated to incandescence, it glows more visible light than infrared light. Basically, the standard light bulb problem. Incandescent bulbs (i.e. the standard light bulbs being phased out for fluorescent and LED bulbs) are filaments of material that when heated via electric current incandesce, i.e. put out visible light. The problem is that they also put out infrared, i.e. heat. As someone once described them, incandescents are heaters that emit light. Energy efficiency strives to find ways to emit more light and less heat. Rare earth metalic oxides seem to have good light to heat ratio. Key words [Incandescence](http://en.wikipedia.org/wiki/Incandescence): > [@](#): > > Incandescence is the emission of light (visible electromagnetic radiation) from a hot body as a result of its temperature.[1] The term derives from the Latin verb _incandescere,_ to glow white.[2] > Incandescence is a special case of thermal radiation. Incandescence usually refers specifically to visible light, while thermal radiation refers also to infrared or any other electromagnetic radiation. [Candoluminescence](http://en.wikipedia.org/wiki/Candoluminescence): > [@](#): > > Candoluminescence is the light given off by certain materials at elevated temperatures (usually when exposed to a flame) that has an intensity at some wavelengths which can be higher than the blackbody emission expected from incandescence at the same temperature… > > The modern scientific consensus is that candoluminescence does occur, that it is not always simply due to selective thermal emission, but the mechanisms vary depending on the materials involved and the method of heating, particularly the type of flame and the position of the material relative to the flame.[1] > When the fuel in a flame combusts, the energy released by the combustion process is deposited in combustion products, usually molecular fragments called free radicals. The combustion products are excited to a very high temperature called the adiabatic flame temperature (that is, the temperature before any heat has been transferred away from the combustion products). This temperature is usually much higher than the temperature of the air in the flame or which an object inserted into the flame can reach. When the combustion products lose this energy by radiative emission, the radiation can thus be more intense than that of a lower temperature blackbody which has merely been inserted into the flame. The exact emission process involved varies with the material, the type of fuels and oxidizers, and the type of flame, though in many cases it is well established that the free radicals undergo radiative recombination.[3] This energetic light emitted directly from the combustion products may be observed directly (as with a blue gas flame), depending on the wavelength, or it may then cause fluorescence in the candoluminescent material. … In any case, the key feature of candoluminescence is that the combustion products lose their energy to radiation without becoming thermalized with the environment, which allows the effective temperature of their radiation to be much higher than that of thermal emission from materials in thermal equilibrium with the environment. A very complicated way of saying that apparently certain materials, when they burn, break into particles that get hotter than the surround air in the flame, and thus emit a higher frequency than expected by a black body at the temperature of the air in the flame. There is debate over whether this factors into latern mantles. [Limelight](http://en.wikipedia.org/wiki/Limelight) uses _quick_lime (i.e. calcium oxide) specifically because it is the first material found to incandesce when heated. Thomas Drummond saw the effect and realized it would be useful in surveying, and developed the effect into a light source. Calcium oxide could be heated with a flame enough to glow without melting. I can’t definitively answer _why_ some materials melt before glowing and some glow before melting. I suspect it has to do with the material’s melt temperature (i.e. the molecular bond strength) vs the black body radiation temperature (i.e. the temperature when thermal emission is in visible wavelengths). But I’m no expert. ---
**Author:** ![Derc](https://avatars.discourse-cdn.com/v4/letter/d/dfb087/32.png) [@Derc](https://boards.straightdope.com/u/Derc)\ **Post date:** [March 21, 2013, 8:50pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/652991/20 "2013-03-21T20:50:01Z")
I’ve seen thin copper sheet actually burn with green flames on the application of heat from a blowtorch, with neither glow nor melt, initially. Thanks for your reply to my question Irishman, still not convinced on the sublimation of a solid to a gas, as in the case of a normally non flammable solid, ie; flour, if a handful were thrown in the air and a naked flame applied the suspended particles would explosively ignite. I too am no expert but have done this ‘trick’ several times over the years. Dust explosions were a common hazard in the early industrial age.