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Fire & Freezing: Blowing out candles

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328898.  Fri May 02, 2008 8:05 am Reply with quote

Question: How does blowing out a candle work?

Forfeit: creates a vacuum, CO2 in your breath, removes oxygen

Answer: Fire needs three things to work: heat, fuel and oxygen. The one which is removed when one blows out a candle is heat, a sudden drop in temperature causes the flame to go out.

Any fire needs three components to burn: 1) Oxygen in this case air. 2) Fuel the candle wax. And 3) Heat enough to keep the fuel above its ignition temperature. Removal of any of these three will cause a fire to go out, and in the case of blowing out a candle, it is the temperature that one loses.

What is fire? It is not a solid, liquid or a gas in fact, the Ancient Greeks considered it one of their primary elements alongside Earth, Air and Water it is a chemical process that takes place between oxygen and a fuel. Flames, on the other hand, are bodies of hot gas and superheated soot that contain and sustain the temperature of the reaction.

The tradition of blowing out candles on cakes is centuries old, and has superstitious beginnings as you might expect however the practice became greatly popularised after a Kodak advertisement used the custom to show its cameras capturing such a happy moment.

According to anthropologists, man (and his ancestors) has known how to use fire for 500,000 years, but only learned to make it himself 12,000 years ago. One of the four tribes inhabiting the Andaman Islands is one of the only two tribes in the history of the human race who have never learned how to make fire. The other is the Pygmies of Central Africa. Migrating groups of Andamanese of all sizes routinely carried their fire with them, the glowing embers protected in clay pots and wrapped in large leaves. Additional smouldering logs were often deposited in protected dry places under tree roots as reserves and perhaps also for religious reasons.

Further sources:

Picture ideas:
Someone blowing out a birthday cake.
General flames

328908.  Fri May 02, 2008 8:27 am Reply with quote

Are we saying that these people still can't use a box of matches in 2008, or just that they didn't work this out until they were shown how to do it by outsiders (presumably some years ago)?

328917.  Fri May 02, 2008 8:46 am Reply with quote

No, sorry, it was when they were discovered by westerners. I suppose they can manage to use lighters now.

s: Historical Dictionary of the British Empire

329006.  Fri May 02, 2008 12:03 pm Reply with quote

Is it worth Stephen having a note about candles that re-light themselves? I would have thought someone is bound to bring that up and Stephen would need something up his sleeve to preserve his omniscience. I'm no expert, but I presume that they work by having something impregnated at points into the wick that burns at a lower temperature. As the wick slowly smoulders it reaches another of these points & re-ignites.

Someone would need to check this.

329019.  Fri May 02, 2008 12:27 pm Reply with quote

Yes, you're right. If no-one else has done so in the meantime, I'll look into this early next week.

336933.  Thu May 15, 2008 7:57 am Reply with quote

Part 2 in the series "I wish I'd read this last week":

The World Record for blowing out candles in a single puff has been beaten, V Sankaranarayanan blew out 151 candles in one go in January. He put it all down to yoga breathing exercises.

I knew yoga would turn out to be useful for something.

651273.  Thu Dec 31, 2009 5:16 am Reply with quote

And 3) Heat enough to keep the fuel above its ignition temperature. Removal of any of these three will cause a fire to go out, and in the case of blowing out a candle, it is the temperature that one loses.

So if I had a hair dryer that blew out air at a temperature of around 1000 degrees c I wouldn't be able to use it to blow out a candle? Even if it blew the air really quickly?

651368.  Thu Dec 31, 2009 11:39 am Reply with quote

I think you might have a problem from keeping the material from which the hairdryer was made from melting from the temperature it generated before it could even attempt to blow out a candle.

651373.  Thu Dec 31, 2009 11:53 am Reply with quote

Remember that 1000 degrees C is very hot, as in Bunsen burner hot if not quite blow torch hot.

I don't know what the ignition temperature of a candle is, but I'd imagine that's hot enough.

651380.  Thu Dec 31, 2009 12:08 pm Reply with quote

The earlier theory about the sudden drop in temp causing ignition to fail may be a contributing factor, but probably not the main one.

I suspect the cooling effect causes the wick to "freeze" - at the same time blowing away any residual wax vapour at the top of the wick (which is the real thing that burns).

So, IMHO, it's the loss of fuel that causes candles to go out caused by 1) a restriction in the supply, and 2) the rapid removal of any existing combustible vapour.

On reflection, the two effects may be inextricably linked, as the rapid removal of the vapour would quickly sap thermal energy from the wick... causing it to "freeze".


651386.  Thu Dec 31, 2009 12:19 pm Reply with quote

The autoignition temperature of paraffin wax is said to be in the region of 245 C, and we are warned that a paraffin wax candle will explode before any smoke is seen. Beeswax is said not to autoignite.

A site about hairdryers tells me that the air that emerges from them should never be hotter than about 60 C, because the hair and scalp would start to burn if it were much above that.

So all in all, it would probably be as well not to try the experiment described. But the candle wouldn't go out.

651388.  Thu Dec 31, 2009 12:23 pm Reply with quote

I suspect the entire candle would spontaneously combust at that temp...

651398.  Thu Dec 31, 2009 12:42 pm Reply with quote

Yes indeed - if it's a paraffin wax candle, then it will do so at around 245 C as noted above.

If it's a beeswax candle, then it's less clear. One site I looked at reckons that beeswax does not autoignite at all, while another reckons that it happens at 299 C. It's probably as well to choose to believe the latter!

651983.  Fri Jan 01, 2010 11:40 pm Reply with quote

It's not removal of fuel, as there is still contiguous fuel from the the wick.

Human flesh burns at about 55 degrees.

652006.  Sat Jan 02, 2010 3:32 am Reply with quote

Davini994 wrote:
It's not removal of fuel, as there is still contiguous fuel from the the wick.
The fuel is the vapour - and how do you know there is still stuff coming up the wick?


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