
What’s a fire, and why does it burn? - pizza
https://qchu.wordpress.com/2016/05/26/whats-a-fire-and-why-does-it-whats-the-word-burn/
======
leetrout
When I was a volunteer firefighter circa 1999 we were forced to learn the
definition of fire that would appease a lawyer / court in case we got
subpoenaed.

Apparently somewhere else there had been an arson fire and the fire chief was
on the stand and the defense asked him what fire was and all he could come up
with was "it's hot" or something similar. I don't know if it caused the case
to be won or lost but our Chiefs decided we wouldn't chance the embarrassment.

I'll now forever remember that "fire is the rapid oxidation of a combustible
material resulting in the release of energy in the form of heat and light".

Still waiting on that subpoena and my chance to put this to good use. Or at
least trivia night or something...

~~~
sandworm101
Heat and radiation, some of which is called light. Infrared energy isn't
really heat, nor is it generally called light as it isnt visible.

Similarly, many chemical fires release substantial amounts of UV, but i doubt
anyone would see much of it in an emergency through all the smoke and ir.
Think rocket fuels.

~~~
nsxwolf
I've frequently seen "light" used to refer to the entire electromagnetic
spectrum in physics discussions.

~~~
swift
Yeah, I typically hear visible light referred to as, well, "visible light". It
may depend on the whims of the particular textbooks and professors you learned
from, but in my physics classes we _always_ used "light" to refer to the
entire electromagnetic spectrum, and I only realized relatively late in life
that some people don't use the terminology that way.

~~~
nsxwolf
Plus I like to think of all light as being "visible" to some material or
apparatus. Our eyes just happen to be tuned to a particular part of the
spectrum. You don't even have to look very far to find other animals that can
see wavelengths we can't, like ultraviolet. Radio antennas "see" radio waves,
film emulsions and CCDs "see" X-rays, etc.

Also with radio we talk about different materials being "transparent" to one
wavelength or another.

~~~
hermitdev
If you ever want to have some fun flying, carry some lead crystal in your
carry on luggage. A number of years back, I was visiting my parents for
Christmas, and they gave me a set of lead crystal tumblers. Apparently due
mostly to the lead and partly to the cut of the tumblers, they appeared as
hand grenades on the luggage scanner. The look of absolute puzzlement on the
TSA agents' faces were priceless. They could not reconcile what they saw on
their scanners with what they held in their hand, even when I explained what
it was. I've had similar experiences flying with lead crystal ornaments.
Which, while always a gift, I never have wrapped while I'm flying, because I
know, inevitably I'd need to open it, anyway.

~~~
nsxwolf
That's great! I wouldn't have helped them! I would have just said "Those are
drinking glasses."

------
JshWright
Two videos I'd like to share (well, one video, and one playlist)...

The first is a 'flashover simulator'. This is a training tool for firefighters
to help them recognize the signs of a 'flashover'. A flashover occurs when the
radiant heat of the fire is sufficient to cause pyrolysis and ignition of more
or less every surface in the room simultaneously (the floor, furniture, etc).

I'm mostly sharing this because sitting in one of these is where it first
clicked for me that fire burns gasses, not solids. Watching the fire snake
lazily through the smoke overhead, until it felt like I was sitting on the
bottom pool, looking up at the surface.

[https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s](https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s)

The second is Bill Hammack (The Engineer Guy) delivering Faraday's lecture
series on The Chemical History of a Candle. This is a truly excellent lecture
series, despite its age (they did choose to keep most of the original
language, which can take a little getting used to)

[https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR...](https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR-
wlvWkLan1_r)

~~~
developer2
That first video is visually stunning, and also gives a complete noob a good
idea of what to expect from fire. It's one thing to have some generalized idea
that "fire spreads quickly"; it's quite another to see how a seemingly low-key
flame will almost instantly burst into an unmanageable fireball. It's
interesting to see that there is a pre-emptive warning to a "flashover" \- you
can see the edge of the fire, for lack of better terms, "licking" or "curling"
along the edge. Of course that visual warning is of little consolation if it
comes with less than a couple of seconds of time to react; especially without
a fire retardant suit.

Imagine how many lives would be saved if every citizen could experience this
kind of training first hand. I think inexperienced individuals are likely to
think there is time to gather belongings "while the fire is still scaling".
How many of us, upon seeing a fire "slowly" taking over a corner of the room,
would believe we have plenty of time to collect our most prized or expensive
possessions before running out the door?

Random side thought: I wish we'd come up with a better way of testing
evacuation drills. All drills do is teach people that every alarm is fake, and
so everyone spends minutes packing up their laptops, grabbing their coats,
etc. It's disconcerting to watch people "finishing a block of code", drinking
their coffee, or texting on their phone at an office when an alarm goes off.
And I've seen that at a company where the executive team planned an
_unannounced_ drill 2 months after the scheduled annual drill. As in, nobody
knew it was a test. And yet it took 15 minutes to evacuate a small building.
_facepalm_

~~~
TeMPOraL
> _Random side thought: I wish we 'd come up with a better way of testing
> evacuation drills._

A company-wide policy that if you're still inside 5 minutes after the drill
starts without a _good_ , drill-related reason (e.g. crowded stairwell), you
get your performance bonus cut this month, and if you're still inside 15
minutes after the drill starts, that's ground for firing?

I can't think of anything else that would make people take drills seriously.

An example I remember from my high-school times that particularly annoyed me
was teachers who didn't want to let the class go because they were having a
test, and test is obviously more important than a fire drill...

~~~
woliveirajr
I liked the pun in the argument: if you're inside the building after 15
minutes, you're fired...

------
RangerScience
This is definitely the most detailed description of fire I've ever seen.

Note - from an artistic perspective, and AFAIK, wood (and kerosene, and other
sooty fuels) produce a "deep"-looking flame, while propane (and other 'clean'
fuels) produce a "shallow"-looking flame, due to the black colors introduced
by the soot.

Compare the look of the fire in these propane effects:
[http://www.effectspecialist.com/special_effects_Propane_flam...](http://www.effectspecialist.com/special_effects_Propane_flame_cannons.htm)

to the look of the fire in this kerosene effect:
[http://mentalfloss.com/article/71663/watch-mesmerizing-
fire-...](http://mentalfloss.com/article/71663/watch-mesmerizing-fire-tornado-
spin-slow-motion)

This is also (AFAIK) why you generally don't see "sooty" colored flame effects
- the amount of the coloring agent that is needed to overcome the "darkening"
of the soot is impractical, at least for large-scale effects.

Source: That one time I got really excited the talked to people who'd made a
(gigantic) flame tornado art piece.

~~~
JshWright
The differences in the examples you provided have more to do with the exposure
settings of the camera than the carbon (soot) content of the flame. It's
actually the heated carbon particles that give flames their yellow color. The
soot itself is what's glowing.

~~~
RangerScience
Hmm. I mean, your first point _is_ straight up wrong - I've spent far too much
time looking at (and taking pictures of!) different kinds of fire to think the
difference in appearance between propane and kerosene is just the camera
settings.

But your second point seems pretty reasonable; I'm still not sold. Can you go
into more detail as to how the presence of glowing soot in one kind of fire -
and absence in the other - would create the appearance of solidity (or lack
thereof)?

Edit: Sorry, that came out real aggressive. I mean, can you throw some more
words at how the glowing soot changes the look of the fire - regardless of the
artistic interpretation of the difference?

~~~
mtreis86
If you want to provide visual evidence as an argument, you should really have
the same camera and lens and exposure settings. Not some random images off a
couple websites. Insisting that there is a difference doesn't reinforce your
previous statements at all. The images here
[https://en.wikipedia.org/wiki/Colored_fire](https://en.wikipedia.org/wiki/Colored_fire)
all have the same background and setup to them.

To get back on topic: "Glowing soot" or Blackbody radiation emitted by solid
particles in a flame, is by pyrotechnic standards, a washed out flame. Smoke
as a background is best not burning. Pretty simple math - to get a red flame
of 50 lumens to appear as red when background has gone up to 100 lumens
requires a 150 lumen flame. This is why we don't use fireworks in the
daylight.

Source: Chapter 11 and 21 in fireworks practices and principals
[https://exploders.info/Files/Library/eng/pyrotechnics/firevo...](https://exploders.info/Files/Library/eng/pyrotechnics/firevorks_printsiples_and_pratstitse__ronald_lantsaster__-_1998_462s..pdf)

~~~
RangerScience
> same camera and lens and exposure settings

Well, yeah, that would make a way more effective argument, but I'd have to
obtain a a) manual camera, b) sizable propane fire c) sizable kerosene/etc
fire, d) b & c having nearly identical brightness, and e) a place and time to
do it. These are doable, but not on an internet-debate timeline.

Think about the differences _in your experience_ with propane and wood fired
fireplaces. Most people have had at least one experience with both, and those
are the only things I can think of where the flames in both are of comparable
size/brightness.

> Smoke as a background is best not burning I'm sorry, I don't understand. Are
> you missing a word, maybe?

> [link] Amazeballs link, thank you! Do you know the name of the fireworks
> that produce large quantities of _specifically_ fire? I know them as "gas
> bombs", but I have no idea if that's the correct technical term.

~~~
ygra
> Think about the differences in your experience with propane and wood fired
> fireplaces.

Our eyes and brains are very, very good at working at vastly different
lighting conditions and presenting the results similarly. That's why they
noted that one should use a camera to compare because our eyes are lying to
us. (Seriously: Try to guess proper exposure needed for a photo just with your
eyes. Even with experience it's hard and you're likely to be off by one or two
EV (i.e. a factor or two or four in brightness!)).

However, if you have EXIF info you can probably still compare brightness of
flames across photos, as an image's brightness only depends on the actual
scene brightness, ISO, aperture and exposure time. And the latter three are
merely factors.

------
jimmyswimmy
That was awesome. Reminded me of the Simpsons episode where the PTA disbanded
and Professor Frink teaches a nursery school class: playing with a baby toy,
he tells the children they can't play with it because they won't appreciate it
on as many levels as he does.

I had a quantum mechanics professor explain to us the difference between a
classical mechanics fireplace and a quantum one. Because in classical
mechanics energy is distributed continuously, it would emit gammas,x-rays,etc
and would kill you if you could see it. The quantum fireplace would behave as
fireplaces do, a much less destructive manner. Fun read.

------
nishs
Thoroughly entertaining read.

This article does not cover the history of scientific theory around fire,
which is quite fascinating too. Scientists in the 17th century theorized
(incorrectly, of course) that fire was an intrinsic property of
"phlogisticated" materials [0], which could be burnt to release the phlogiston
contained in them. The theory prevailed for over 100 years, until Lavoisier
proved that a gas (oxygen) must be present for combustion by measuring the
increased weight of phosphorus upon burning [1].

[0] Phlogiston theory:
[https://en.wikipedia.org/wiki/Phlogiston_theory](https://en.wikipedia.org/wiki/Phlogiston_theory)

[1] Oxygen theory:
[https://en.wikipedia.org/wiki/Antoine_Lavoisier#Oxygen_theor...](https://en.wikipedia.org/wiki/Antoine_Lavoisier#Oxygen_theory_of_combustion)

~~~
hexane360
I love the sheer inertia that theory showed. Even after we knew burning added
mass, many scientists believed that it didn't disprove phlogiston, only prove
that phlogiston has negative mass.

------
janvdberg
I thought this would be a link to a Richard Feynman video where he 'explains'
fire. But it isn't, so here you go:
[https://www.youtube.com/watch?v=N1pIYI5JQLE](https://www.youtube.com/watch?v=N1pIYI5JQLE)

~~~
JoachimS
Dang you beat me to it.

------
mncharity
My "What is flame?" \- a contest entry:
[http://www.clarifyscience.info/part/L1Uko](http://www.clarifyscience.info/part/L1Uko)

Skimming OP, one thought. My very fuzzy understanding is that, while flame has
a spectra shaped like a blackbody curve, it isn't one. Instead, that's the
soot emission spectra (due to its diverse mess of bonds). The BBR spectra is
much redder and dimmer. This misconception appears widespread.

Meta-observation: Creating fun and correct science education stories is hard,
requiring multi-year collaborative effort from many people. It seems a pity
there's nowhere on the web to build such.

... Wikipedia has "no original research". The US national science education
wiki was/is? just a search engine for high-barrier low-quality paywalled
science education magazines. Here in the wretched past, the best we manage to
do is accumulate scattered blog and discussion threads. Disrupt, please?

~~~
jcranmer
> ... Wikipedia has "no original research".

But they do have a "List of common misconceptions" page.

------
shmerl
It could also mention that fire can be considered plasma[1].

1\. [http://www.askamathematician.com/2013/05/q-is-fire-a-
plasma-...](http://www.askamathematician.com/2013/05/q-is-fire-a-plasma-what-
is-plasma/)

------
shmageggy
> _Combustion is in some sense the opposite of photosynthesis, an endothermic
> reaction which takes in light, water, and carbon dioxide and produces
> hydrocarbons._

This is a neat idea that I hadn't heard before!

~~~
bcbrown
I've heard it said that Buckminster Fuller described fire as "sunlight
unwinding from tree trunks", which struck me as a very poetic way to phrase
it.

~~~
photogrammetry
Quite beautiful. Thank you for the quote.

------
joeevans1000
well, then...
[https://www.youtube.com/watch?v=edQ08Eze0jM](https://www.youtube.com/watch?v=edQ08Eze0jM)
.

~~~
JoachimS
Love the way he sort of laughs at these ideas. And his dialect. Sounds like a
friendly mobster.

------
falcor84
I came here for the Little Mermaid reference and definitely got some answers.
Thank you for writing this!

~~~
fennecfoxen
I'm disappointed in whoever effaced the _Little Mermaid_ reference from the
title of this article. I thought we were supposed to use the titles the source
article used. :b

------
adriand
This reminds me of growing up in a really conservative, religious household
(think of it as a typical Bible Belt upbringing, except that instead of the
south-east part of the US, I grew up in Canada, of Dutch heritage). God
created the universe 6,000 years ago, etc. Anyway, to the detriment of their
desire for me to grow up believing the same things they did, my parents were
big readers and really encouraged me to read, and as a budding nerd, mainly
what I read was science fiction and science non-fiction, which led me to have
intense arguments with my parents.

I vividly remember my father saying to me at one point, as his response to me
making some point about the validity of modern science, that "no one knows how
fire works", and that therefore science really wasn't so knowledgeable after
all. I remember thinking, "surely if they've figured out atomic fusion, they
know how fire works", but I really didn't have a proper response at the time.
I had no idea how fire worked. It still seems magical!

Some 30 years later, I finally have the answer...perhaps I should send it to
him.

------
mirimir
> The heat produced by combustion can be used to fuel more combustion, and
> when that happens enough that no additional energy needs to be added to
> sustain combustion, you’ve got a fire.

Sure, but what does "sustain combustion" mean? Well, it provides activation
energy for reactions. And here's the thing: lack of activation energy is all
that keeps us and other combustible stuff from burning.

------
flexie
This post is a good reminder that innovations are so often made by people with
zero or little education.

Scientists don't agree on when humans learned to control fire. Some say
200,000 years ago, some almost 2 million years ago. But clearly, man learned
to control and to make use of fire for heating and cooking, for agriculture
and as weapons and even for machines long before anyone understood what it is
or why it behaves like it does.

------
3chelon
I remember when I was about 10 I asked my teacher this question. He had always
been a good teacher, encouraging open-minded debate, so I was quite shocked
when he told me not to ask stupid questions. (In retrospect, he'd probably
just had his fill of me asking unanswerable questions that day.)

Glad to finally get a detailed answer, 39 years later!

------
castratikron
If this was written by someone who "isn't a physicist", this is an excellent
summary of statistical physics.

~~~
loeg
He's a pure math guy.

------
SagelyGuru
Particles is the wrong word to use when trying to explain radiation heat of
fire. Particles usually mean subatomic particles. Particles in general sense
are too vague. Molecules would be better.

~~~
pstch
"Particle" is used at all scales, most usually subatomic, but also microscopic
and macroscopic. A car can be viewed as a particle, so I'm not sure it's so
wrong to label a group a molecules as "particles".

Also, the author was only referring to particles when trying to explain the
concept of thermal radiation where he is actually speaking of subatomic
particles, not of molecules.

------
Pica_soO
I expected to be Feyn-mannered.

TL,DR; Stored chemical energy can be released, by adding to its jiggling- and
produces enough energy to cause a jiggling chain reaction to other storages
nearby.

------
falsedan

      > Thermal radiation is produced by the motion of charged particles:
    

Agreed.

    
    
      > anything at positive temperature consists of charged particles moving around
    

This does not follow: infrared radiation != temperature. Neutron stars have
positive temperature…

~~~
falsedan
Dear downvoters, please comment to let me know what I should improve. Thanks,
falsedan

~~~
philipkglass
You are being downvoted because you appear to be trying to correct a mistake
that does not exist in the source material.

I can make sense of your attempted correction by reading "thermal radiation"
as "infrared radiation," but those terms don't mean the same thing. Thermal
radiation refers to radiation emitted due to a body's temperature, but the
peak emission frequency can go up into the visible and beyond if the body is
hot enough. (Like the sun and larger stars.) Examples of non-thermal radiation
would be e.g. excited state transitions in molecules that produce light in
fireflies or the blue light from the lower portion of a candle flame. (The
blue part of a candle flame is hot relative to the surrounding air and liquid
wax but not hot enough that its thermal emissions would peak in the blue.)
Fluorescent and LED light sources also emit visible light by non-thermal
processes.

~~~
falsedan
I think the error does exist; infrared radiation is a type of electromagnetic
radiation and relies too n charged particles (electrons) absorbing energy,
going through various excitation levels, and then emitting photons with
particular frequencies.

The error in the source is claiming that charged particles are necessary for a
body to have a positive temperature (i.e. what I quoted). The error in the
downvoting is a kneejerk correction without explaining + not reading in good
faith.

~~~
philipkglass
If you had left out "infrared radiation != temperature" it would have been
clear. I didn't understand why you said "infrared radiation != temperature"
when the source material did not say or imply "infrared radiation ==
temperature" and I incorrectly guessed at the point you were trying to make.

I was not one of the downvoters, btw.

~~~
falsedan
Thanks for responding and clarifying, it's helpful.

